1fz-fe EGR disable ? (2 Viewers)

[Pin_Head]

I'd say that dumping less crap into the engine will increase the engine lifetime. I'd say that not blowing a headgasket is a performance benefit (ever seen how well an engine runs with a blown HG?). I'd even say that not melting your main engine wiring harness might be important for performance (and possible the life of all sorts of stuff, including the ECU).

So what is the connection between the EGR and head gaskets? It isn't obvious.

The EGR just recycles exhaust gas, so it isn't clear that it is dumping anything into the engine that wasn't there in the first place. A tiny bit of it gets a chance to go around a second time. There is a lot of carbon that builds up and breaks off and gets spit out, so engines have to deal with this whether they have an EGR or not.

 

[alaskacruiser]

Interesting article by Kevin McCartney , who is a "five-time Ford Motor Company Certified Training Program award winner. His experience in automotive computer diagnostics spans more than 28 years as a Master Technician, Manager, Instructor, Master Trainer, Consultant and Technical writer/editor. McCartney also has done extensive research and development of alternative fuel, hybrid electric and human powered vehicles."

Decoding EGR Strategies

One quote of interest:

"Combustion chamber temperatures
EGR reduces NOX formation by reducing combustion temperatures. The exhaust gases that are recirculated slow the combustion process and reduce peak temperatures. Modern engines that use EGR systems are designed to perform very well with the slower combustion that EGR causes.

When the EGR system on one of these engines is disabled, combustion chamber temperatures can rise dramatically and actually melt components. The advanced computer-controlled systems on newer cars prevent this from happening, but it still occurs on many older cars."

 

[alaskacruiser]

Another interesting read from Toyota Training Docs that, from what I gather, appear to have been scanned in 2001:
Automotive Training and Resource Site

If you click on the link for:
Technical Articles
Toyota Series - Engine Performance OBD-II
61 Emission#7 - Exhaust Gas Recirculation.pdf file size 227KB

You'll get this file on EGRdirect link)

Another interesting bit of text:

"EGR Impact on the Engine Control System
The ECM considers the EGR system an integral part of the entire Engine Control System (ECS). Therefore the ECM is capable of neutralizing the negative performance aspects of EGR by programming additional spark advance and decreased fuel injection duration during periods of high EGR flow..."

 

[Pin_Head]

Interesting article by Kevin McCartney , who is a "five-time Ford Motor Company Certified Training Program award winner. His experience in automotive computer diagnostics spans more than 28 years as a Master Technician, Manager, Instructor, Master Trainer, Consultant and Technical writer/editor. McCartney also has done extensive research and development of alternative fuel, hybrid electric and human powered vehicles."

Good stuff!

I liked this part that is often overlooked by people that want to disable the EGR:

"Most technicians assume that EGR is controlled independently to reduce NOX emissions and has no other purpose. That is incorrect. While EGR was originally introduced for NOX emission reduction in computer-controlled cars, EGR also is used to increase fuel efficiency, allow increased ignition advance and prevent heat damage to combustion chamber components.

Fuel efficiency mode
EGR has been used to increase fuel efficiency on many cars since the mid-1980s. During steady throttle cruise conditions, EGR is used to reduce throttling losses. Throttling losses are energy that is used to create intake manifold vacuum and overcome the intake restriction of the throttle. It takes significant horsepower to create the intake manifold vacuum that occurs under cruise conditions.


EGR commands are controlled partly according to calculated load. As load increases beyond the level required by federal emission testing, EGR commands drop off. Performance cars reach this point at a very low calculated load.
During light-load cruise conditions, the EGR valve is gradually opened much more than is necessary for NOX emission control. The open EGR valve reduces intake vacuum and replaces some intake airflow with EGR flow. The reduced intake airflow reduces engine power, but the throttle is gradually opened to replace the lost airflow. Either the cruise control or the driver does the throttle opening.

