the_knightrider

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:

• Before this, replaced the battery, crank sensor, cam sensor, both O2 sensors, new headlights, and air filter

• Cleaned and tested IAC, saw it was not moving properly and replaced it (it was a chinese part and you know how those work)
• Tested the MAP sensor, saw it was reading back bad values and replaced it -- this helped it run better but still wouldn’t start
• Checked the CCV/PCV valve and replaced those two gaskets on the valve cover since they were super hard
• Tested the TPS, gave correct values
  • Also tried running with the TPS unplugged and manually actuating the valve. This made it so that I could idle if it was cracked open a tiny bit. This was the case when it was either connected electrically or disconnected. In my mind, this pointed away from the IAC and air as being the root of the issue.
• Checked for vacuum leaks, couldn’t find a single one. I have a connector with a tiny crack that got replaced but didn’t change how it ran
• Cleaned and checked the throttle body
• Tested major ground points, all are < 1ohm
• Checked fuses and relays, all are good. Checked ASD relay by swapping it with a good one and it still behaved the same
• Checked for gas leaking from injectors (only on the outer O-ring) and there wasn’t any. I pulled the injectors early on and didn't notice any O-rings that were visibly damaged
• Replaced the cat (more like installed a new one since the dude before me straight-piped it) to pass emissions
• Confirmed I’m getting fuel pressure, ~50psi at the fuel rail when running and a little bit lower when primed
  • This lead away from my idea that either the fuel pump or mesh filter were bad since it was getting enough pressure to the rail
  • It is possible that the injectors are bad, but unlikely because it ran fine until I reset the PCM
• Checked all fluids and I’m good on all of them
  • Oil pressure sensor is finicky from time to time but no codes and it runs even when it reports a low reading (pressure is fine; the connector needs to be cleaned)
    

At this point, I started moving from thinking that fuel/air was the problem to spark. I had fuel and air, but maybe spark was not great? I also thought that maybe if I had bad spark, a normal mixture was too lean for a weak spark and by opening the valve, it would lean the mixture out such that combustion could occur. I then:

• Checked spark plugs, they were blacker than black, so I replaced them. Makes sense if the mix was super rich, right? Anyways, this made it run better, but still not stay running.
• Checked the color of the spark, it was a weak orange so I moved on to the wires and distributor
• Tested the each spark plug wire’s resistance, they were all well within spec
• Checked the distributor and noticed the cap had a little bit of corrosion on the contacts so I replaced it with a new MOPAR one
• Moved on to the ignition coil and tested it both through the distributor and with a spark plug attached and saw a weak spark still. I then checked the resistance and it was super out of spec (2.5 ohms on the primary coil). I thought I had found my issue! I replaced it with a MOPAR coil, which made it run much smoother but it still didn’t solve the original problem.
  • After replacing the coil, it ran better but could only rev up to ~2500 with a bit of a misfire near that limit. Didn’t seem like limp mode, but I’m hesitant to try and put it in gear since I have to hold the accelerator to idle haha
• I tested the spark with the new coil and plugs and noticed it had a nice fat spark…but it was still orange so somewhere power is being lost
• I've been probing around the wiring harness for shorts but haven’t come across any

• Checked battery while running and when not running and got normal values (alternator seems to be fine)

• It’s possible that the fuse box internals are messed up somehow, but it seems unlikely

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!
​​​​​​​

DavesJeep101

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.

Roler

Could someone summarise this please?

Big David

and change OPs screen name to stephenking

cruiser54

What about a clogged catalytic converter?
This can be diagnosed with a simple vacuum gauge.

the_knightrider

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.

the_knightrider

@cruiser54 It is possible but I installed a new one right before it wouldn’t start. I’ll give it a shot this evening when I get home. Thanks

the_knightrider

@DavesJeep101
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.

Roler

TPS check with analogue meter
clean throttlebody and IAC

the_knightrider

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.

the_knightrider

@Roler Done, done and done. The issue still happens when the tps is either mechanically or electrically disconnected. Those were my first ideas 😅. Even if I manually turn the tps when it’s not mechanically connected it doesn’t change anything and I was getting good readings from it.

Roler

Doesn't that suggest a faulty TPS...?

the_knightrider

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.

Roler

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?

the_knightrider

I don't remember the voltage off the top of my head, but I'll try and find it. I do remember that it was within what I was seeing online for other 97's with the 4.0. And yes, measured with the engine off but key on. I'll add it to my list of things to check. Last night I got it to a steady-ish idle @ 2K but it was super rich. Going to order some NGK O2 sensors tonight. I also checked compression last night since its pretty quick and easy and everything was withing range, albeit on the low side. It was a cold engine though, since, you know, it doesn't run