How does the EGR 'sense' insufficient flow (P401 code)?? (2 Viewers)

[IdahoDoug]

I'm wondering if anyone has gotten to know the 95+ system well enough to answer this question? Forget about the 93/94 for the moment - lets focus on the 95+ because I notice on these models there is a California-only model with an EGR temp sensor. We'll tackle that next, OK?

I'm wanting people to THINK before they reply on this - I'm specifically asking 'how does the EGR system sense that it's not getting enough flow?' I'm not asking how does the system work as I've read through it enough times I understand it, and am going to explain that in case I'm wrong:

At low temps, the EGR valve is kept closed by the EGR's VSV (vacuum sensing valve). When engine temp raises, the VSV simply clicks open and sends vacuum to the EGR valve which opens and admits exhaust gases into the combustion system.

If I'm wrong, please advise but that's simplistically how it works.

So, if the VSV works, but the system is still not "getting enough EGR airflow" then what I'm SPECIFICALLY asking is what is the system using to detect airflow? Clearly there's not an air-mass flow sensor there (like the intake side has for example). Clearly there's not an EGR valve position sensor there (like the intake throttle position sensor for example). So what is it that's telling the EGR system it is not getting enough flow?

I'm guessing it's a temperature sensor that at full airflow should be sensing hot exhaust and when it's just a fraction below its expected temperature the system assumes it is not getting full hot exhaust airflow enough to heat the sensor fully? Am I right? I can think of no other way the system can detect lower airflow.

So, please no longwinded posts on the entire system - just tell me how 'low EGR airflow' is detected by the system. Thanks!!!

DougM

 

[tomatopants]

My '94 has a temp sensor. And a code 71, which is EGR insufficient flow.



And it's a California truck.

 

[Walking Eagle]

thanks for explaining how it works, now can you show me how to fix my P401 code?

 

[tomatopants]

thanks for explaining how it works, now can you show me how to fix my P401 code?

I've tried a bunch of things to get my code 71 fixed, which is the same thing, cept OBD-II.

I replaced the VSV, some other vacuum valve I don't remember the name of, the modulator, and I took out the temp sensor to see if it was covered in carbon, which it wasn't.


So I'm stumped now. I think once I get some money, I'm going to replace all the EGR hoses.

 

[CJF]

You guys are gonna get it when Doug gets back.

 

[MDarius]

You guys are gonna get it when Doug gets back.

Yeah...91-94 =

Hmmm...reviving the P0401 debate...sounds dangerous...let's get closer!

















Sorry I have nothing technical to add, but I'm waiting for those who do!

 

[landtank]

I'm pretty sure it has nothing to do with temperature controlling the VSV. While being at a certain temperature is needed to activate the circuit it's not "controlled by it. It is controlled by the engine's RPMS. At idle the circuit is off and then as you accelerate the circuit is on and that when there is a timed response needed by the temp sensor getting to a certain temp. If getting the temp sensor to that point takes to long or doesn't happen at all the error code follows.

The VSV turns on the circuit but the modulator controls it. This is done by the hose at the base of the unit. Because there is only pressure in that pipe when the exhaust valve is open on the number 6 cylinder, we get a fluctuating condition which is used to control the opening of the EGR valve.

So with the system up to temp and not idling the EGR valve needs to be flowing enough exhaust gas to heat that sensor enough to satisfy the ECM.

 

[MDarius]

So, does the code get tripped because the symptoms indicated an insufficient flow, or is there something that is actually measuring air flow directly? What I'm getting at is, is the computer making an assumption based on various readings, i.e: the temp sensor not indicating a the right temp at the right time, or is it from a direct reading?

 

[CJF]

I'm guessing it's a temperature sensor that at full airflow should be sensing hot exhaust and when it's just a fraction below its expected temperature the system assumes it is not getting full hot exhaust airflow enough to heat the sensor fully? Am I right? I can think of no other way the system can detect lower airflow.

Actually, I think you just nailed it.

 

[landtank]

The ECM only cares what the temp sensor reports the EGR gas temp is.

Too low a temp=insufficient flow and too high a temp=excessive flow.

With everyone having this problem and some having changed everything electronic I'd look into making sure that metal tube that feeds the base of the modulator is clean and unobstructed. A restriction ther would effect how much the EGR valve opens and give an insufficient flow error.

 

[knorrena]

Perhaps we are misinterpreting the meaning behind the phrase `insufficient flow`. Could it be that the low flow condition is sensed by the vsv as a continuous current draw, i.e., the valve is not closed, thus draws too much current, and simultaneously does not permit enough vacuum to open the egr valve because the vsv has not closed? Is there a position switch on the vsv?

