My brother owns a 2003 Tahoe with a 5.3L with 145k miles on it. 2 weeks ago it started running really bad (misfiring) and then it would eventually die out. His wife was driving it at the time and she persisted to try to start it. Eventually she got it started but it had no power and she continually tried forcing the pedal to the floor. All the while she was dumping gas that was on its way down to the oil. When I dropped the oil, it had nearly 3 qts of gas in it and metal and aluminum shavings - toast!

So, I picked up a 2004 engine with 66k miles and I have it installed. Well, I have severe misfire, so I started diagnosing the problem. Here is what I have done so far:
- checked spark from all coils and found 1 that was dead, so I replaced. Still misfire P0100, p0106
- verified all injectors were firing with screwdriver to ear test. Also pulled injectors out and inspected them, no carbon and visually look new
- after running awhile, I pulled plugs and the entire passenger bank was white (lean condition of sort), Driver bank was all sooty, but still firing well
- replaced plugs and still running the same
- Catalytic converter on passenger side about 3X hotter than cat on driver side.
- checked fuel rail and both sides are getting fuel, but did not verify system pressure because do not have a gauge.

I am thinking that the passenger catalytic converter is completely plugged and thus creating backpressure and hindering fuel ignition and probably forcing fuel back into intake. I pulled the O2 sensor on that side and it appeared to run better while idling, but not sure.

Anyone think I am crazy by the diagnosis? I have already ordered a new cat assembly.

 

Those codes relate to the mass air flow sensor and the manifold absolute pressure sensor.

 

Yes, It also read the p0300 code. I don't believe these are the true issues, but created more by the backpressure from the plugged exhaust? remember that this is a new (different engine).

 

The p0300 is just a general misfire code which can be caused by anything that causes a misfire.

You may be completely right about the problem being the catalytic converter.

On the other hand if the MAF is not working correctly, and/or the MAP is not working correctly the problem at the cat may be a "result" as opposed to the "cause." With two sensor codes it could also be the connections at the computer.

 

I have had many cats stopped up on this engine. Remove both upper oxygen sensors and start the engine and let it run. if all misfires disappear replace the cats.

 

I have tried only the one O2 sensor unplugged. I will try both and see if anything changes. How long until I should expect the misfire to disappear? I have put a new set of plugs in it now.

 

Well, I finally removed the driver O2 sensor so that both upstream O2 sensors were out of their ports and you were right, the engine purred without a misfire. Once it warmed up to temperature it started running poorly again by trying to further lower the RPM, but I suppose that was due to the O2 sensors not reading the expected thresholds.

The new CAT assembly is in transit and hope to have it here by Wednesday.

Lt1Porsche, can you explain the technical details behind what is going on when a CAT is mostly blocking exhaust on one side?

I would think that the engine would begin to pressurize and thus not pull in fresh air sufficiently. I am still somewhat perplexed by the fact that one entire bank was leaned completely out and the other bank was sooty. Is this common in the CAT failures you have seen?

 

http://www.aa1car.com/library/converter.htm

 

Darkman, that is an excellent technical guide for diagnosing the problem. I have paid attention to your previous guidance that the problem likely occurred elsewhere which led to the CAT failure.

From what I gathered from this technical reference, a faulty O2 sensor could have been constantly telling that side of the engine to burn lean which may have led to the "white" plugs. I suppose that I should replace the O2 sensor, but I do remember reading about measuring the voltage differential in order to determine if it is functioning in the proper range.

The other problem that my brother had was a slow consumption of oil from the previous engine that I just removed. He mentioned that it burned about 1 Qt every 3,000 miles or so. This too could have led to the CAT failure.

Thanks for the guide!

 

We do custom exhausts and normally when a cat is blocked you will hear alot more intake noise or drone a half to wot and make shur that if it has a cat that they dump into after that one that it isn't getting blocked from the one that was bad incase it would of started breaking down

 

You need to check manifold vacuum ,,,,,,,if it's a cat vacuum won't go above 10 or 12 course it's a little trickier to tell with dual exhaust/cats . Yes bad gas , out of tune engine , small head gasket leaks allowing a little coolant into exhaust and oil consumption and ho2 sensor problems and hot engine running temps all are cat killers ,,,,good luck

 

The easiest quickest way to check for a blocked cat is to remove the air check valve and put a pressure gauge on the air fitting. Start the engine and rev it up, there should be less than 2 psi.

 

Well, the new CAT assembly arrived and I installed it and everything runs great! The engine RPM at idle is right around 500-600 where it should be and all misfires have gone away.

Thanks for everyone's help and the lessons in diagnostics for this problem.

 

It can be difficult to understand why the cats clog. Most cat failure are due to secondary issues. Bad misifres, Poor fuel delivery causing to much heat. But im my experience I have seen too many of those sets go bad and have never had any failures of replacement cats. I have spent hours looking for issues only to find everything was operating exactly like it should have. I have personally decided that it is some sort of a design flaw with the factory design. Longevbity issue.

When a cat fails, they can fail in many different ways. They can either just fail to store oxygen, fail to allow exhaust to flow through, or normally just weaken and break apart. In your case, as they get older they flow decreases until the exhaust backs up into the engine and does not allow the cylinders to vent properly causing uneven cylinder mixtures which cause the misfire.