03' Tahoe 5.3 rough idle

[tahowner1]

I have a rough idle and codes PO300, PO151 and PO155. The truck runs great other than at idle. It bounces between 500-700 in park and at stop lights, but runs fine once moving. Things I've done so far:
It threw PO171 and PO174, so I changed intake gaskets. Codes gone.
Changed plugs, wires, MAF, fuel filter, all 4 O2 sensors. Cleaned the throttle body and intake manifold. No change at all.
When I pulled the spark plugs the passenger bank were completely black and the drivers bank was perfect. I assumed the passenger cat converter was plugged because I've read other threads about that. To test it I took out the front O2 and it made no difference in the way it idles. I'm thinking about dropping the exhaust behind the manifolds to make sure or go get a back pressure test at a muffler shop. Anyone have any idea what else to look for? It's got to be something specific to the passenger bank since all of the plugs on that side were full of soot. Is this definitely the cat?

 

[MO Viet Vet]

Signs point to a malfunctioning passenger cat that is not clogged.

 

[tahowner1]

I took it to the Chevy dealership and their recommendations were changing 02 sensors and to do their 4 stage fuel system flush at a cost of $325. I got it back from them and changed the 02's and nothing. I've run injector cleaner through the fuel tank twice with high octane fuel in the last month, while on a 2000 mile trip. I don't think it's an injector issue.

 

[tahowner1]

Signs point to a malfunctioning passenger cat that is not clogged.

I'll drop the exhaust from the manifold and see if it runs any different and post up the results. Thanks for the input.

 

[MO Viet Vet]

If it is a malfunctioning cat that is not clogged then dropping the exhaust will not effect the idle. You could also try a Seafoam treatment to the fuel system.

 

[tahowner1]

If it is a malfunctioning cat that is not clogged then dropping the exhaust will not effect the idle. You could also try a Seafoam treatment to the fuel system.

Can a cat go bad without throwing a cat efficiency code? PO420, I think it is. I haven't had that code.

 

[MO Viet Vet]

But you had oxygen sensor codes. We would be set if every malfunction "ALWAYS" threw a code. Not saying the cat is for sure but it points that way. I would exhaust all other possibilities first though. Try the Seafoam and look again at the plugs that you just installed and see how they are doing. If you have a mechanics stethoscope you could remove the probe on the end and listen for a vacuum leak. Doing intake gaskets does not guarantee all vacuum leaks were addressed. By the way, did you do the knock sensors while in there?

 

[tahowner1]

But you had oxygen sensor codes. We would be set if every malfunction "ALWAYS" threw a code. Not saying the cat is for sure but it points that way. I would exhaust all other possibilities first though. Try the Seafoam and look again at the plugs that you just installed and see how they are doing. If you have a mechanics stethoscope you could remove the probe on the end and listen for a vacuum leak. Doing intake gaskets does not guarantee all vacuum leaks were addressed. By the way, did you do the knock sensors while in there?

I'll Seafoam it and see if it helps. I assume a vacuum leak wouldn't only affect one bank, right? Didn't do the knocks. Pulled and looked fine, maybe I should have changed them just to eliminate that possibility.

 

[MO Viet Vet]

Even if the knock sensors are not causing a problem it is always best to replace them and the harnesses while under the intake. Yes, a vacuum leak can effect just one back. Knock sensors are not going to cause the problems you are complaining about, I just recommended because of the labor aspect. Again, before you do the Seafoam, look at the plugs.

 

[2004ChrisTahoe]

I'll Seafoam it and see if it helps. I assume a vacuum leak wouldn't only affect one bank, right? Didn't do the knocks. Pulled and looked fine, maybe I should have changed them just to eliminate that possibility.

Did you ever get this fixed...