Because the EGR valve is opened very gradually, the driver doesn't even notice that he is opening the throttle to maintain speed. The end result is that intake airflow and therefore, the fuel flow are both slightly reduced, but intake manifold vacuum and throttle losses are significantly reduced. The benefit is reduced fuel consumption under freeway conditions. This EGR based fuel efficiency mode is common on EGR equipped vehicles built during and after the mid-1980s."

You get better fuel efficiency with a functioning EGR.

 

[semlin]

interesting discussion

i own a "federal" 93. toyota didn't give it an egr temp sensor. only california models have them for 93 (and i think the same goes for the 3fe in 91-92). when i bought the truck the egr valve was shot, the plenum was completely clogged, all the vacuum lines were done, the modulator was bad, the vsv valves was bad and the t-body ports were plugged up. it ran fine untit i tried to pass emissions

at the very least, the egr modulator is a part that is intended to be replaced regularly. the whole system is also prone to blockage and failure. toyota was well aware of this by 1993. yet the only system failure monitor installed is the temp sensor and they only installed it where required by law in california.

so if the operation of the egr was critical to engine health and longevity, i am pretty sure they would have put a sensor and CEL in there without beind required to do so by law.

so my view is that the ecu of the 93 is not tweaked to assume the egr valve will be there. for a 95+ ymmv.

 

[Pin_Head]

so if the operation of the egr was critical to engine health and longevity, i am pretty sure they would have put a sensor and CEL in there without beind required to do so by law.
.

They did do just that in the later years, probably for the reasons you suggest. On all modern engines, the O2 sensors will detect the EGR failure as an increase in O2 and richen the mixture to compensate, but burning more fuel is not an ideal response to the problem. If the EGR and the O2 sensors aren't working properly, then you may have some risk of damage. On later engines, the ECU can detect the EGR failure by the temperature sensor, but if you fool the ECU into thinking that it is flowing fine, who knows what happens? I don't, but the Toyota engineers do and they aren't telling us.

What I am saying is that it is probably an acceptable risk to disable the EGR as long as the ECU is aware of it and the O2 and knock sensors are working, but that fooling the ECU into thinking that everything is working properly may be risky.

 

[Ebag333]

You completely missed my point (and Pin_head's before that) about why the EGR plugging itself up is ok, but not with the mod.

Except you never actually addressed it.

You claim that when the EGR plugs up, the ECU detects it and compensates. Except that the ECU does not detect it much of the time. Which you keep skipping over.

You also missed a major point about this not being personal as in "he has to prove this and I have to prove that", but rather exploring the pros and cons of a mod thoroughly and as a group.

So you never said that the onus was on me to prove that it didn't decrease engine longevity? Seems to me that's saying I have to prove something.

No, we don't know that the ECU leans the fuel mixture, and you don't know that it doesn't. That's the point- figuring it out, not some pissing contest you seem to want to initiate for some unknown reason.

Exactly. And there is zero evidence to show that it does for the 1FZ. You can quote docs from new Toyotas or other brands, they're about as applicable as me quoting fuel injection specs for someone who's running a carb. They're completely different engines, completely different applications, under completely different standards, and built for completely different purposes.

Newer engines also have a lot more EGR monitoring systems than the 1FZ does. In order to achieve the behaviour you seem to think the EGR system has the engine needs a lot more monitoring than a single temp sensor can give. Trying to do something like lean out the fuel mixture when the EGR system is flagged green (but non-functioning) is a recipe for disaster when the ECU has no way of telling if it's actually disabled or not.

They did do just that in the later years, probably for the reasons you suggest.

The primary why all OBD-II trucks come with a temp sensor is because Toyota added the EGR system to all N/A spec trucks, largely due to increasing emission requirements that were passed (or threatened to be passed). Basically they made all trucks Cali spec.

I'm sure standardized distribution had a reason for it.

On all modern engines, the O2 sensors will detect the EGR failure as an increase in O2 and richen the mixture to compensate, but burning more fuel is not an ideal response to the problem. If the EGR and the O2 sensors aren't working properly, then you may have some risk of damage. On later engines, the ECU can detect the EGR failure by the temperature sensor, but if you fool the ECU into thinking that it is flowing fine, who knows what happens? I don't, but the Toyota engineers do and they aren't telling us.

The difference is that the failure for the O2 sensors is much much much much more sensitive than the EGR sensor. My wife's O2 sensors are throwing a CEL, yet technically they are within resistance (granted just barely). The EGR system does not have that level of detail, I ran for almost 1k miles with the temp sensor line jumped (same method as the OEM method of disabling the EGR system in OBD-I trucks) without a single CEL and the EGR system flagged green every time I reset it.

Since the resistance was no where near where it should have been, I should have been throwing a P0402 code like crazy, yet I wasn't. I eventually did throw the code, but again not until after 1k miles.

The point is that jumping it mimics a situation that'd happen if the EGR system was stuck wide open, and the resistance is basically off the charts. But the ECU failed to pick up on it, and the engine did not run any different at all. It's basically telling the ECU the opposite of plugging up the EGR system but leaving the sensor intact.

So I've been at both ends of the spectrum, where I should have been throwing a P0401 every 2 drive cycles and where I should have been throwing a P0402 every 2 drive cycles. But I wasn't in either case.

What I am saying is that it is probably an acceptable risk to disable the EGR as long as the ECU is aware of it

Again, even with the temp sensor not disabled the ECU is often not aware of it and thinks everything is just dandy. Which is a giant hole in your theory that you could drive a Cruiser through.



I've asked several times before, but I'll ask again:

Situation A:
EGR system plugged up.
Temp sensor in acceptable resistance range.
EGR system flagged green.

Situation B:
EGR system disabled.
Temp sensor in acceptable resistance range.
EGR system flagged green.

Situation C:
EGR system disabled.
Temp sensor in acceptable resistance range (due to resistor).
EGR system flagged green.


Please explain why A and B are OK but not C? Remember that in all three situations the ECU sees the exact same number from the temp sensor.

 

[semlin]

They did do just that in the later years, probably for the reasons you suggest. On all modern engines, the O2 sensors will detect the EGR failure as an increase in O2 and richen the mixture to compensate, but burning more fuel is not an ideal response to the problem. If the EGR and the O2 sensors aren't working properly, then you may have some risk of damage. On later engines, the ECU can detect the EGR failure by the temperature sensor, but if you fool the ECU into thinking that it is flowing fine, who knows what happens? I don't, but the Toyota engineers do and they aren't telling us.

What I am saying is that it is probably an acceptable risk to disable the EGR as long as the ECU is aware of it and the O2 and knock sensors are working, but that fooling the ECU into thinking that everything is working properly may be risky.

you have the same risk of damage from bad o2 sensors with or without an egr sensor.

i think you missed my point. toyota did put the sensor in in california from 1991-1993, but they did not put it in for the other 49 states because they did not think it was necessary and there was no law requiring it. same ecus. they just use a jumper on the harness.

if ecu was an integral part of engine cooling i would not expect toyota to go cheap on a $5 part.

so i think your concern is unwarranted for the 80 series. maybe different for more finely tuned and newer models.

 

[Pin_Head]

Please explain why A and B are OK but not C? Remember that in all three situations the ECU sees the exact same number from the temp sensor.

Situation A and B report actual EGR flow values (none), while situation C may report a value that the ECU thinks indicates that the flow is appropriate, when there is no flow. It would depend on the value of the resistor. The resistance varies with the EGR gas temperature, which is a function of flow. The ECM uses this temperature to adjust the timing and fuel delivery. The FSM indicates that the ECM should set DTC P0401 after 2 drive cycles with the EGR stuck closed. If not the ECM may be bad.

