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Unread 12-28-2007, 05:11 AM   #1
twmattox
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1997 ZJ Multiple Cylinder Misfire please help ASAP

Driving home on Christmas night and ended up stranded. The Jeep just started misfiring (completely out of the blue with no symptoms) with a very heavy fuel odor in the exhaust. It was all I could do to limp it along the side of the road to a safe area; no power, stalling, backfiring, misfiring, etc.

I quickly did the on-of-on-off-on trick and pulled:
Code 12= Battery disconnect - input to the PCM was disconnected within the last 50 starts.
Code 43= Multiple cylinder mis - fire.
Code 45= Trans temp sensor volts too low or too high, Governor pressure sensor volts too low or high, O/D switch pressed (LO) more than 5 min - overdrive off switch input too low for > 5 min.
Code 55= completion of fault code display.

I then rented an OBD II Scanner and pulled:
P0301 Cylinder 1 Misfire
P0302 Cylinder 2 Misfire
P1762 GOV Pressure Sen Offset Volts Too Low Or High
P0300 Random Multi Cylinder Misfire

Oddly, when cold, it would start and run fairly normal (rough but still had power). As soon as it reached operating temperature...missfires constantly. I cleared the codes and it now seems to run just fine, both cold and at operating temperature (I still haven't trusted it to a longer trip).

From my limited understanding, this can be caused by:
spark plug failure
spark plug wire failure
Distributor cap/rotor failure
ignition coil failure
Powertrain Control Module failure
Crankshaft Position Sensor failure
Camshaft Position Sensor failure
Battery failure

I will replace the plugs, wires, cap, rotor tonight. I will have the battery checked tonight. I will test the coil tonight too. The local Jeep shop has offered to check the PCM (for free if I remove and bring to them). This leaves the Crankshaft Position Sensor and Camshaft Position Sensor... Am I correct that if these were faulty, a code would have popped up? Is there anything else I should check???

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Unread 12-28-2007, 06:21 AM   #2
greasefingers
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Yea a basic tune up is the most common cause to the multiply misfires.

Then clear the codes and see if the rest still pop up.

No crank & cam sensors are not emission items so there are not mandatory to store a code. However, if one completely fails then a code will be stored. Of course if it completely fails then the engine will not start. So an intermittent cps failure is what is tricky.
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Unread 12-28-2007, 08:16 AM   #3
twmattox
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So, it is possible for the CPS to fail intermittently? If I run a diagnostic on them (per the manual) and they test fine, it is possible for them to still be "bad"??? Is it just not better (and safer) to just replace both of these?

Any other sensors I should be looking at? The vehicle has 200k miles on it and never had any of these things checked/replaced. I just want to make sure my wife doesn't break down in the future...
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Unread 12-28-2007, 08:18 AM   #4
twmattox
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Oh, one more question:
I have read that non-MOPAR oxygen sensors don't work well in these Jeeps. Is that the same with these sensors? Or, will common AutoZone replacements work just fine???
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Unread 12-28-2007, 11:14 AM   #5
greasefingers
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The Cam sensor is not nearly as troublesome as the Crank sensor. I have never replaced a Cam sensor. But they will sometimes fail. If you are going to take a guess at one of these then just replace the Crank sensor.

Intermittent failing of the Crank sensor is common and will cause stalling (seems worse at idle)

Non-Mopar O2 sensors are not recommended. Some people here have had no issues with Bosch ones, but their track record is not good on Jeeps.

However, a bad O2 sensor will always throw a code. Also a bad O2 sensor will never cause the engine to stall.
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Unread 12-28-2007, 06:30 PM   #6
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The behavior I was experiencing was more along the lines of timing being off. I have an older Scrambler. It woluld be like advancing the timing considerably (missfire) to the point that at idle it would stall and had no response to throttle (would just misfire more as the throttle was opened).

I went to the Jeep dealership today with the PCM and they indicated that they had no recollection of telling me they could verify if it worked... They were kind enought to discuss the problem with me. They indicated I should look for a grounding issue (intermittent problems are usually due to poor or intermittent ground)? Also said to look at the Throttle Position Sensor.
I would think that if the TPS was faulty, the idle speed would fluctuate...

What is really throwing me is that once I cleared the codes, no more problems....
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Unread 12-28-2007, 06:50 PM   #7
greasefingers
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Quote:
Originally Posted by twmattox
The behavior I was experiencing was more along the lines of timing being off. I have an older Scrambler. It woluld be like advancing the timing considerably (missfire) to the point that at idle it would stall and had no response to throttle (would just misfire more as the throttle was opened).


I went to the Jeep dealership today with the PCM and they indicated that they had no recollection of telling me they could verify if it worked... They were kind enought to discuss the problem with me. They indicated I should look for a grounding issue (intermittent problems are usually due to poor or intermittent ground)? Also said to look at the Throttle Position Sensor.
I would think that if the TPS was faulty, the idle speed would fluctuate...

What is really throwing me is that once I cleared the codes, no more problems....
This almost sounds like it is starved of fuel?

To bad the dealer has memory loss!

Not sure what to say about the TPS being bad?

Does your year have an external fuel filter? If so then has it been changed lately?
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Unread 12-28-2007, 07:27 PM   #8
twmattox
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No, internal fuel filter. I replaced the fuel pump about 2 years ago and used a new sock. I would agree, it fealt like it was fuel starved...however, started to backfire and knock horribly when I gave it gas trying to limp to a safe stopping area. Not to mention the horrible fuel smell in the exhaust.
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Unread 01-01-2008, 09:38 AM   #9
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Just noticed I didn't give info on engine etc. 1997 GC Laredo with 4.0L...

