Hi Scotty!
I have a 2003 Chevy Tahoe Z71 5.3 4x4 230,000 miles… Car is stock except exhaust is flowmaster… always filled 91 octane, until recently I am filling 89 (it performs better, the 91 seems to not be combusting well anymore)
No codes specific to this issue… only codes that come up are things that go bad because of this mysterious problem. I believe my transmission went bad because of this, and I have put a new transmission, transfer case and rear differential.
All functions normal, car just performs like running “lean” and creating back pressure. But I believe I am not getting proper combustion because when I downshift, I am getting blackish smoke from exhaust (unburnt fuel) and my O2 sensors keep going out, and likely the reason my cats went bad.
Only notable thing I have seen is when going full throttle or downshifting the RPM goes up to 3500 as it should, but then immediately drops to 3200 as if there is bogging or something and then picks up momentum and continues. I can upload a video if you want to see it?
Compression check was good… no oil or water leaks… bearings all clear and free, no binding of brakes… fuel tank and fuel are clean.
Parts changed (all OE, except air filter and catalytic):
Air filter (I had a K&N, now regular and I change it with every oil change around 4,000-5,000 miles with synthetic oil)
O2 sensors front and back (5-6 times)
Fuel Pressure regulator (4-5 times)
Mass Air flow (3-4 times)
Throttle body
Intake manifolds
Spark plugs (2 times)
Spark plug wires (2 times)
Catalytic converters (only aftermarket item, it is Magnaflow)
Fuel Pump
Fuel Filter (2 times)
Fuel injectors
Coils
Alternator
Battery (4 times)
Coolant temp sensor
Map sensor
Throttle pedal sensor
Crank sensor
Knock sensor
a few more things... but I dont remember anymore 🙂
Any ideas or help would be greatly appreciated... I think it is an interesting issue that a diagnostic computer cant even figure out.
car just performs like running “lean” and creating back pressure. But I believe I am not getting proper combustion because when I downshift, I am getting blackish smoke from exhaust (unburnt fuel) and my O2 sensors keep going out, and likely the reason my cats went bad.
black exhaust indicates a rich mixture, not lean. Rich mixture would also explain the cats burning out.
You need to put it on a diagnostic scanner and read out the fuel trims, MAF & MAP, ECT, misfire count... stuff like that. You won't fix this by spraying parts at it, and it will end up costing you FAR FAR more than a proper diagnosis from a qualified mechanic.
One thing I'm not seeing here is an analysis of live data. Looking for codes of course is only the beginning of the diagnostic process - is there anything suspicious going on in the data, particularly when those symptoms are happening?
the only live data code given to me by one mechanic: Cat 1 running lean on idle AND Cat 1 & 2 running lean on load... he then changed the O2's and readings improved, but performance not really...