 

[tblume]

I'm with Semlin on this one- mostly because I have a "dumb" '93 that has the factory jumper. (but I do have that sexy dash...)

Question for you Semlin; Is the 'federal' moniker for the non-CA trucks an industry or EPA designation, or equivalent? All this posturing and argument seems to only effect the OBD-2 trucks that for all intents and purposes are CA emissions anyway if I read in between the lines.

Follow-up question for anyone who knows off the top of their heads: Are ROW vehicles using OBD-2 or is that just here in the litigious states of America?

I seem to recall South American carb'd 80's (and 105's?) so maybe that answers my question.... http://en.wikipedia.org/wiki/On-board_diagnostics or this helps me too.

 

[alaskacruiser]

While I think I'm done repeating myself to Ebag (hint: among a ton of other things, you ignored my sample points of P0401 codes, that you have the same (lack of) data the rest of us do yet you seem sure of what's happening while I am looking to explore a possibility, possible changes in the 95-97 ECUs including different logic to trip EGR CELs, oh and not turning this into a pissing contest, etc etc etc), I have a question for the forum:

Can you guys think of any way to test if the ECU is leaning out the mixture on a '95-97 EGR-modded truck (EGR blocked, EGR temp sensor bypassed with resistor)? I have a Scangauge II. I'm thinking perhaps I can compare LTFT (long term fuel trim) and Ignition Timing numbers (to look for a lean condition) between 2 cases (Note: I'm still reading up on all this stuff, so someone please correct me if I have the wrong idea here):

Case #1. EGR blocked, temp sensor connected, P0401 CEL on
Case #2. EGR blocked, temp sensor bypassed by 10K resistor, no CEL

Does anyone have any suggestions on a better/different way to check for a lean condition? It would be best if I could log/plot this data. Is the Scangauge capable of doing this?

No doubt Ebag will ask "but what about the case #1 where the CEL is off, since half the 80s are running that way anyway?"
Well, since I don't believe that 'lean fuel mixture immediately = "engine go boom"', I'm guessing the ECU is preventing long term damage by catching this eventually. I also do not believe the EGR CEL is as insensitive as he claims, or that a lot of 95-97 80s are running around with blocked EGRs and no code. So I will take many sample points of data on how long my truck takes to identify a P0401 in Case #1 and following the driving operating/environmental conditions in the FSM, because IIRC my results were much more sensitive than those Ebag reported. That way I'll have some concrete data on it.

 

[7schulz]

Well I haven't driven mine extra far (50 miles) but I have no codes and I still haven't found a temp sensor.

 

[Ebag333]

Situation A and B report actual EGR flow values (none), while situation C may report a value that the ECU thinks indicates that the flow is appropriate, when there is no flow. It would depend on the value of the resistor. The resistance varies with the EGR gas temperature, which is a function of flow. The ECM uses this temperature to adjust the timing and fuel delivery. The FSM indicates that the ECM should set DTC P0401 after 2 drive cycles with the EGR stuck closed. If not the ECM may be bad.

This is a complete misunderstanding of the way it works.

The EGR temp sensor reports a resistance value, and the ECU interprets that to a temp. Fairly simple.

If the ECU flags the system as green the resistance must be within the correct range. Simple enough?

So the EGR sensor never reports flow. It reports the temperature.

So situation A, the ECU says "Hmm, the temp sensor is reading in the right range. The EGR system must be flowing as normal."

So situation B, the ECU says "Hmm, the temp sensor is reading in the right range. The EGR system must be flowing as normal."

So situation C, the ECU says "Hmm, the temp sensor is reading in the right range. The EGR system must be flowing as normal."

In all three situations the ECU see's the same temp range, so it thinks that it's working properly. Yet you and alaskacruiser seem to believe that somehow it magically knows more accurately how well the EGR system is working than it actually does.

