Stumped the dealerships today need help

[iamdub]

Could do a leak down test too. Pump should hold pressure for a little while after it shuts off. My only concern with it being a regulator is that would show rich instead of lean. Only way to be lean is not enough fuel or too much air. Since it's not feasible to get too much air we have to assume the latter.

On a cold start and in open loop, the PCM isn't monitoring the mixture so it wouldn't report rich or lean. By the time it's in closed loop, the puddled fuel/flooding would be gone and it's running well enough to take in and burn the fuel leaked from the regulator (or elsewhere in the injector spider).

I know he had a lean code, but I was focusing on the other factors he mentioned: burning 1/4 tank of gas during diagnosis (either the mechanics put a lot of miles on it or it's burning a lot of gas), the cats failing (hence their removal), and it acting up just during the first 30 seconds. Those three points sound like it's running very rich which would burn a lot of gas and clog the cats. Also, if running rich enough to damage or clog the cats, the O2 sensors can just as easily get clogged with soot, making them read the exhaust as being lean since it's not "seeing" much of the exhaust any more. The PCM will then try to richen up the mixture, exacerbating the issue.

The discount to this is that it acts up "for the first 30 seconds" before "running fine" after feathering the gas. But, what is the OP considering "running fine"? Does the OP track his fuel mileage? Etc...

***Disclaimer: I must confess that on odd issues such as this, I tend to go far off into left field with my thinking and miss the simpler possibilities.

 

[jiggie]

On a cold start and in open loop, the PCM isn't monitoring the mixture so it wouldn't report rich or lean. By the time it's in closed loop, the puddled fuel/flooding would be gone and it's running well enough to take in and burn the fuel leaked from the regulator (or elsewhere in the injector spider).

I know he had a lean code, but I was focusing on the other factors he mentioned: burning 1/4 tank of gas during diagnosis (either the mechanics put a lot of miles on it or it's burning a lot of gas), the cats failing (hence their removal), and it acting up just during the first 30 seconds. Those three points sound like it's running very rich which would burn a lot of gas and clog the cats. Also, if running rich enough to damage or clog the cats, the O2 sensors can just as easily get clogged with soot, making them read the exhaust as being lean since it's not "seeing" much of the exhaust any more. The PCM will then try to richen up the mixture, exacerbating the issue.

The discount to this is that it acts up "for the first 30 seconds" before "running fine" after feathering the gas. But, what is the OP considering "running fine"? Does the OP track his fuel mileage? Etc...

***Disclaimer: I must confess that on odd issues such as this, I tend to go far off into left field with my thinking and miss the simpler possibilities.

Yes they did put a bunch of miles on it and i believe it pretty much was running all day lol. I was ok with that i believe they put about 75 miles on it and with the constant start shut off idle and mix of highway and in town driving trying to diag id expect it to use some gas. As far as milage goes im getting anywhere from 300 to 375 on 3/4 of a tank(i never let it get below 1/4 if i can help it) depending on driving over a 2 week period. I always fill up every other friday regardless of how much gas it has. If i do 100 miles or so of highway driving and the rest in town I can get 375 on half a tank. Got to love a 42 gallon tank lol. Yes cats were removed due to new o2 sensors that were installed about 6k ago when i first bought it not making the codes for efficiency go away.

When i say running fine i mean it is quiet runs and idles smooth and doesn't hesitate when you leave a light or get on the freeway. And left field is usually where the answer is lol.




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[jiggie]

Talked to the dealer again and its going to go back next week for its oil change tire rotate, ac charge and they are going to have another look at it. After talking with yall, im going to have them look at the fuel system closer to see about the leaking possibilities. I also gave them permission to pull the upper intake if need be. Gave them the budget and told them not a dollar more. Fingers crossed. I changed the fuel filter and coolant temp sensor tonight so we will see if that changes anything.


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[east302]

Re: fuel issues, if it passed a pressure and leakdown test, that usually rules out a fuel issue. GM does have test procedures for testing the actual injectors (injector coil test and the other is called a balance test, I think) that will let you know if you have a poppet issue. They shouldn't be "guessing" that you have bad injectors, in other words.

I'd hate to see them throwing parts at it on your dime. What do they want to look at by pulling the lower intake...the lifters?

Did they do a compression test on it?


