'97 4L Strange Idle Behavior
Hey everyone, been lurking for a while but have to ask for some help because I’ve reached the end of my technical knowledge with this issue. Little background: I'm used to working on toyotas, fords, and subarus but a few months back I got a ‘97 XJ 4L 4wd with only 150k miles to play around with. Had the all-too-common exhaust manifold crack so I replaced that first, which is when my crankshaft position sensor went out. That was replaced and everything was running smoothly until I had a code show up for the camshaft sensor. I tested the old one and got no readings back so I replaced it and the code went away, but the misfire/stalling didn’t until I went to reset the PCM, which is where my problem really started XD. My wife wants her parking spot back so I’ve gotta get it driving soon haha
TL;DR-- The jeep will start and idle as long as I crack the intake slightly, otherwise it stalls right after firing up (runs for ~10 seconds) and has a rev limit of ~2500 rpm. It happened after I reset the PCM after installing new cam and crank sensors. Throttle body sensors, cam and crank sensors, fuel pressure, and O2 sensors are good and report expected values. I have a weak spark coming from the ignition coil but it is a new MOPAR one that has coil resistance values within tolerance. Current theories are below. The main problem I have is that it will start but it won’t stay running unless I hold the accelerator (or butterfly valve on the throttle body) a little bit open, which started right after I reset the PCM. I also have no codes/CEL. I started with debugging and found plenty of sensors reading back incorrect data. Here’s what I’ve done so far:
My neighbor is a big XJ/MJ/JL guy and I told him about my issues and he came over for a look. We did some debugging and he left with no idea what could be the cause. My current theories are that maybe the signal from the PCM to the coil is losing strength somewhere along the way, the PCM fried due to a short when I reset it, some other electrical issue is causing bad spark, one of more of the injectors is faulty (stuck open) and dumps fuel, or that the compression is low in one or more of the cylinders. On the plus side, my rear hatch closes and locks just fine. I’m going to check compression in the next day or two and any advice or ideas are more than welcome! Thanks in advance! |
Are you using an OBDII scanner? With all this going on there would be codes or some data out of spec. Also you said "reset pcm" does that mean clearing codes and adaptive memory with a scanner/reader, or shorting the battery terminals together? Both will do the same think I think.
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Could someone summarise this please?
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Originally Posted by Roler
(Post 3679480)
Could someone summarise this please?
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What about a clogged catalytic converter?
This can be diagnosed with a simple vacuum gauge. |
Originally Posted by Roler
(Post 3679480)
Could someone summarise this please?
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Originally Posted by cruiser54
(Post 3679499)
What about a clogged catalytic converter?
This can be diagnosed with a simple vacuum gauge. |
Originally Posted by DavesJeep101
(Post 3679477)
Are you using an OBDII scanner? With all this going on there would be codes or some data out of spec. Also you said "reset pcm" does that mean clearing codes and adaptive memory with a scanner/reader, or shorting the battery terminals together? Both will do the same think I think.
Back at the beginning I checked the codes via OBDII and cleared them after replacing the sensors and they didn’t come back. Then I did a hard reset by shorting the terminals so that the pcm could relearn with the new sensors. |
Originally Posted by the_knightrider
(Post 3679501)
The engine will start and idle as long as I crack the intake valve slightly, otherwise it stalls right after firing up (runs for ~10 seconds) and has a rev limit of ~2500 rpm. It happened after I reset the PCM after installing new cam and crank sensors. Throttle body sensors, cam and crank sensors, fuel pressure, and O2 sensors are good and report expected values. I have a weak spark coming from the ignition coil but it is a new MOPAR one that has coil resistance values within tolerance.
clean throttlebody and IAC |
Little update: Checked the cat and it tested fine. I also checked the battery when it was on and when it was running. Both times it showed good voltages. I also had a thought to try it without the upstream O2 sensor unplugged again and it ran better. It still died but it was more stable when idling. I wonder if since its been running so rich the O2 sensors are all fouled up? Gonna try cleaning then tonight when I do the compression test and see if that makes a difference.
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Originally Posted by Roler
(Post 3679590)
TPS check with analogue meter
clean throttlebody and IAC |
Originally Posted by the_knightrider
(Post 3679597)
@Roler ... The issue still happens when the tps is either mechanically or electrically disconnected..
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Sorry forgot to include that I backprobed the tps to capture readings when it was turned and got a smooth linear response they was within tolerance. It’s always possible that it’s bad but I figured since I had a good reading it was fine.
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Originally Posted by the_knightrider
(Post 3679624)
Sorry forgot to include that I backprobed the tps to capture readings when it was turned and got a smooth linear response they was within tolerance. It’s always possible that it’s bad but I figured since I had a good reading it was fine.
Doesn't make sense that it makes no difference with TPS unplugged or not; if the TPS would be good it should. I take it you measured with engine off, key on? Can you verify what you have at idle? |
Originally Posted by Roler
(Post 3679655)
Sounds like that. Do you remember the voltage?
Doesn't make sense that it makes no difference with TPS unplugged or not; if the TPS would be good it should. I take it you measured with engine off, key on? Can you verify what you have at idle? |
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