Case #1. EGR blocked, temp sensor connected, P0401 CEL on
Case #2. EGR blocked, temp sensor bypassed by 10K resistor, no CEL

Does anyone have any suggestions on a better/different way to check for a lean condition? It would be best if I could log/plot this data. Is the Scangauge capable of doing this?

No doubt Ebag will ask "but what about the case #1 where the CEL is off, since half the 80s are running that way anyway?"
Well, since I don't believe that 'lean fuel mixture immediately = "engine go boom"', I'm guessing the ECU is preventing long term damage by catching this eventually. I also do not believe the EGR CEL is as insensitive as he claims, or that a lot of 95-97 80s are running around with blocked EGRs and no code. So I will take many sample points of data on how long my truck takes to identify a P0401 in Case #1 and following the driving operating/environmental conditions in the FSM, because IIRC my results were much more sensitive than those Ebag reported. That way I'll have some concrete data on it.

It's been my experience that there are a lot of 80's running around with blocked EGR systems. Just search for P0401 on the board. Many of those people are getting an EGR code infrequently, despite having plugged up systems.

Even if it's not as insensitive as I believe it is (which you've offered no actual evidence of) in a normal situation the engine could be causing itself damage easily at least half the time (if not more).

Here's some evidence for how not sensitive the ECU is to the temp sensor. Go read Bear80's thread and compare what people are running to what the FSM says. Technically even the 1k resistor is out of spec....but by running that people are not getting an ECU code. I ran without a resistor (or temp sensor, line jumpered) and didn't get an ECU code for over 900 miles. That resistance is no where near the nominal range, but the ECU thought that the EGR system was operating just fine (and I know it was because it flagged the system green multiple times after being reset).

If the resistance is out of range the ECU should throw a code because it sees it as too much or too little flow (temp too high or too low). But it didn't. And you don't have an explanation for it other than "it must not be true."

 

[semlin]

the egr temp sensor has nothing to do with controlling the egr system and afaik the ecu does not use its signal for anything. it is just a rather crude feedback tool to tell the operator when the egr is not functioning.

as far as responding to fuel trim, the fact that toyota engineered this truck for third world use and sells a supercharger kit for this engine, plus the proven longevity of the engine, tells you that the o2 sensor and ecu together are quite capable of adjusting to a modest lean condition. i am sure the ecu just throws fuel at the problem.

 

[Ebag333]

the egr temp sensor has nothing to do with controlling the egr system and afaik the ecu does not use its signal for anything. it is just a rather crude feedback tool to tell the operator when the egr is not functioning.

Exactly! Some people seem to think that the ECU has some sort of insight into the EGR system that it does not.

as far as responding to fuel trim, the fact that toyota engineered this truck for third world use and sells a supercharger kit for this engine, plus the proven longevity of the engine, tells you that the o2 sensor and ecu together are quite capable of adjusting to a modest lean condition. i am sure the ecu just throws fuel at the problem.

Bingo. The FSM tells us that the ECU uses the O2 sensors to adjust the fuel trim, but it never says that it uses the EGR system to do the same. Why would Toyota tell us one scenario that causes the fuel trim to adjust but not another? It just doesn't make sense.

 

[96r50]

the egr temp sensor has nothing to do with controlling the egr system and afaik the ecu does not use its signal for anything. it is just a rather crude feedback tool to tell the operator when the egr is not functioning.

Exactly! Some people seem to think that the ECU has some sort of insight into the EGR system that it does not.

While I am starting to see this as viable and quite possibly true, I think it's probably impossible to prove it 100% since there isn't much/any documentation about the inner workings of the ECU (not that anyone has to prove anything, we're all just here for the fun of it!). It is similar to the question of whether or not the ECU uses the transmission temperature sensor for anything other than setting off the A/T Temp warning light. I'm (currently) of the opinion that it does not, and I'm leaning that way with the EGR temp sensor as well.

