Stumped the dealerships today need help

***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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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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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)

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.

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.

iamdub said:

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.

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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.

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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jiggie said:

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.

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


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


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