I have a 1998 F150 4.2L V6 .It has a vibration at an idle in gear only . It recently had a complete tune up. My friend says it could be the flywheel but we have to remove the tranny.Could it be something else?

 

Assuming your truck is an automatic.

If so, is it a shaking vibration, or a noise?

 

Yes it's auto. And yes it's a shaking vibration,You can feel it on the steering wheel and slightly in the seats.

 

We changed the fly wheel and it still has a vibration.We then removed the fan belt. I started it,put it in drive,and it ran smooth.
The belt tensioner seems to be siezed.I'll change that today.

 

I changed the belt tensioner,it's quieter,but still vibrates.
Anyone have any suggestions?

 

How many miles?

If more than 50K, I'd look at the wires and plugs. I know they're supposed to be 100K plugs - but I wouldn't leave any plugs in for that long. Wires can get dry cracks, and let moisture in too. You can also make sure you don't have any vacuum leaks around the manifold or intake, PCV valve/hoses, etc.

Personally, I thought a flywheel was pretty severe for a first step...

Good Luck

++++

Read that you found it smoothed out with the belt off. Check the fan clutch and water pump too.

 

Thank You.The tune up is fresh,but I like your idea about the possible vacuum leak,I'll check that next.Thanks again.