I have an 07 with the 5.3 flex fuel engine with 85.000 miles that misses and runs rough for 30 to 45 seconds after the first start up of the day. The engine light comes on then goes away once the engine smooths out. Performance is fine after that. No codes are stored. I checked fuel pressure and it is fine and the regulator is working. I suspect plugs but don't understand why it would be just at first start. Give me some clues where to look.

 

Welcome to the forum.

Try a couple of bottles of Techron. If that doesn't help, I'd check/replace the plugs.

 

+1 on the fuel additive, also consider BG products, though the Techron can be more easily found, pull the plugs and inspect for oil fouling esp #1 and #7...

A link with some helpful info
https://chevroletforum.com/forum/tah...07-5-3l-63215/





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Installed new plugs and wires. No plug fouling seen on old plugs, still missing/rough for 30 to 60 seconds on first start up in the morning. Dumped some Techron in it will see what happens.
Engine light doesn't stay on long enough for me to get to the auto parts store to put a code reader on it while it is misbehaving and no codes are stored.

 

I have a similar issue. 95 with a 2.2. When first started it runs on 3 cylinders until the temp reaches normal op temp. After that runs beautiful. Plugs & wires have about 20k on them temp sensor is new as well as the 02 sensor is new the thermal sensor in the air cleaner housing is new but still the problem remains. Any ideas.