How does the EGR 'sense' insufficient flow (P401 code)?? (1 Viewer)

[Bear80]

well you're lucky, but I'm pretty sure others here have also concluded that the '97's have a sensitive egr trip system. Mine ran for about 10k after the initial code and modulator replacement but came back and won't go away no matter what I've tried.

 

[gray]

Yes, I have the '95 version. This is a huge section with lots of details. I imagine if I actually read the FSM I wouldn't have to ask so many questions in tech.

''Ding! Ding!''

But back to my previous question, what are we buying by fixing this? I know - we all like our 80's to be like new and the light is embarrassing, but are we wearing anything out or losing significant MPG's? or, is it just a light?

My gas mileage went up my 1 -1 1/2 mpg immediately after I replaced the the EGR and vac mod (already had put in a new VSV), and stayed that way for the 2500 miles that the MIL was off. But then the MIL came back and haven't checked my mpg since.

But now after more fiddling, cleaning, checking connectors
and applying the dielectric grease,etc; it has stayed off for
about 250miles.

g

 

[RavenTai]

So raven,
I just read your thread, thanks for all the research and sharing it with the rest of us. I noticed you did not test the VSV. Were you able to perform all the other tests without removing the intake manifold? I've been there done that and what a pain. I'm thinking I'll try your tests along with the tests in the FSM and see if I can solve my code issues, but if I have to remove the intake I'll wait till the weather warms up. I'm a sissy without a garage. Thanks.

Actually I did remove and test the VSV the FSM way, it tested good per the FSM even though it was intermittently bad, I also tested it with my own "wile driving vacuum test" and here is where it showed up bad, replaced it and p0401 gone, intermittent problems can be a real bear to find via the FSM tests.

You can remove the VSV in about 15 minutes without removing the intake manifold, search for a post by beowulf that has details.

defiantly do the FSM tests of the EGR valve first. vacuum test is only meaningful if the EGR valve is known to be good,

briefly

Run the engine at idle, check the EGR pipe it should not be that hot, (about 200F) if it is very hot the EGR valve is leaking, plug one port to the EGR valve and apply vacuum to the other the port,

The engine should stall and the pipe will be very hot (instant finger tip sear like bacon) this proved the valve is not stuck and opens when vacuum is applied, the valve should hold that vacuum without leaking down, if it leaks down the diaphragm is leaking

after you know the EGR valve is good you know when EGR gas is flowing by watching what kind of vacuum is applied to it

 

[RavenTai]

well you're lucky, but I'm pretty sure others here have also concluded that the '97's have a sensitive egr trip system. Mine ran for about 10k after the initial code and modulator replacement but came back and won't go away no matter what I've tried.

have you tried watching it with a vacuum gauge?

 

[MDarius]

Thanks!

 

[Bear80]

have you tried watching it with a vacuum gauge?

I've left out a lot of the trouble shooting I've done for my p401 and it would appear I gave up rather quickly and slaped a resistor in the temp sensor's place. That however, is not the case. I have tried gauges, testing intermently while it should be operating, etc. I'm inclined to belive my temp sensor while testing correctly is the culprit. If my assumptions are correct then all else should be within acceptable function with the resistor. Again the EGR is not a critical system but I'm not one to discard the whole of it because it does do some good. This thread brought it back to the front of my mind, so I might try to mess with it again but I'm not expecting to find the smoking gun.

