Guys,
Please help. I'm new here, but not to cars/trucks. I have a 1991 Cherokee with the 4.0 that's treated me pretty well for years. I think it's got about 190k on it. Lately though, I've been trying to chase down a demon. I've googled every term I can think of, in every combination I can think of and read pages and pages of threads. I commonly see "long start" and "rough idle". But I can't seem to find a thread with symptoms like mine or think of the right terms to search for to produce results with symptoms like mine.
Symptoms:
Initially, the first symptoms were driving home and it was lacking power and didn't seem to be shifting correctly. Didn't want to go over like 25-30 mph. Would nearly die at stops.
Then when I looked at it:
1. long start (I've found the trick of priming the pump a couple times, which has worked). It would turn over for like 30-45 seconds before firing. but then when it did....
2. It would idle up to like 1200-1500 rpms and settle back in to like 800. Then a while (30-60 seconds) later, it would drop to like 400rpm and nearly die, but it would save itself and go back to 800. Then it would jump up to like 1500 again and repeat that cycle over and over. Sometimes it would go from 400 right to 1500 though. Throttle response was death if it wasn't at 800 (400 or 1500 it would just crash), but at 800 it seemed normal.
I have replaced the TPS with a BWD unit (I'm told that's Bosch now) after multimeter testing yielded weak results. I cleaned the IAC. However, I realized that I didn't unplug the battery when I pulled and reinstalled them. I cleaned the throttle body. I did mess around with the spintle when cleaning the IAC, but it never moved, only the sleeve around the spring rotated. To test the IAC after cleaning, I have unplugged it and had no change in symptoms. I have since unplugged the battery for 1/2 hour to "reset" the pcm with no change in symptoms.
My parts store guy has a Cherokee and suggested the TPM after I mentioned shifting issues, as did many, many forum threads. I haven't checked the MAP or Crankshaft Position Sensor, but my reading hasn't led me to believe either would be the fault.
I've tried pulling codes with the ON-OFF-ON-OFF-ON key method and it's only giving me a '12', or "battery disconnected in last 50 key cycles".
I have also recently replaced plugs, wires, cap, and rotor before symptoms with a kit from Summit Racing for the 4.0.
Other things I see pop up in search are fuel pump, check valve, leaky injector, etc.
Thanks for any help,
Chris
Please help. I'm new here, but not to cars/trucks. I have a 1991 Cherokee with the 4.0 that's treated me pretty well for years. I think it's got about 190k on it. Lately though, I've been trying to chase down a demon. I've googled every term I can think of, in every combination I can think of and read pages and pages of threads. I commonly see "long start" and "rough idle". But I can't seem to find a thread with symptoms like mine or think of the right terms to search for to produce results with symptoms like mine.
Symptoms:
Initially, the first symptoms were driving home and it was lacking power and didn't seem to be shifting correctly. Didn't want to go over like 25-30 mph. Would nearly die at stops.
Then when I looked at it:
1. long start (I've found the trick of priming the pump a couple times, which has worked). It would turn over for like 30-45 seconds before firing. but then when it did....
2. It would idle up to like 1200-1500 rpms and settle back in to like 800. Then a while (30-60 seconds) later, it would drop to like 400rpm and nearly die, but it would save itself and go back to 800. Then it would jump up to like 1500 again and repeat that cycle over and over. Sometimes it would go from 400 right to 1500 though. Throttle response was death if it wasn't at 800 (400 or 1500 it would just crash), but at 800 it seemed normal.
I have replaced the TPS with a BWD unit (I'm told that's Bosch now) after multimeter testing yielded weak results. I cleaned the IAC. However, I realized that I didn't unplug the battery when I pulled and reinstalled them. I cleaned the throttle body. I did mess around with the spintle when cleaning the IAC, but it never moved, only the sleeve around the spring rotated. To test the IAC after cleaning, I have unplugged it and had no change in symptoms. I have since unplugged the battery for 1/2 hour to "reset" the pcm with no change in symptoms.
My parts store guy has a Cherokee and suggested the TPM after I mentioned shifting issues, as did many, many forum threads. I haven't checked the MAP or Crankshaft Position Sensor, but my reading hasn't led me to believe either would be the fault.
I've tried pulling codes with the ON-OFF-ON-OFF-ON key method and it's only giving me a '12', or "battery disconnected in last 50 key cycles".
I have also recently replaced plugs, wires, cap, and rotor before symptoms with a kit from Summit Racing for the 4.0.
Other things I see pop up in search are fuel pump, check valve, leaky injector, etc.
Thanks for any help,
Chris
as well, but not new to cars--if you're checking pressure with the key in on, and getting 12psi, you may have a fuel pump/filter problem, which could help explain the idle problem. 
