Inconsistent Idle, up and down when cold?

[Myasylum]

Does anyone know why this might be?

Anything under 40 degrees the idle jumps up and down.
It doesn't run rough, just up and down.

I thought I may have fixed it by replacing the plugs and wires, but it still does the same thing.

I have a 98 4.0L.

Thanks!

 

[XLTrunner]

Does it go away once the engine is warmed up? If so, you probably have an intake vacuum leak.

 

[Myasylum]

Yes it does!
engine light is on too.

How do I find that sucker?

 

[XLTrunner]

If you have a Check Engine light on, the first thing you should do is run a scan and see what's stored. May not be a vacuum related problem. Which 4.0 engine do you have...OHV or SOHC? Typical intake vacuum leaks will be caused by old gaskets/seals that shrink just enough to cause the leak in colder weather...then, swell back up to seal the leak once the engine warms up. You'll need to remove the intake manifold to access the gaskets. Might get lucky and just need to do the upper intake gaskets if this is the problem; although on the SOHC engine, it's not much more work to remove the lower intake and replace those while you're in there.

 

[J_C]

"IF" it's not a vac leak, or if you haven't done it in the last few tens of thousands of miles anyway, you might clean out the Idle Air Control valve.

I too would go ahead and replace the lower intake gasket if it's torn down to the point of doing the upper gasket.

 

[Myasylum]

I'm sure both need to be done.
I'll have to look into both of these.

It's a SOHC.

Thanks!