 

[baxter650]

Raven Tai: i made the assumption that because the EGR valve was opening and the engine stalled at idle that the system was OK, i then failed smog, came home and found i had lost vac from manifold. I then teed into ported vacuum from throtle body and ran that to the EGR valve and it was then opening, retested and failed again. I got the system components working but the EGR piping itself was clogged up so i was not getting enough flow to dilute and cool the combustion chambers at cruise, after cleaning i retested and sailed through.
Vince

 

[RavenTai]

Raven Tai: i made the assumption that because the EGR valve was opening and the engine stalled at idle that the system was OK, i then failed smog, came home and found i had lost vac from manifold. I then teed into ported vacuum from throtle body and ran that to the EGR valve and it was then opening, retested and failed again. I got the system components working but the EGR piping itself was clogged up so i was not getting enough flow to dilute and cool the combustion chambers at cruise, after cleaning i retested and sailed through.
Vince

Sorry if that was not clear, it was not my intent to state that if the EGR valve works then you have no problem,

my statement is that you must verify the EGR valve via the FSM tests before further troubleshooting of the rest of the system per the link I posted in post #54. it was a confirmation and detail to this MDarius's comment:

"I'm thinking I'll try your tests along with the tests in the FSM and see if I can solve my code issues"

 

[MDarius]

Question: Do old vac hoses collapse under the pressures related to this system? I'm thinking the walls of the hose weaken over time. Is this an issue or non-issue?

 

[gray]

My impression is that the vac hoses just do not seal well enough when they get old.

Example: The hose that runs from the DS rear of the manifold up to the top of the EGR valve on my 80 was not cracked; looked good, but the ends were hard, and I noticed that I could pull it off the EGR nipple/pipe with little effort; ie: it did not "stick" like they do when you first try to remove them, assuming they have not been previously pulled off.

I then looked closely at the ends of the hose; the end holes had opened up a bit, and it was stiff.

As discussed in this thread and others, I think the EGR system is not "robust" enough; ie: not enough extra vacuum to make up for small leaks; like from any old hoses that are not stuck/sealed well enough to their nipples/pipes/ports.

My thoughts; if you fiddle with a 10-15 year old hose that is not clamped on, like all the small ones, it may not reseal itself well enough to keep a good vacuum. This goes doubly for the EGR to vac modulator hose (more of a pressure hose) which actually takes quite a beating from heat and pressure. For grins, pull that hose off from the bottom of your vac modulator and run the engine; you will be impressed by the amount of pressure being pumped out that little hose; just have a replacement handy when you do that.

G

 

[landtank]

I think the EGR system is not "robust" enough; ie: not enough extra vacuum to make up for small leaks; like from any old hoses that are not stuck/sealed well enough to their nipples/pipes/ports.

210,000 miles and never had a P0401 code. My impression is that I do extensive highway driving so there is always good flow through the system which helps keep it clean. Like so many things with automobiles short runs are the most damaging.

 

[gray]

Agree with Landtank; however (not to jinx his vehicle) my 80's P401 saga started just around 200K; most of my miles are also highway.

I now have approx 300 miles and multiple trips since my last attempt to cure my P401; we will see what happens on my next trip: 1300 miles out and back.


g

 

[landtank]

I've posted several times that people should be measuring the temp sensor and VSV with their VOMs at the ECU plugs. I hope you guys are doing this as I doubt that only injector wires are taking the abuse where the harness passes over the EGR valve. This could very well be a different symptom of a very well know problem.

 

[CJF]

WHO HAS ACTUALLY REPLACED THE TEMP SENSOR IN AN ATTEMPT TO CURE THE "INSUFFICIENT FLOW" ISSUE?

I feel like I've still gotta throw this out there:

1) David Dearborn (professional mechanic for those who don't know) says he'd had success by doing this when the other components check out.

2) Bear80 (if I'm interpreting this right) is convinced that's his problem has well, but hasn't tried replacing it to verify his hypothesis.

No one???

I swear, I feel like buying someone a freaking sensor in the interest of science. (And to put these dang threads to rest...)

Curtis

 

[MDarius]

I feel like I've still gotta throw this out there:

1) David Dearborn (professional mechanic for those who don't know) says he'd had success by doing this when the other components check out.

2) Bear80 (if I'm interpreting this right) is convinced that's his problem has well, but hasn't tried replacing it to verify his hypothesis.

No one???