 

[IdahoDoug]

Rick,

Where do you think the ECM gets its "EGR gas temp" reading from? That's the mystery I'm trying to solve at the moment. My 97's not here in the garage, but while my '93 (California model) has an EGR temp sensor right on it, I don't believe the 97 does. If I'm right (someone with a 97 could go look) then I'm still on that point - what is the temp reading coming from on a 95-97?

On my 93's EGR valve there's a wire on the right side of the EGR valve under the 'hat' shaped circular vacuum valve that's atop it. The wire terminates into a sender that's screwed into a large protuberance that's cast for this purpose. I seem to recall on my 97 this casting is not threaded - just blank.

DougM

 

[subdave]

IDoug ,

My 97 has an EGR temp sensor located very close to the EGR valve. It's function is exactly as you described.

The sensor is a resistance temperature detector. It basically a variable resistor where resistance varies directly with temperature. Works great, last long time.

Now why do I still have this P0401 code....

-Dave

 

[IdahoDoug]

Knorrena,

No offense, but I'm really wanting to focus this thread on figuring out how the system 'knows' (or thinks) there is insufficient EGR flow. What's it using to decide that? If I'm right that the 95-97s don't have the EGR temp sender then it's a mystery worth solving.

Just briefly, I'm going to agree with the findings of others that the VSV is such a simple device that it's unlikely the cause of any of the check engine lights. So I'm leaving it alone in favor of working backward from the title of this thread. Understanding WHY/HOW the engine thinks its getting low flow is key.

So, please let me run with this thread's current focus. Thanks in advance.

I'm in a mood to hammer this thing. I've already figured out how to remove the VSV in 15 minutes and thanks to Alia's idea mine is external at the moment. Once I've 'fixed' my P401 code I'll post it all together. I do have a prediction, which is stupid simple. Evaluating the EGR system for the first time today, I think that the system's tolerance for precise vacuum is such that hardened hoses are the problem. In the end I suspect that new hoses will be the cure.

Anyhow, what say ye on the way the engine decides there is insufficient flow? The diagrams don't show any temperature probe interfaces so I'm at a loss on that.

DougM

 

[IdahoDoug]

Dave,

Thanks - saved me an hour waiting for my 97 to come home. OK - back at it....

DougM

 

[Darwood]

Well considering there are only two things in the system that are electronic, the VSV and the temp sensor, I would think it has to be one of those. I like landtank's explanation best.

Oh and my 97 also has a temp sensor and indicates itself to OBDII as a California Emissions vehicle. The truck originated in Las Vegas.

 

[Walking Eagle]

IDoug ,

My 97 has an EGR temp sensor located very close to the EGR valve. It's function is exactly as you described.

The sensor is a resistance temperature detector. It basically a variable resistor where resistance varies directly with temperature. Works great, last long time.

Now why do I still have this P0401 code....

-Dave

ok, so if it doesn't last a long time or, a long time is the 195K on my truck, how would one go about evaluating if it is bad?

I'm thinking one obvious reason it's saying insuficent flow, is there really is insuficent flow! So, how do you fix that - unless I'm taking doug's thread too far to the left.

 

[IdahoDoug]

OK, looking at the EGR temp sender circuit, etc. Taking a break to shoot BBs in the back yard with the kids.

DougM

 

[knorrena]

No offense taken, and thanks for being cordial. I was just speculating that flow at the egr valve may not be measured at all, in other words, the ecu never really knows, or cares that there is low flow at the egr valve. Perhaps the comment 'low flow' is related to something else, like not enough vacuum to actuate the egr valve and the logic of the ecu concludes that since there is not enough vacuum to actuate the egr, thus its not open and there must be a low flow condition.

My comment, in a perverse way, seems to fall in line with your comment on how precisely vacuum is measured, and that leaking vacuum hoses might be to blame; however, I do not have the familiarity you have with cruisers, and I have no qualms about deleting my comment if you think I am off topic.

Karl

 

[gray]

IdahoDoug;
PLEASE take a lot of photos (or an MPEG movie; can we get that capability on the forum?), if you know a method to replace your VSV in 15 minutes.

This may add a tiny amount to what you are after (if not, sorry).

You may recall on my P401 situation, I had replaced the vac mod, EGR and VSV. Then I performed the on-vehicle test of the temp sensor with the engine running at 4000rpms (EGR should be open); the resistance never got down to where it should have, which means it never got hot enough, which means the EGR never opened.

Subsequently, I checked the temp sensor out of the vehicle; my "freezer-room temp-stove" test; it went exactly by the book, and got down to below 4K ohms when heated to 302 F degrees.

So it follows what others have said above, if the temp sensor is not hot when it should get hot, the ECM sees that, along with ? other engine parameters (throttle position, rpms, time??) and bingo; P401.

But I guess that is what you are after; what are all the parameters does the ECM put together to send a P401, correct?

I don't know.

g