I have an annoying problem.While idleing the truck has a stumble,tach doesn't move and it only does it while in gear.When it is in park it idles smooth.It idles....stumbles,idles...stumbles,etc.Any ideas?I have replaced the spark plug wires and plugs.It has a new distributor,cap,and rotor.

 

Sounds like the idle air adjustment screw is out too far. Which truck is this happening on? When I had my 90 V6 pickup the idle air adjustment screw on the throttle body had backed it's way out some and the idle got higher and would stumble when it was in gear and I had my foot on the brakes. I just turned it back in a turn or 2 and the idle went down and no more stumble. Check that first.

 

It's on the 91.It does idle a bit high,about 1000-1100 rpm

 

Seafoam!!!!

 

automatic or stick? if the clutch is out of adjusment it might be partially rubbing or not disengaging causing a quick hit and htus the stumble.... if i idont make sure my clutch is pushed in 100% it idles rougher than neutral

 

Auto.I have done seafoam 2 times and it mad a huge difference in other ways.

 

Try what I suggested. Adjust the idle air screw on the throttle body and your idle should drop down and it might make the stumble go away.