Engine Misfire on Cold Start - 2003 Tahoe 5.3

[mcorbitt52]

Symptom: Engine will run rough on a cold start. This is an intermittent problem that is totally unpredictable. If it happens it only happens on a cold start. It will continue to run rough until the engine has warmed up, and has been turned off about 30 seconds and restarted. Then it runs fine.

The engine may cold start fine for a couple of weeks and then the problem suddenly surfaces on a cold start and the problem may last for a day or two. Always corrected by doing a restart one the engine warms.

This has been happening for about a year but now happening more often. Foxwell code reader only indicates a P0300 – Multiple cylinder misfire.

What I have done: I have changed spark plug wires, air filter, cleaned MAF, replaced upper O2 sensors and installed a new throttle body.

Any thoughts? Could this be a fuel pump issue? If so, can anybody instruct me on how to test the electrical draw of the pump? To effectively test, would I need to be experiencing the rough running?

Hoping someone can help.

Mike

 

[afpj]

What are your fuel trims? Vacuum leak?

 

[mwilson62000]

Had P0300 recently with a rough idle. Would get better the hotter the engine got. Used dashcommand app with added GM PIDS and showed misfires on cylinders 3 and 4.more on 4. I put new plugs in 3 and 4, no change. Swapped ignition coil on 4 with number 6 and misfire followed the original number 4 coil. Replaced that coil and issue solved. Coils were ACdelco I put in only 15k ago. I would definitely get it scanned with a scanner that can show which cylinders are misfiring. Might help you track it down.