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Head scratcher for today

Paul Ruggles

Member
Supporting Member
Apr 13, 2019
53
Midland, MI
I do not think an ignition misfire. It would most likely be 2 cylinders because of the waste spark setup.

A fuel injector caused misfire maybe.

I would suspect many 05 and 06 PCMs get swapped because of the bad press they get on the forums.

Is this a test? Do you already have it sorted out?
 

Mike_H

Rust Belt Heavyweight
Supporting Member
Feb 28, 2017
4,436
Grand Rapids, MI, United States
I agree with Paul. One coil per two cylinders means to me that if the PCM were the cause, you'd have issues on at least two cylinders. But, I really don't know. That is just a theory.

Not sure if its helpful or not, but I had a nagging P0301 code. I tried swapping injectors and new plugs. Didn't fix it. Before I went too far investigating internals, I knew my fuel pump anti-siphon valve was shot, so I replaced the fuel pump assembly. That fixed it. I don't really know why, as the pump was putting out the correct pressure, it just wouldn't maintain when the engine was off. I do know that since I've put the new pump in, I've not had a misfire code of any type.
 
OP
mrblaine

mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
5,349
Quail Valley, CA
I do not think an ignition misfire. It would most likely be 2 cylinders because of the waste spark setup.

A fuel injector caused misfire maybe.

I would suspect many 05 and 06 PCMs get swapped because of the bad press they get on the forums.

Is this a test? Do you already have it sorted out?
Not really a test, just some info. I helped a guy who was told his SKIM was messing up and locked them out so they couldn't start the rig. Mechanic wanted 2 grand to fix it. He sent the PCM to us with the ignition key, lock cylinder, and SKIM module. We plugged it into a good vehicle, found out the PCM was bad, located another one, added 2 SKIM keys to the existing single and programmed it all to work. The labor plus the new parts was about 450ish out the door back to him.

He gets it back, installs it and says we sent him a PCM that is causing an ignition misfire on #5. I explain that isn't possible for it to have died in transit and it was good when it left. He says mechanic checked by putting test leads at PCM. I explain it is a waste fire system and it would have 2 dead cylinders if the PCM were causing the misfire and the mechanic needs to look elsewhere. He says the mechanic checked the other cylinder at the PCM and it has 2 that aren't being fired. No, again, there are only 3 triggers at the PCM that fire the coil.

He wouldn't listen, kept insisting that I sent him a bad PCM. I tell him to send it back, he does, we fire it up in the test rig and send him screenshots of the DRB scanner screen showing all 6 cylinders running fine with zero misfires detected.

He replies back, okay, good, now we know to look someplace else. Grrrrrrr!
 
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toximus

TJ Addict
Supporting Member
Mar 29, 2018
2,248
Northern WI
Is the other guy doing the work or mearly handing the PCM over to this mechanic without listening for the misfire himself?

He replies back, okay, good, now we know to look someplace else. Grrrrrrr!
I hope he's paying you for that...
 

Sundowner

Sheepless Hobo
Supporting Member
Feb 6, 2019
828
Republic of Dave
He replies back, okay, good, now we know to look someplace else. Grrrrrrr!
The basic problem, here, is that he trusts the mechanic and doesn't trust you...which is why he needed a machine to verify what you were saying. Don't expect that to change.

That said: no, the PCM couldn't cause the single-cylinder misfire on the spark end of things, but yeah...it still controls injectors, and those are individually fired. Besides, the one thing that cannot be the problem is the PCM: it's the one piece between the two rigs that's common, and it's "working" in one and "not working" in another (i.e. something else must be at fault).
 
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mrblaine

mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
5,349
Quail Valley, CA
The basic problem, here, is that he trusts the mechanic and doesn't trust you...which is why he needed a machine to verify what you were saying. Don't expect that to change.
I'm done with it. I have two resolutions in place. If he still insists the part is bad, I'll send him back what he paid for it and he can start over. If he insists that we did something wrong again, I'll send him the money he paid for labor and he can start over. There were 2 phone conversations where I tried to explain this to him and he would not have it. He insisted that the part was bad and causing a misfire. I explained that we had installed the new to us PCM, fired up the rig, ran the scanner on it, found it acceptable and then programmed the SKIM to it and then programmed two more keys to the SKIM. There is no way it died in transit. He still would not have any part of it. Fine, send it back, I will put it back in the rig and test it again. I'm done now, all the way done.

The worst part? He got it back 3 months ago and it has taken that long to fix a leaking transmission cooler line.
 

Paul Ruggles

Member
Supporting Member
Apr 13, 2019
53
Midland, MI
I think this is how shitty mechanics without troubleshooting skills like do do stuff.... "Well, I can't figure it out so I think that there computer thingy is bad "

I have personally seen 1 bad pcm on a chrysler. It was like that when I bought my jeep. I had a wire chafe through and shorted out on the firewall. Caused the IAC motor to only increase idle, not decrease. Troubleshot the wiring , fixed the wire, reman pcm, fixed.

Do PCMs fail for no reason? Sure, but not as often as people are led to believe.
 
OP
mrblaine

mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
5,349
Quail Valley, CA
I think this is how shitty mechanics without troubleshooting skills like do do stuff.... "Well, I can't figure it out so I think that there computer thingy is bad "

I have personally seen 1 bad pcm on a chrysler. It was like that when I bought my jeep. I had a wire chafe through and shorted out on the firewall. Caused the IAC motor to only increase idle, not decrease. