Runs rough couple miles after starting

[04envoyxlv8]

Original owner of an 04 Envoy XL V8 with 131k on it. Check engine light has been on solid for 7-8 years now. Code shows it's an O2 sensor but can't indicate which one(s). Vehicle has run fine all this time so didn't bother fixing.

Experiencing an issue last couple weeks. Starts up fine cold but begins running rough/sluggish 1-2 miles down the road and the check engine light starts flashing. If I keep driving after that, it begins running normally again and the light stops flashing. Sometimes happens when stopped at a traffic light and idles rough. I turn the ignition off and restart...runs fine after that.

Recall, in my deep grey matter, having a similar issue when the vehicle was still under warranty...took to the dealer. The service receipt said all they did was replace the gas cap. Thing has run fine all these years until recently.

Is it possible replacing the gas cap again could resolve the issue?

 

[Mike534x]

Never seen a bad gas cap cause idle, or sluggishness on this platform.

I think your most likely problem lies in the faulty O2 sensors, and there's a good chance your cats are toast or partially plugged leading to back pressure that is most likely causing the misfire. Since you've got the 5.3, I'd tackle all 4 of the O2 sensors and see how it is afterwards.

 

[Mooseman]

1. Address the O2 sensor issue. The code(s) will tell you which one(s).
2. You may have fouled plugs which are causing the misfire, which is indicated by the flashing CEL. Once the other issues are resolved, replace the plugs and clear the codes.
3. Your cats may have been damaged by the improper air/fuel mix caused by the bad O2 sensor. There are tests do determine this (backpressure and temp gun test)
4. A bad gas cap would throw a code for the Evap system. It would not cause drivability issues.

It is never a good idea to let such issues fester as it can cause more problems down the road.

 

[MD_Envoy_Driver]

I would pull the intake off and completely clean the Airflow sensor and the entire intake and throttle body at this point.

 

[04envoyxlv8]

Thanks for the suggestions folks. I'm not a mechanic and didn't want to get into that stuff.

Installed a new OEM gas cap last weekend. Believe it or not, have not had the issue since.

No idea why that fixed the problem. Know of other gas/tank/cap issues. Friend had a Dodge Shadow in the late 80's, early 90's. If he filled the tank, the car ran like crap until he burned a couple/few gallons. He started not filling the tank completely and it ran fine.

Have a 20 y/o John Deere L118. Died in the middle of the lawn maybe 5 years ago. Called the dealer I bought it from to inquire about getting it fixed. Explained circumstances of what led to the stall. Was told to insert the tip of a pin into a small opening in the top of the gas cap to clear fine debris so the tank/fuel system could breathe. It worked! Fired right up. Really nice folks at that dealer.