Sensors and loading up question

Ive got a few questions. They all relate on my truck running crappy and loading up when I sit in traffic. First off, I tested my CPS and it has 250 ohms. I know its supposed to test no continuity. Also when I tested my IAT sensor it tested about 900 ohms when it was warmed up. That number was dropping slowly. The haynes manual says it should run at about 4000 ohms or less and slowly drop. I borrowed a MAP sensor from my dads work and it was still doing the exact same thing so I can pretty much rule that out. When I floor it it hesitates for a few seconds and then it goes normal. It also backfires out of the intake when it hesitates. Every once in a while when I start it it doesn't start. I know thats from the CPS but would it also make it run crappy? If I sit in traffic(or anywhere) and let it idle in about 2 minutes it will load up and run like crap. It unloads if I floor it when I go. Any help would be appreciated.

Haven't you been around long enough to know that you need to tell us what year model, etc? aperwork

First off CPS won't cause it to load up. Sounds like an O2 sensor, possibly a MAT sensor.

Duh...Sorry about that. Its a 88 MJ. 4.0L, AW4, NP231.

If you have had the distributor out, maybe indexing.

Im not sure what indexing is but a guy at my dads work told him about a service bullitin that he got that explained a timing problem that they may have. He said I need to get the #1 cylinder at TDC and then take out the distribulotr and cut off two little ears so I can adjust the timing. After I do that I use a distributor cap and adjust it so that the #1 spark plug is aligned with the rotor. Im not exactly sure what to do so im going to wait til my dad can help me.

Here is the pix'

thanks old man, ill have to try that after work tomorrow

I'm not understanding the whole indexing thing.....if the timing is non adjustable(only adjusted by the computer)why would you want to adjust it?

Is it due to worn out parts.....like slack in a streched timing chain?

Once it is indexed correctly, it won't change. That is why I asked if you had been messing with the distributor. Unlike on an old points type distributor, the computer changes the timing. You just have to make sure that for the range of timings, the rotor is in proximity to the contact it is supposed to deliver the spark to. If you get the rotor off by a tooth, it still is within reach for much of the range of timing, but has to arc a long ways for the other portion of the range. If the system isn't really up to snuff, it simply cant fire the arc that far, or it delivers a real weak spark to the plug. Indexing just gets the rotor and cap lined up correctly. It is a common problem if people pull the distributor, to get it back in, but off a tooth.

The resistance reading you got for the CPS on the Renix system (88 included) is correct. Only the later ones should read no continuity. No continuity on a Renix CPS = no go at all! You have a healthy reading.

Check the throttle position sensor. It could be bad, or the connections to it could be. I found on an 87 that connections were really critical to good running, and it would occasionally need to have all its connectors unplugged, cleaned and replugged. Mine had the symptom of stalling out when you stabbed the accelerator, and two things made it better: cleaning connections, and adjusting the TPS correctly. Too high an output voltage on the TPS (reading the transmission side, on an automatic version) made it worse.

The ignition indexing is also important, though. On mine it would run pretty well with the distributor one tooth advanced, but it would start badly. The computer was able to adjust the timing once it was going, but it wasn't ever quite right. One tooth retarded and it would start perfectly but spit through the intake, and be unable to rev up. If, when starting, it occasionally bogs down, almost as if it were hydrolocked, this is a definite symptom of one tooth too advanced.

When dealing with engine electronics be very careful with Haynes. They mix things up a little, and even when they don't they make it easy for you to.

I havnt got around to doing any of this yet but this weekend im going to put a new cps in my truck. I can get it new from the dealership for 25 dollars so it cant really hurt anything. I am also going to check into the tps. thanks a lot again

I guess I havent been on here for a while but I figured I'd ask a few more questions. This MJ stopped running about a year ago and I recently started to mess with this again and ive done a few more things. It will start and run for a few seconds but it acts like the timing is really off(backfires out of exhaust and sometimes intake). I can get it to run longer if i feather the throttle but it doesnt have enough power to move it. I replaced the timing chain, swapped tps/map sensor/iac/coil/plugs/wires with another running cherokee. I also replaced the crank position sensor. Fuel pressure is at 35psi. Im starting to think that this may be a computer problem. Ive never had the distributor out. Any other suggestions?