Section 24: EGR Systems

I was looking into the E-Book to see if I could help weigh in on a topic posted by one of the members in the repair section.
scannerdanner.com/forum/post-your-repair...ng-at-idle-only.html
It was about a Subaru with a misfire at idle that cleared up under part throttle.
The long term fuel trim was -23%, indicating a rich condition. My interpretation of this is that the recirculated exhaust displaces a certain amount of oxygen which is accounted for by the o2s as a rich condition.
Come to Page 6 of section 24 with me if you will, and we have a data capture of a 92 GMC with a 4.2l v6 suffering from a stuck open linear egr valve.
RPM-913 idle
o2 mv 482
Integrator 108
block learn 108. I don't fully understand this pre-obd2 terminology, but it's labeled as "Lean Command"
Loop status-closed
exhaust oxygen- lean.
My question is this: Why on this Chevy are we seeing a simultaneous lean condition AND lean command?

Good question but none to answer ooh!i think it's oxygen sensor manufuntioning.

Two things.
1. The only reason the stuck open EGR is causing a rich condition on this vehicle, is because it is a speed density engine (no MAF sensor). So at idle, the vacuum in the engine is super low from the stuck open valve and the MAP sensor is indicating a heavy load situation which is causing the rich condition.
2. The O2 signal you are seeing is one single moment in time, you cannot determine the average state of the O2 signal here. (I promise you it was rich)

To sum this up. A stuck open EGR does not cause a rich condition. The statement that EGR gas displaces oxygen and therefore makes the mixture rich, is false. (It is what I used to believe and even taught for years)

I'm going to find a case study I did with a stuck open EGR on a MAF engine and post it here.

Also, go through my week 1 playlist on SD Premium and look for a video titled "the tomato sensor". This changed everything for me.

These are both SD Premium videos. If you're not a subscriber, let me know and I'll give you a coupon good for 30 days.

Oh shoot. Noah! I didn't realize this was you. Is this a question from someone on the forum or was it yours?

DANNER! I love your reasoning oooh!no maf,true.map was reporting 84 kpa before a fix after fix 45 kpa.Thank u

I like learning and watching premium vedios but not available in kenya. Vpn?i know less abt it oooh may be i will ask my mobile phone network provider.Thank u a follower of Jesus Paul d.

ScannerDanner wrote: Oh shoot. Noah! I didn't realize this was you. Is this a question from someone on the forum or was it yours?

Hey paul,
It's my question. Another member had a Subaru with a rough idle that cleared up under part throttle, I linked the thread in my first post. The problem was a stuck open EGR valve, and his LTFT were max negative, like -23.
I'm gonna have to watch that tomato sensor series again.
I watched it before, but it honestly left me with more questions. I think now, I may be able to absorb the material better.
Thanks for the replies on this topic man, I appreciate it!

I get that it's only one frame in time, and that it's going to change. I believe you that it was rich, that one particular capture just wasn't showing it.
Also the Subaru is MAF (or at least it should be). Trust me, I'm not trying to turn this into a debate, I hope you don't think I'm questioning you for the sake of argument. I really just want to understand this the best I can to be the best tech I can.

Noah wrote:

ScannerDanner wrote: Oh shoot. Noah! I didn't realize this was you. Is this a question from someone on the forum or was it yours?

Hey paul,
It's my question. Another member had a Subaru with a rough idle that cleared up under part throttle, I linked the thread in my first post. The problem was a stuck open EGR valve, and his LTFT were max negative, like -23.
I'm gonna have to watch that tomato sensor series again.
I watched it before, but it honestly left me with more questions. I think now, I may be able to absorb the material better.
Thanks for the replies on this topic man, I appreciate it!

I get that it's only one frame in time, and that it's going to change. I believe you that it was rich, that one particular capture just wasn't showing it.
Also the Subaru is MAF (or at least it should be). Trust me, I'm not trying to turn this into a debate, I hope you don't think I'm questioning you for the sake of argument. I really just want to understand this the best I can to be the best tech I can.

