Hi Scotty, I have an 03 GMC Sierra 5,3L with auto trans and about 150k miles. Noticed the other day after a warm start up a miss at idle but no check engine light. I hooked up my scanner and looked at live data and noticed when it was missing stft on bank 2 was -29. It went to normal, close to zero after less than a minute and quit missing. Still no codes. I have in the past few weeks cleaned the maf and throttle body but this started a week or so after cleaning those. Not sure if I maybe have a O2 sensor going bad or the sensor heater maybe not working or an injector issue. I have heard also the manifold gaskets go bad but that shows up on cold starts I think till things expand a bit and shows a code. Any idea what to look at?
I'd guess at a failing fuel injector.
You probably have a fuel injector or injectors dribbling a little fuel when the engine is off. So when the engine first starts up the mixture is too rich and the computer compensates by leaning it out. Once the excess fuel is burned the mixture returns to normal.
I am going to try some gumout that Scotty recommends in one of his videos and run thru a tank of fuel. I looked at injector prices and wow they are like 75 bucks each.
Wow, are they gold-plated? Hope the cleaner works for you, but those injectors are nearly 20 years old. If a cleaner additive doesn't help there are mechanics that have a device to run solvent directly through the fuel rail and into the injectors which may do the trick.
I was thinking it could be an O2 sensor too but a leaking injector could do it also. It seemed the last trip I made it took about 30 seconds to start missing but still only lasted a short time, If it was a leaky injector I would think it would be right at start up,
If you can check live data you should be able to see what the O2 sensors are doing.