Stumped the dealerships today need help

[jiggie]

***lengthy post just forewarned!! Lol***

Hey all! So we're discussing my 99 obs gmc k1500 burban with the 5.7 4wd sle. Had the cat issue "taken care of"ie removed till i can afford a full exhaust with high flow cats, because i was told that they were the cause of my catalyst efficiency below threshold bank 1 and 2 codes and the reason why its boging down backfiring and acting crazy for the first 30 seconds when I go to drive after work in the morning or anytime it sits for a while. After i feather the gas a bit it goes away and runs fine. We Changed the o2 sensors about 6000 miles ago when i first bought it along with intake gaskets, t stat, belt, transmission issues fixed u joints,shocks and other things. This was all done at the chevy dealership I worked at.

So i took it to the chevy dealership in my new town to get it diagnosed and they found 5 codes. 4 of the catalyst efficiency codes and one dealing with the fuel system running lean. Vac leaks is the first thing that came to mind said the tech So they hooked up things to check fuel pressures, checked plugs wires vac lines etc. everything came back normal. They used 1/4 tank of gas driving it today lol. So they called and said they couldn't figure it out so they took it to the gmc dealership and had their tech check it out and nothing different. They reproduced the issue but no new codes show up when it happens or anything to alert them of whats going on.

So They want to just throw a bunch of parts at it and see what happens. The parts include a upgraded fuel spider conversion, fuel pump and fuel filter(the fuel filter was done 6k ago) plugs wires and cap or what ever i have and checking and removing every vac line and replacing as needed and all new sensors ie tpms iac mass air flow etc. and thats along with a brake booster and master cylinder for the brakes because of the hissing noise it has and that separate issue. They said if they replace all of this and and it still does it then ill be looking at a engine tear down to check for cracked heads and other engine related things. Even though i don't think i have any symptoms that would justify that.

The cost is in the thousands and i cant afford that even at non dealership mechanic rates nor can i do a lot the work my self due to lack of tools work schedules etc. Does anyone have any idea or experience with this issue? Its getting bothersome and want to get this thing mechanical good but its one thing after another. My pocket book might not be able to afford this thing if things keep going wrong. And of course i cant get it to act up to post a video either. Go figure. Sorry if this is all over the board tried to give as much info as possible.


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[east302]

If there are no catalytic converters installed then the computer is going to throw the efficiency codes.

What were the five code numbers? I don't remember if you had it in your other thread or not.

I'd ask around for recommendations on a local mom and pop shop. I suppose your dealer has tried to do the right thing by admitting that they cannot find the issue, but it's a machine, so there are only so many things that can go wrong with it.




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[Snowbound]

Low efficiency codes don't do anything other than get you a fail at emission test station. The pre-cat O2's control fueling. They tell computer if the air fuel ratio is at stoichiometric which is Lambda 1 or 14.68:1 and adjust fueling to achieve stoich.
Stumble or bog with a lean code is most likely fueling. Change the fuel filter, it's cheap. What's throwing me off is your saying it's doing it cold but only for 30 seconds. Could point to something as stupid as a coolant temp sensor. If it's bad and telling PCM your up to temp (closed loop) it will run lean. When it's cold (open loop) it should run rich based off short term fuel trims set in a table within the PCM until the engine reaches normal operating temp. The gauge for coolant temp is seperate from the PCM input for CTS.

Any other situation I would say replace cap and rotor. They suck, they go bad but if it were spark, it would do it reguardless of temp, moisture would be a different story. I will say this, I had a chevy truck one time that I was working on and complaint was low power. I tested everything and then even brought it in to a shop to look it over. They couldn't figure it out either. Fuel pressure was within spec and I only found the issue when I did a flow test. Replaced pump and truck ran perfect.

Not saying these are your issues. Just trying to point you in the right direction. Just remember K.I.S.S. (Keep it simple stupid)

 

[ivin74]

Sell it and get you a NBS they are more realiable and easy to trouble shoot, take ur loses now before you start dumping more money into it.

 

[iamdub]

I agree with the last three replies. I'll add that it also sounds like the fuel pressure regulator is leaking. It's inside the manifold so it's nothing visually obvious. This would cause it to be flooded when first started, but usually also causes it to require longer cranking before it starts due to the fuel system losing it's prime. Replacing the injector spider assembly would fix this if it includes a new regulator (and I think it does) as well as improve the fueling control. It'd be easy to check: test fuel pressure after it's been sitting and before you start it and pop off the intake bonnet and look inside for puddled gas.

 

[Snowbound]

I agree with the last three replies. I'll add that it also sounds like the fuel pressure regulator is leaking. It's inside the manifold so it's nothing visually obvious. This would cause it to be flooded when first started, but usually also causes it to require longer cranking before it starts due to the fuel system losing it's prime. Replacing the injector spider assembly would fix this if it includes a new regulator (and I think it does) as well as improve the fueling control. It'd be easy to check: test fuel pressure after it's been sitting and before you start it and pop off the intake bonnet and look inside for puddled gas.

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.

 

[jiggie]

Thanks for all the replies!! Ill see if i can get through them all.

I don't have the exact codes handy. The place i go is about half the price of the dealer so im talking with him to see what he thinks.

My temp gauge is reading correctly so would the coolant temp sensor sill be bad or are their 2? The tune up route is affordable and i can do that myself so ill def do that.

If i could afford it i would get a nbs id be all over it! But the catch 22 is i cant afford a car pmt but cant really afford all the repairs either lol. But that option is on the table if need be.

It cranks with the bump of the key even if i just get in and start it. Being a truck driver its habit to turn the key wait a few sec then crank it. But ill see about having the fuel pressure after sitting checked as well.


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[east302]

My temp gauge is reading correctly so would the coolant temp sensor sill be bad or are their 2? The tune up route is affordable and i can do that myself so ill def do that.

It is separate. The sensor in question is by the thermostat. If it's bad, it can make it hard to crank, especially if it's only been shut off for a few minutes. Flooring it while cranking usually gets you around it.

You can test the resistance of the sensor to see if it's functioning. Also, a scanner will read the temperature value being reported. A bad sensor will usually give some oddball value like negative 30 or something like that.

You could download a scanner app and use a Bluetooth OBD2 adapter to get that reading, check and clear codes, look at misfire counts, etc.



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[jiggie]

Took a video but cant figure out how to upload it lol


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[jiggie]

Got scanned for codes and came up with the P0420 code. The fuel code from yesterday is gone


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[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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