Here's the run down, i'll stay as brief as possible. I have a 96 grand Cherokee, 4.0, straight 6, 4x4, automatic, 155k. Bought it from a friend for $600, water pump leaked, he over heated it too bad and cracked the head. He also always messed with the pcm plug, as the truck would die if the plug wasn't positioned just right. I have replaced the head, radiator, water pump, fan clutch, thermostat, coil, cap, rotor, plugs, wires and muffler (it exploded from a backfire before I bought it). The tail pipe was pinched so I decided to basically weld the muffler right behind the cat and eliminated the rest of the pipe. I was able to remove the pcm connector that was super glued in place and salvage the pcm, what a nightmare! I had to wire in a new plug from pick n pull. The motor no longer dies if the connector or pcm is moved, it doesn't even effect the idle. After a day of driving and running great, I am getting constant, random missing. I checked the wires at the tps (pcm was reset after installing btw) and everything appears in order. One wire a constant 5.14 volts, the other would vary with the throttle position (seeming smooth on my digital meter) and I even tried splicing in a new ground wire in a desperate act. The rest of the motor sounds great, I just need to figure out this misfire and get some ball joints, then off to the hills I go! Any help would be GREATLY APPRECIATED. Thanks.

 

Can't believe I forgot to mention it, showing error code p0122, throttle position sensor.

 

If all your tps readings check out and its throwing that code I would assume your computer isn't recieving its info correct and would go back through he connector.

 

Check for continuity between the TPS connector and PCM connector.

 

^^ :-) what he said ^^

 

I'll be checking continuity from plug to plug after work, I'll post my findings, thanks.

 

Ran some new tests today on the constant and the variable power wire to the tps. I have no resistance from pcm plug to tps plug on either wire. I put the old tps back in just for giggles and my code went from p0122 (low voltage) to p0123 (high voltage). Went and bought another tps, no improvements. Code hasn't reappeared, but engine is still missing. With the ignition on, my constant to the tps is reading 5.14 volts. The variable reads .89 when stationary and seems to increase smoothly to 3.89 when throttle is fully opened (I just opened the trottle externally where the cable connects, didn't pull the air hose and move the butterfly this time). Because of the previous pcm issues, I'm thinking more and more that it's the problem. Any thoughts?

 

Anxious as can be to get this baby rolling again, just don't want to drop the money for a pcm to find out its something else. I'm pretty sure that's got to be it after 3 tps's ran the same... Can anyone offer other ideas before buying the pcm? Maybe I'm just being wishfull and should shut up and order the damn computer already.

 

Well, you can try to see if the misfire is a injector misfire. Back probe the injector with a test light, start it, and see if it is missing, if its a injector mis fire I would assume it could be the pcm. I would also check compression on your new head just to make sure...

 

So I've replaced my pcm, still having the problem. The engine sounds really smooth, I hear this "miss" from the tail pipe more than anything. I was thinking my converter may have melted and blown out when the previous owner overheated the motor and cracked the head, but I've yet to see an exhaust-related code. I actually haven't seen any codes since the pcm swap about 30-40 miles ago. Previously, the tps code was thrown pretty quickly after a pcm reset, any ideas? I'm stuck right at the finish line and am losing my mind!

 

You should have misfire codes if there is in fact a miss.

 

No engine codes and like I said, the motor sounds great. I used the wrong with "miss", if you listen at the tail pipe, it sounds like it just randomly cuts out for a second, then a muffled puff, much like a very minute backfire. I don't feel it much while driving, but I do see a little fluctuation in the rpm's at idle or if held at higher rpm's while parked. Could it be a small enough exhaust leak that it doesn't throw a code or put out a noise loud enough to hear?

 

A leak or an old muffler or cat.

 

I just put a new muffler on and I figured if it was my cat, it would throw a code. Yesterday I popped open the month old distributor cap and out fell the center contact from the coil and blade at the end of rotor was tweaked. I noticed I had some play in my distributor shaft, so a new distributor, cap and rotor.... no change. Still this slight, random "thumping" in the exhaust, present at all rpm's but less noticeable when driving, rpm's bounce a little at idle. and it smells like it's running fairly rich. IAC?

 

Mines doin the exact same thing.:-( I'm working on tracking it down. I figured cat. Mines renix so it doesn't throw codes.