Semlin, what happened to all your hair?

as far as responding to fuel trim, the fact that toyota engineered this truck for third world use and sells a supercharger kit for this engine, plus the proven longevity of the engine, tells you that the o2 sensor and ecu together are quite capable of adjusting to a modest lean condition. i am sure the ecu just throws fuel at the problem.

Bingo. The FSM tells us that the ECU uses the O2 sensors to adjust the fuel trim, but it never says that it uses the EGR system to do the same. Why would Toyota tell us one scenario that causes the fuel trim to adjust but not another? It just doesn't make sense.

I agree that the ECU and O2 sensors should be quite capable of adjusting to a lean condition produced by disabling the EGR system by dumping more fuel. However, I wonder if that leaves a smaller margin of error for a case where the ECU has to compensate even more for an additional lean condition brought on by weak gas or an air leak? Injectors can only squirt so much fuel. Obviously an AFR gauge/wideband O2 would allow this to be monitored, but that seems like a lot of additional hassle.

I think in the end, disabling the EGR is a personal decision and it should be done after considering both the pros and the cons of it and deciding which you agree with more. In my case, I was able to troubleshoot the EGR system on my truck and get it operating properly again without much hassle so I'll leave it alone for now. YMMV!

 

[Ebag333]

Semlin, what happened to all your hair?

Hmm, I wondered about the same.

I agree that the ECU and O2 sensors should be quite capable of adjusting to a lean condition produced by disabling the EGR system by dumping more fuel. However, I wonder if that leaves a smaller margin of error for a case where the ECU has to compensate even more for an additional lean condition brought on by weak gas or an air leak? Injectors can only squirt so much fuel. Obviously an AFR gauge/wideband O2 would allow this to be monitored, but that seems like a lot of additional hassle.

IF the ECU leans out the engine when the EGR is operating (which there's zero evidence for) then yes it'd give you a slightly smaller margin of error.

But which do you think is going to have a bigger effect, strapping on a blower or disabling the EGR system?

Considering that a blower will be operating when the engine is at the highest amount of risk for damage--WOT, heavy accelleration, heavy loads, etc--then I'd say the blower for sure.

The EGR is only supposed to operate under conditions which will be the least likely to damage the engine if you run slightly lean--very little load, highway cruising speeds--basically the situation where you have the greatest amount of room to play with.

Even if you had both a blower and disabled the EGR system I'd say that you are at no more risk than just one or the other, as they operate at different points in time (basically mutually exclusive).

 

[Pin_Head]

This is a complete misunderstanding of the way it works.

The EGR temp sensor reports a resistance value, and the ECU interprets that to a temp. Fairly simple.

If the ECU flags the system as green the resistance must be within the correct range. Simple enough?

So the EGR sensor never reports flow. It reports the temperature.

This is pedantic. The temperature is directly proportional to EGR gas flow, so the ECU uses the temperature as a means of monitoring flow. This is clear from the FSM trouble shooting guide. When there is no flow, the temperature of the sensor is 100F less than ambient and the ECU is supposed to set the CEL and P0401. If it doesn't, there is something wrong, such as the temp sensor and it needs to be fixed.

 

[Ebag333]

This is pedantic. The temperature is directly proportional to EGR gas flow, so the ECU uses the temperature as a means of monitoring flow. This is clear from the FSM trouble shooting guide. When there is no flow, the temperature of the sensor is 100F less than ambient and the ECU is supposed to set the CEL and P0401. If it doesn't, there is something wrong, such as the temp sensor and it needs to be fixed.

My temp sensor specs out according to the FSM. It's not a problem. My ECU specs out according to the FSM. It's not a problem.

Why do you keep insisting that the simplest explanation (and most logical one) must not be true?

When there is no flow, the temperature of the sensor is 100F less than ambient and the ECU is supposed to set the CEL and P0401.

From the FSM:

122 (F) - 64K -97K Ohms
212 (F) - 11K - 16K Ohms
302 (F) - 2K - 4K Ohms

Again, you seem to not understand the way that the EGR system works unless you think that ambient is regularly around 250* F.

