91 Cherokee

[flyby night]

I have a 91 cherokee with the 4. 0 liter L6. It is my beater that I just bought to commute back and forth to work in. It ran fine for the first week, then it started to run crappy whenever it went over 2500-2800 rpms. Once it hits that magic point, it falls on its face or bogs all the way down. It runs great and drives good as long as I keep it below 2500 rpms.



It has NO engine codes

I just put spark plugs and wires on it.

the cap and rotor look good.

The fuel pressure runs at 45 psi at all rpms idle and when it is crapping out.

The back pressure in the exhaust is good.



Anyone have any ideas? It is kind of weird to me how it is ok up to the magic rpm.

 

[pmartin]

I have a 91 cherokee with 4. 0 engine (owned for several years). Mine would miss at idle but run OK at higher RPMs. After several sets of plugs, wires, injector cleaner, etc, I finally replaced the oxygen sensor - problem solved. Appearantly it was causing a faulty fuel/air ratio. Don't know if that would help your problem but if oxygen sensor hasn't been changed in recent years you might try it. They don't cost much. Hope you find your problem.

 

[Flexiheep]

I know you said you did the plugs and wires etc, but my 89 just ate up the ignition parts, maybe a year if I was lucky. I once had exactly the symptoms (wouldn't rev over 2500) you described and pulled the plugs to find the gap had widened to almost 120 thousandths! (measured with dial calipers) It ran much better when I got new plugs in gaped to spec (35 thou I believe).

Other than the ignition needing the wear parts replaced every year or 15K miles, I never had any engine problems, the thing is still on the road with a friend with 300K+ on it now.

 

[wcjp]

If oxygen sensor doesn't fix it try unplugging/plugging in the Map sensor. Usually found just below the middle of the hood. When my Jeep did this it was a vacuum leak.