I swear, I feel like buying someone a freaking sensor in the interest of science. (And to put these dang threads to rest...)

Curtis

I'M IN!! I'll be happy to PM you my ship to address.

I have changed the EGR, VSV, cleaned all the hoses and the through-pipe on the air intake, and cleaned out the throttle body, but that's it. I haven't actually done any testing, so I'd be a GREAT candidate!!

 

[Bear80]

I think Rick hit it right on the head. Although my 'cruiser is a weekend driver, it doesn't see much highway use overall. Also tolerances can and do change in electrical systems, increased resistance in the wires etc. I really hate to sound like I half ass stuff on my 'cruiser but this p401 has been a time consuming pita that isn't worth the time, other the piece of mind knowing it's exactly right.

 

[Beowulf]

I swear, I feel like buying someone a freaking sensor in the interest of science. (And to put these dang threads to rest...)

Curtis,
As you know, I follow these P0401 threads fairly closely. I believe Raven has done the best job so far with the proper testing and documentation. I sincerely believe there is no SINGLE fix. It is a relatively simple system with 4 components and the interconnecting hoses and tubes.

First, those hoses and tubes have to be clear of blockage and should not be leaking.
Second, the individual components should be tested. Raven and I both had intermittent VSV issues that were resolved with the replacement of the VSV for EGR.

I do not believe you can include Dearborn's "fix" by replacing the EGR temp sensor for his customer. There are a lot of reasons and just look at the many, many P0401 posts. I cannot tell you how many (but a LOT) of guys have taken their trucks to multiple shops. If the first doesn't fix it then they either take it to another or try to fix it themselves. Over and over again, the same story. My guess (and I would bet $$ on it...) is that customer had a recurrance and either took it somewhere else, fixed it himself, or is living with the MIL.

I have not read of a single confirmed, permanent fix of a P0401 by replacing the temp sensor. Lots got cleaned though, including mine about 5 times.

-B-

 

[IdahoDoug]

Had time to play with the 97 with the CEL on for the 401 code. VSV checks out OK on the blow test and resistance and no stray ground. I pulled it and it is now sitting on the outside of the intake manifold while I pursue the 401 problem.

Found a very loose vacuum line that did not agree with the FSM diagrams and replaced it. For those who have looked under there, the diagram shows a T where the EVAP and EGR systems are connected very close to the EVAP's check valve. The diagram shows the line they T into going to the low vacuum area of the throttle body. On my truck, the line actually goes into a full vacuum port on the intake manifold. As loose and dry as this short 3" 90 degree elbow was, I'm glad I found it.

I also pulled the EGR temp sender and found it coated pretty uniformly with enough carbon it could be reading low or slow due to the insulation effect. Cleaned it to bare metal and put it back in.

As part of moving the VSV outside the intake for ease of access, I also bypassed that strange hose from the EGR valve that goes "through" the intake. Informationally, my EGR modulator is new via Cdan a couple months ago as a quick/cheap fix that did not prove to be the solution. I'm down to very few remaining problems now if the code returns.

So, I'll let you know if it comes on again.

Kudus to RavenTai for an excellent post (he references it above) that clarifies everything you could POSSIBLY want to know about the EGR system to definitively fix it, and you cannot fix it without knowing what he knows. So read the thread.

DougM

 

[baxter650]

Is it stated in the FSM what the vacuum reading should be at cruise.
Has anyone else teed into their EGR vac line and done a reading at cruise.
I did and found that at cruise i had squat for vac, that was why i teed into vac from my throttle body.

 

[CJF]

I have not read of a single confirmed, permanent fix of a P0401 by replacing the temp sensor. Lots got cleaned though, including mine about 5 times.

Lots of good info as usal Beo, but I still haven't gotten one single person other than Dearborn to say they they've even replaced one period, let alone whether it did/didn't solve the problem.

Call me a conspiracy theorist if you like, and of course I'm probably wrong anyway, but it's still bordering on bad science.

Curtis