Not at all Noah! I honestly am okay with still being wrong with the EGR gas and how it actually affects the air/fuel ratio and the combustion process. This is chemistry man, and to be honest with you, I am not good with chemistry.
Are you saying that the stuck open EGR on a MAF design engine was the cause of the -23% trim? If so, I guess I need to go back to the drawing board on this subject again

This is chemistry man, and to be honest with you, I am not good with chemistry.

I'm right there with you man! There's a reason I'm not a chemical engineer

I just finished watching the Mazda case study video, thanks for putting that up. Man that initial vacuum reading was scary! But it does reinforce the rich command on a MAP engine, I totally get that.

I'm going to see if I can plug into my truck after dinner and force the egr open to see what the trim numbers look like.

Are you saying that the stuck open EGR on a MAF design engine was the cause of the -23% trim?

You know, I can't say for certain that it was the cause. That was just the information posted with the help request. I never thought to ask if the LTFT corrected after the EGR repair. That could blow the whole thing wide open if there is another unrelated fault causing the negative trims.

I'm going to see if I can plug into my truck after dinner and force the egr open to see what the trim numbers look like.

Sweet! That's why I like you Noah. Keep me posted okay?
Question:
Does your truck have both a MAF and MAP or just MAF by itself. I think this can also add a variable if it has both.

Noah wrote:

Are you saying that the stuck open EGR on a MAF design engine was the cause of the -23% trim?

You know, I can't say for certain that it was the cause. That was just the information posted with the help request. I never thought to ask if the LTFT corrected after the EGR repair. That could blow the whole thing wide open if there is another unrelated fault causing the negative trims.

So it was a stuck open EGR, that fixed his problem though right? But are still questioning the fuel trim after the fix?

Yes, the EGR fixed the complaint. I didn't think to ask him about the fuel trim after the fix honestly. Looking back, I guess it's not fair to say that the fuel trim was 100% a reaction to the egr gas. He didn't post any data from after the repair. Which is fine, a fix is fix, in the shop you gotta get on to the next one.

I "think" I have both, I'll have to check for sure. This is exactly the variable I was contemplating while writing this post to begin with. I imagine these results won't be the same across the board from one manufacturer to the next just because they don't all run just a MAF or just a MAP.

Drops dead lean, it's just a giant vacuum leak as far as the 02s are concerned. They don't give a darn about the exhaust gases. I'm trying to resize the data capture to post.

1999 Lincoln Navigator DOHC 5.4l Vin A
EGR commanded open @ idle by grounding EVR solenoid control wire.
Truck stalls immediately. The only way I could get it to stay running without manipulating the throttle was to activate the solenoid during the initial start up high cold idle phase.
o2s drop dead lean, STFT goes to +45.
There does not appear to be a MAP sensor on this engine, MAF only

Nice capture, Noah! The design of your engine works well for this test, too, since there's no EGR position sensor.

Noah wrote:

Attachment not found

1999 Lincoln Navigator DOHC 5.4l Vin A
EGR commanded open @ idle by grounding EVR solenoid control wire.
Truck stalls immediately. The only way I could get it to stay running without manipulating the throttle was to activate the solenoid during the initial start up high cold idle phase.
o2s drop dead lean, STFT goes to +45.
There does not appear to be a MAP sensor on this engine, MAF only

Awesome Noah! Changes everything in our understanding of EGR gas doesn't it?
I still don't fully understand the O2 sensor and combustion gases like I should. But I do know enough to fix the cars. I wrestle with how much theory I actually need. I still resort back to my old way of thinking when troubleshooting as its never really gotten me into trouble. Certain types of misfires though, I think it can help us to ID the type if we study this subject. But we have other ways to ID the type of misfire right?

But I do know enough to fix the cars. I wrestle with how much theory I actually need. I still resort back to my old way of thinking when troubleshooting as its never really gotten me into trouble.

That's kind of why I didn't get too upset about not quite following the Tomato Sensor series at first. They were the first videos I watched when I got the premium subscription.
I think now that if I go back and watch them, (now that I've been able to apply what what I learned from the earlier content), I may be able to absorb the theory much better.
Thanks for the help on this one.