Intermittent misfiring

[Paul Kendall]

So I have an 03 Silverado 1500 4.8, I just put a new tranny in it, went to fire it up and take it for a test drive and it's all dandy for the first couple km, then it starts misfiring like an SOB, loses power etc... Getting a random multiple cylinder misfire code, but I'll drive along then the SEL with quit blinking and all will be fine for a couple more km then it's back to the misfiring. If I stop and sit for a couple minutes it will be fine. It is also fine when in park, the motor will Rev up with nothing out of the ordinary, held it a 2000 rpm for a couple minutes and no misfiring. Any ideas???

[Jsdirt]

The intermittent nature of the problem suggests it's not intake gaskets, or a large vacuum leak. My money is on an electrical problem.

 

Since a new tranny just went in, that would be the very first place I'd be looking. Possible pinched wires between the bellhousing and block, or something left unplugged, or routed somewhere where it chaffed, or melted.

 

The engine and transmission move around quite a bit when under load, so a check of wire slack would be helpful too. Sometimes a tight wire can be torn when the engine and transmission torque to one side under load.

 

Get a mirror and a bright light, and check that area out real good.