88 comanche 4.0 5speed. It likes to randomly idle around 1100-1700 rpms. Vacuum lines were replaced back when I put a new head on and was fine all summer. I can also make the rpms rise by pumping the brakes. I was never able to solve the problem with the 89 Cherokee I had.

 

Squeeze off the line to the booster and see what the idle does.

 

I've blocked off the line to the booster and it still does the same thing. My 89 would rise rpms pumping brakes with a new booster also.

 

Check to see if your intake manifold bolts are loose.

Vacuum leaks can also be found by spraying throttle body cleaner on to suspected areas at idle. If it's leaking, you'll know it.

 

and intermittent idle problem could be caused by lose manifold bolts? They are all tight. Checked them a month back when I did the valve cover gasket.

 

Yes, especially if the leak seals up or opens up when the engine is hot or cold.

Gotta eliminate the cheap and easy possibilities first.

Have you sprayed for vacuum leaks?

 

I have sprayed to try to find a leak and never found one along the injectors and where the manifold meets the head. Also have retighten all the bolts today. Even does the idle rise will all the vacuum lines capped off. Including the brake booster. So far the only thing that seems to help is if I unplug the idle air control.

 

Good work. You have cleaned the throttle body and IAC, right?

Is it possible someone might have screwed with the butterfly stop at some point? If so, see post 14

 

Throttle body and the intake manifold was cleaned very well back in February while the head was replaced. Throttle body still appears clean. The butterfly adjustment looks untouched. I have a extra set of throttle body gaskets to throw on since I have a throttle body spacer. I need to pick up a set of factory throttle body linkage and bolts one of these trips to the wrecking yards.