Hey everyone I have been fighting my xj for a while and cannot figure the issue. I have reached the end of my diagnostic abilities and dont want to just start throwing parts at it. Any help you can provide would be awesome because it is just sitting out front of my house enraging my HOA.

Background: I picked up an 88 cherokee 4 speed auto with a blown motor from a friend that who was moving. The blown motor was a HO but still used all the renix components. So I put a new HO jasper motor in it and everything was great for the first 500 miles.

Problem:
About a month ago it started to stumble on acceleration every once in a while and has gotten worse. Now it consistently stumbles during acceleration but once it gets cruising is fine. There are no issues if it is in park and I rev the engine. I tried seeing if putting it in 3 and 1-2 would make a difference and when it is in 1-2 the problem is worse and will even back fire. The exhaust may also have more soot than I remember but that is not for sure.


What I have tried:
New plugs and wires
cleaned throttle body
new IAC
New TPS (adjusted using volt meter)
New 02 sensor
IAT tested good
Tested MAP (good)
Grounds have been checked (they had been moved to the battery when purchased)
Distributor & cap cleaned
Distributor is not loose
No vacuum leaks I can find

Thoughts:
If it was electrical why is it fine when cruising and reving in park?
Maybe fuel pump but if it is backfiring and has sooty exhaust that would indicate running rich.
Something in the transmission that is causing the issue under load

Maybe unrelated but the AC compressor has a loud whine like the bearings are going bad

 

My initial thoughts: I would test fuel pressure and exhaust back pressure (clogged cat).

 

Im not sure how to check the fuel pressure and the cat is gutted

 

Fuel gauge from auto parts store

there's a port on the fuel rail to plug it in


Should see in the 35 psi range

 

Thanks ill look into it

 

just for my education if it is fuel pressure how come it is backfiring?

 

Also fuel pump check valve could have failed if it's running rich

 

Your issue sounds like something that was happening to me. I was throwing a code for the TPS but after changing it several times it turned out to be a bad wire splice. On my Jeep it was the purple and white power wire to the TPS and a few other sensors. I ended up replacing the bad splice and haven't had any issues since.

Hope this helps,

Mark

 

Maybe your IAC is going bad. And yes, as mentioned about, check for exposed wires .

 

If you are using the original Renix engine controls and manifold on a HO engine and head then there is less sealing area because of the difference of port heights. You may have some leakage. Check for loose bolts on your intake/exhaust. Some guys test for intake leaks with a unlit propane torch turned on and with the engine idling, pass it around the intake. Listen for a change in how it's running. You may pinpoint a leak.

 

So my first thought would be secondary ignition being it seems to only happen under a load. This happens due to the change in density within the combustion chamber. More load equals higher density. If there were say a crack in one of the plugs along the porcelain the spark would find the path of least resistance, in this case not the electrode. However you said you replaced plugs and wires. Can I ask what type and brand?

My next thought would be the condition of the cap and rotor, the process of failure is a lot like above. Is there any white lines inside the cap, that would indicate carbon tracking? Whats the seal look like, is there moisture inside the cap? Build up on the electrodes?

Another question that comes to mind can you reproduce the failure/issue while power braking the vehicle in drive? If so this would make it possible to do a power balance test. This would be the best scenario being you could narrow the problem to the appropriate cylinder and back track from there.

My last thought would be something in the valve train. I have seen worn/broken/weak valve springs cause all sorts of issues that mimic ignition problems. Once ignition is ruled out and you have a cylinder pin pointed you can either remove the valve cover or use a vacuum gauge to test valve train. If you have a vacuum gauge handy it wouldn't hurt to plug in to direct manifold vacuum (brake booster line comes to mind) and see what the needle does. At idle you want to see a steady needle between 18-24 in of vacuum.

 

Tested Engine Coolant Temp sensor?

Are all those sensors you replaced good?

Gone through these? https://www.cherokeeforum.com/f51/cr...x-tips-153657/

 

BINGO. The Renix intake/exhaust manifold gasket must be used.

Fuel pressure is 31 at idle, jumping to 39 when the vacuum hose is removed from the fuel pressure regulator.

 

I've never had this problem but I would also look at the PCM or whatever controls the firing. My f150 had this exact problem and it turned out the pcm was causing misfire and it was shorting out parts in the process. This might be totally useless info but hey if it helps.

 

In my Jeep ^