I just found this web site, so I'm new here. I drive a 1991 F-150 w/4.9 straight 6,as my work truck and lately it's beening running rough at idle when cold. I replace the spark plugs, cap/rotor and the fuel filter every thing else is fine. I just can't figure this one out. Could it possibly be the 'warm up regulator"? Any help would be great. Thanks

 

Welcome aboard.

Don't know where you're located, but I bet it just turned cold.

Sound like a bad choke pull-off. Did your truck run fine all summer, and now on a cool morning run like a POS? Then the engine warms up and it runs OK?

If so, then probably bad choke. The choke pull-off has the carb set for a warm engine, until the engine actually warms up to match the choke the engine runs rich.

 

I would agree with you if my truck was carb. but my truck is fuel injected. And yes it's starting to get cold here in NJ. It ran fine this summer but now in the cold morning it's a POS! Thanks

 

Mine has always ran rough when cold. I just let it warm up for 3 or 4 minutes if it hasn't been ran that day yet. But I suppose my definition of cold is a little differnt living in the Northern part of ND. It already is 30-35 degrees during the warm part of the day lately.