4.0 HO Runs Uneven/Weird

Been experiencing some issues with the ’91 4.0L HO, lately.

First of all, it doesn’t run like it should while idling. It sort of jumps on the RPM, and it sounds like it doesn’t run on all cylinders, totally uneven. This gets worse when revving at a standstill.

Secondly, while travelling at higher speeds, it randomly feels very weak. Typically while trying a second kickdown (the first one being normal), the car acts like it suddenly has 100 horses less.

I am suspecting the plug wires, could they generate this kind of behaviour? They seemed pretty worn.

Any suggestions? I would appreciate some fast help.

A vacuum leak can cause the problems you describe. The usual culprit is the vacuum reservoir/ball behind the front bumper on the passenger side. Take it out and inspect it. Two screws, that's it. You could probably even test it to see if it holds vacuum, or switch it for a buddy's that is working and go for a drive.

Out of curiousity, do your heater control settings switch back to defrost when you are climbing a long hill? That is what my old Cherokee does. Heater controls are controlled by vacuum switches, and defrost seems to be the default setting. Under load, it switches back to defrost.

I had a vacuum line come off the intake manifold, near the front of the vehicle on the drivers side once. A mechanic i knew found it and plugged the line saying, "You really don't need all those vacuum lines anyway." With that, it would not down shift on a hill.

Geoff

I have a spare vaccum reservoir. I took mine off and have had no noticable problems for about 2 years now, but if you want it PM me and make an offer.

The spark plugs could be worn causing misfires. Or the throttle body could be dirty.