Ambient temp (or more correctly the intake temp at startup) appears to be one of the factors that determines whether the EGR system is tested, but it is not the test to see if it passes or fails. The pass/fail is based on the temps above. Ambient temp is used to enable the pass/fail test.

If the EGR system is flagged green, then the temp sensor is reading a resistance of 11k to 16k Ohms.

But what we're seeing is two things:

1) The range that the ECU considers acceptable seems to be larger than the resistances above. I've had my truck run for almost 1k miles with a resistance of (essentially) 0, and no CEL, despite being far outside the normal range. Others have run their trucks with 1k resistors, and again no CEL's.

2) Even without bypassing the temp sensor the ECU flags the EGR system as green with a plugged up EGR system. Unless you want to suggest that dozens upon dozens of people have a bad ECU, the simpler option is that the ECU sees the EGR system as good because the temp range is acceptable to it.




It would be interesting to drive around with different sized resistors and figure out where the threshold for the EGR system is set at. While you couldn't get it down exactly, you could come close. We already know that the 1k resistor works just fine, which shows that the ECU thinks that anything below 300* is just fine. As I ran for almost 1k miles without a CEL, even above 300* is okay, though how much below is hard to say.

AFAIK no one's gone the other way very far. There's one person I know who was using a 64k resistor without issue (granted over a relatively short period of time) which seems to suggest that temps as low as 150* are acceptable to the ECU.



For those who are interested there's a technical doc that can be found on the TIS site about the EGR system. It details the basics of the three different types of detection methods used by the ECU to determine the state of the EGR system.

There are three types
of detection methods are:

• EGR temperature detection method
• EGR MAP detection method​

• EGR valve position/temperature detection method

The temperature only method is rather crude and basic, and does not give very good resolution into the EGR system.

The MAP and valve position plus temp detection methods are far more advanced and give a nice clear resolution into the operation of the EGR system. Either of those methods could be used for the more advanced EGR operation that you find in newer engines, such as leaning out the fuel flow.

Even the MAP method is not much more advanced than temp only as it relies on a misfire to set a DTC P0402 code.

The valve position + temp detection method can actually see if the valve is open or not, and what the temps are. Unlike the temp only method, this allows the ECU to see if the EGR system is truely functioning or not. That's the difference between newer Toyota engines and the 1FZ, they use very different methods of determining what the EGR system is doing.

 

[Pin_Head]

But what we're seeing is two things:

1) The range that the ECU considers acceptable seems to be larger than the resistances above. I've had my truck run for almost 1k miles with a resistance of (essentially) 0, and no CEL, despite being far outside the normal range. Others have run their trucks with 1k resistors, and again no CEL's.

2) Even without bypassing the temp sensor the ECU flags the EGR system as green with a plugged up EGR system. Unless you want to suggest that dozens upon dozens of people have a bad ECU, the simpler option is that the ECU sees the EGR system as good because the temp range is acceptable to it.

Even if it is true that the ECU does not set the CEL with no flow, it doesn't mean that the ECU does not use the temperature information to monitor EGR flow. Why would they go to the trouble of installing a temperature sensor if they aren't going to use the information? My understanding is that they use this information to adjust the air flow mix, ignition timing and what the O2 sensors expect to see to optimize the conditions to reduce combustion temperatures (and NOx) and to improve fuel efficiency. It is easy to understand that if the ECU has bad information about the EGR flow, that the conditions might result in increased combustion temperatures and detonation. If it has good information about the lack of flow, it can increase the fuel delivery to keep the mixture within the stoichiometric range and the combusiton temperatures in a reasonable range and back off the timing to prevent preignition. This would seem to be counter productive because why would someone go to the trouble of disabling the EGR to get worse fuel economy?

What is up with the head gasket and the EGR? This isn't obvious.