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[Snowbound]

On a cold start and in open loop, the PCM isn't monitoring the mixture so it wouldn't report rich or lean. By the time it's in closed loop, the puddled fuel/flooding would be gone and it's running well enough to take in and burn the fuel leaked from the regulator (or elsewhere in the injector spider)

In open loop it still monitors short term fuel trims. The map for these trims are hard wired into the ECM and do not get corrected by the long term fuel trims until it's at normal operating temperature. It runs rich in open loop but not so rich it would cause cats to go bad. Also I seen where OP said that the O2 sensors were changed so I had to assume the lean code was correct. (I know it's never a good idea to assume but we have to go off what is told to us)

I'm thinking if fuel line or regulator is leaking it's always gonna be leaking especially when the pump is running which would cause a rich mixture. I'm just trying to help the guy out. I don't think this is a who's right and who's wrong thing, I don't have the vehicle in front of me to test my theories. I try to start with the most basic things first as in fuel filter, if it hasn't been changed in a while, just change it. Then I would see if the coolant temp sensor is working, the gauge reads off temp sensor on head where the ECM gets its input from t-stat housing. The techs that are diagnosing the problem should read the temp and maybe they did, I don't know for sure. If it's running lean and all sensor inputs are correct it leads to a fueling issue (lack thereof) or a vacuum leak.

 

[iamdub]

In open loop it still monitors short term fuel trims. The map for these trims are hard wired into the ECM and do not get corrected by the long term fuel trims until it's at normal operating temperature.

I never knew of this "gray area" of monitoring and adjusting in open- versus closed-loop. Makes perfect sense, though.

I'm thinking if fuel line or regulator is leaking it's always gonna be leaking especially when the pump is running which would cause a rich mixture.

I agree, which is why I pressed for a few more details on how it starts and runs.

I'm just trying to help the guy out. I don't think this is a who's right and who's wrong thing.

Absolutely and agreed and I'm not discounting or contradicting you. Actually, I understand and agree with you (and apparently learning a thing or two myself). I'm just throwing in my guesses formed from what I've read and experienced from going in circles with similar issues and finding it was something that would never have been considered from the start. So, I guess one could say I'm starting way out in left field and working my way inward instead of starting with the basics and working up. Probably confusing and possibly counter-productive, but my intent is to get other hypothesis formed so they can be tested alongside the basics.

 

[east302]

Question--

Does the computer "do" anything with the downstream oxygen sensors in terms of adjusting fuel trim? Or are they solely for monitoring the catalytic converter efficiency?


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[iamdub]

Solely for monitoring the converter's efficiency.

 

[Snowbound]

Question--

Does the computer "do" anything with the downstream oxygen sensors in terms of adjusting fuel trim? Or are they solely for monitoring the catalytic converter efficiency?


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Solely for monitoring the converter's efficiency.

Exactly!! That is their only purpose. That's why in my first post I said they do nothing but give you a code for low efficiency convertor. You can trick them by getting an O2 fooler pipe or in other words get them out of the stream but not too far.

@iamdub, don't think for one second I'm some mad scientist when it comes to engines. I've done so much with so little for so long that I'm pretty much qualified to do anything with pretty much nothing. Hahaha. I'm no expert in any sense of the word, and I learn every day from guys that have off the wall problems. Sometimes it's something stupid that I didn't even think of or overlooked. Any input and advise that can be given to someone with a problem is always a good thing. A few people bouncing their ideas or thoughts off one another usually can reveal what the issue is. I like having conversations with like minded people and I respect everyone's ideas. My grandpa told me when I was younger 2 things that stuck with me. One being, every person you'll ever meet throughout your entire life will know something that you do not. It's in your best interest to listen more than you speak. And two, if your the smartest person in the room, your in the wrong room because there's always something that you can learn from others. I guess what I'm trying to say is, please don't think that I was trying to be a know it all smart ass. I know just enough to get me into trouble most of the time.

 

[east302]

Thanks guys, for the clarification on the oxygen sensor.

If y'all get a chance, maybe take a look at this guys thread...he's having trouble with a no-start on a Tahoe in Germany. It kind of sounds like an injector issue. Maybe you can offer some suggestions.

http://www.tahoeyukonforum.com/threads/96-tahoe-dies-while-driving-and-wont-start.96116/


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