Hi, first time posting here, lemme know if doing this wrong lol

I Have a 2001 XJ sport, and I keep having random misfires, usually after I have been pushing the motor a bit, (highway speeds, crawling, etc)
I have replaced---
coil pack
injectors (new 4 port)
spark plugs
multiple ground wires
Injector harness
crank shaft pos sensor
throttle pos sensor
DEI heat soak on the fuel rail, injectors and manifold
All quality parts, not cheap Amazon stuff
I believe it maybe the ECM, but I wanted to check other possibilities before replacing it because it's expensive. The reason I believe it's the ECM is because when the engine is running rough, it also shifts at a higher rpm then normal. I didn't have any problems with misfires until I took it 4wheeling one day and the ECM got smoking hot, and would shut off the engine, even tho it was only running about 220. I know im getting fuel because i can smell it being dumped out the exhaust.

This is getting long, but thanks for any help in advance

 

True that the ECM controls the transmission and a rebuilt one isn't exorbitant. If you have a shop manual, use the procedures to test sensors with a VOM prior to replacing them - save your money for wheeling

 

Both the cps and tps were going out, they needed to be replaced, when the TPS was fading it was almost undriveable, when the CPS died it wouldn't even start

 

When did you replace all of these parts? Before or after the misfire problem occurred?

 

I replaced these parts after they occurred, i just kept going down the list, it would seem to be fixed and then it would come back

 

Its possible that the under-hood temps are much higher than the engine temp. The driver side is the hottest since the exhaust is on that side. Do you still have the stock air box. This would shield the PCM from direct radiation from the exhaust.

Its possible you cooked the PCM, but you say this is intermittent under certain conditions, . That would indicate that the PCM is working, maybe its just getting too hot. Is the PCM dirty? It needs to be clean in order to remove internal heat. Shield it from engine heat if possible and try increasing air flow around it.

I am spit-balling this response as its difficult to know the exact conditions you are creating and how the PCM is behaving. I'm just giving you something to consider based on the facts you provided.

You could try cooling the PCM next time this happens. If the cable connections are good you can pour water on it and not harm it. Cool it while its running to see if the mis-fire goes away.

I'm out of ideas at this point. Report back after trying my suggestions. If nothing worked then it might be time for a new PCM. I hate to say that because there are so many other variables. I just picked the heat angle because you mentioned it as the first time it occurred.

 

The PCM is clean, I have not been in super deep mud or water, I also have hood vents, nothing crazy, pretty low profile. I did pour water on it the most recent time it got hot but that was after we had turned it off. After it sits and cools for a bit it does seem to be a lil better but the mis fires do come back. The bad thing is is that the misfires are worse when I'm crawling, when it gets rougher when I'm in town I can usually give some rpm and it clears up, air flow definitely a factor with hwy driving versus trail driving.

I checked compression this weekend and ruled out that factor.
I have potential access to get cooling fins or custom vents made for it.

 

If this is cheaper than a PCM I'd try that before replacing the PCM.

Another thought in a different direction would be fuel pressure. It may be that loss of fuel pressure is causing the misfires. Connect a fuel pressure gauge and drive it until it misfires and see if the pressure drops. You can get a pressure gauge from O'reileys or Autozone for testing through their loner tool program. It won't cost you a dime unless you don't return it. I believe you can use it for 2 or 3 days. I would not automatically replace the fuel pump until its verified that it's loosing pressure. You don't want to add an unknown variable. Check you fuel line for kinks and restrictions.

 

In the Haynes Shop Manual under "Engine misses": A lot of ignition parts and fuel related parts like the fuel pump are listed. But the PCM is not listed. I agree with and Jones that a fuel pressure test be performed. The fuel pump/pressure regulator/vent and fuel strainer are all in one assembly.

 

It has been having trouble starting recently, just keeps cranking and cranking. If it's a loss of fuel pressure would that explain why when I romp on the throttle and get it to 4 or 5k rpms it seems to run well for a bit?

 

Yes low fuel pressure/ volume would cause high rpm power loss. And its possible it has degrade to the point of a no start or long crank condition. Don't replace the fuel pump without further diagnostics.

First step is to verify low fuel pressure with a gauge. If in fact the pressure is low, then a voltage check should be done at the pump to verify its getting full battery voltage.

I would only replace the fuel pump after rulling out all other items that can affect fuel pressure, both electrical and fuel delivery. I would replace. The fuel filter if you haven't already.