I pulled the PCM, cleaned the connections, re-attached with some di-electric grease. There was some surface corrosion on the external case (passenger side rear). Not sure if that presents a problem.

Pulled and replaced the plugs. The tip color was a light tan but the base was pitch black (from the seafoam treatment I assume). Two were slightly over-gapped but nothing major.

I replaced plug wires. I compared resistance of the old wires to the new ones...no difference. New cap and rotor too. The cap had a little build-up at the terminals, but was clearly still making contact.

I replaced the distributor magnetic pick-up (what I was referring to as the camshaft position sensor). The old one was a touch worn, but nothing major.

I pulled and replaced the ignition coil. The plastic housing on the stock coil was cracked.

After all of this, it ran incredible...until it reached operating temperature. Then, it was all I could do to keep it running. I had the OBD II reader hooked up and even when it was barely running, missing, and acted like it was running rich it was pulling no codes.

It would idle fine, but when I gave it throttle it would bog for a second and then respond (increase speed). When I took my foot off the throttle it would bog and nearly die. Almost like it was dumping too much fuel in.

There are 4 sensors hooked to the throttle body. What are these? I am going to replace the crankshaft position sensor today. What else should I look at? What throws me is that it runs perfectly (until it gets to operating temperature)...
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Unread 01-01-2008, 09:51 AM   #10
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Quote:
Originally Posted by twmattox
Just noticed I didn't give info on engine etc. 1997 GC Laredo with 4.0L...

I pulled the PCM, cleaned the connections, re-attached with some di-electric grease. There was some surface corrosion on the external case (passenger side rear). Not sure if that presents a problem.

Pulled and replaced the plugs. The tip color was a light tan but the base was pitch black (from the seafoam treatment I assume). Two were slightly over-gapped but nothing major.

I replaced plug wires. I compared resistance of the old wires to the new ones...no difference. New cap and rotor too. The cap had a little build-up at the terminals, but was clearly still making contact.

I replaced the distributor magnetic pick-up (what I was referring to as the camshaft position sensor). The old one was a touch worn, but nothing major.

I pulled and replaced the ignition coil. The plastic housing on the stock coil was cracked.

After all of this, it ran incredible...until it reached operating temperature. Then, it was all I could do to keep it running. I had the OBD II reader hooked up and even when it was barely running, missing, and acted like it was running rich it was pulling no codes.

It would idle fine, but when I gave it throttle it would bog for a second and then respond (increase speed). When I took my foot off the throttle it would bog and nearly die. Almost like it was dumping too much fuel in.

There are 4 sensors hooked to the throttle body. What are these? I am going to replace the crankshaft position sensor today. What else should I look at? What throws me is that it runs perfectly (until it gets to operating temperature)...

It could need to re-learn one of the new sensors you added. Remember when cold it is in open loop and is using the programmed A/F mixture and timing to run, just like WOT. Once it reaches operating temp and teh O2 sensors are hot it goes into closed loop mode and uses the sensors to control A/F and timing. So if you have one sensor still bad or replaced one and it has not learned it yet it could run fine cold and like crap warm. Try driving it a few miles to let it learn the new stuff before you go replacing more.
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Unread 01-01-2008, 10:03 AM   #11
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well i was going to start a new thread just to let people know but this thread is covering it. My Crank sensor would intermittently fail but not do so enought to cause the car to die and it would cause multiple misfires....trust me i replaced every spark plug, wire, coil, cap, rotor, pcm, throttle body sensor...to try and get it to stop the misfire. I bought a CPS from Oreill'ys and it still did it....so many months later and hundreds of dollars i tried one from the dealership because i was at the end of my rope and now it works fine. I hope its just spark plugs or something for you but for me the Crank sensor was failing intermittently.
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Unread 01-01-2008, 10:06 AM   #12
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Mine did the same thing when I put in a NEW CPS when I had it changed just because I was switching transmissions out and it had to be taken out anyways, I figured it had 150,000 miles on it it was kind of due but the NEW one (Bosch) made my ZJ miss on ALL cylinders and intermittently at that ( not all the time) but when my mechanic put the old one back in problem solved so it was the "new" CPS.

That taught me to not screw with a Jeep if it ain't broke
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Unread 01-01-2008, 10:13 AM   #13
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Quote:
Originally Posted by twmattox

It would idle fine, but when I gave it throttle it would bog for a second and then respond (increase speed). When I took my foot off the throttle it would bog and nearly die. Almost like it was dumping too much fuel in.

There are 4 sensors hooked to the throttle body. What are these? I am going to replace the crankshaft position sensor today. What else should I look at? What throws me is that it runs perfectly (until it gets to operating temperature)...

This is a shot in the dark, but one of those sensors on the TB is the TPS. I had a similar condition to yours, when my engine reached operating temp, the jeep would have similar symptoms. Once I replaced the TPS, all of my problems went away.

Harry
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Unread 01-02-2008, 02:24 PM   #14
twmattox
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Just replaced the Crankshaft Position Sensor too (what a PITA!!!). Still backfiring and loss of power (after entering closed loop). I guess the only thing left to try is the Throttle Position Sensor. If that doesn't do it, I don't know what else to try...possibly new PCM??? Any other thoughts would be appreciated.
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Unread 01-02-2008, 04:02 PM   #15
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Trouble-shooting sensors can be a real PITA. A scanner that can record voltages over time is the ultimate tool for this. Then a plot can be viewed and you get the output of the sensor over a period of time. Then you for spikes, drop-offs, or periodic flat-lining. This is especially important for intermittent types of failure.
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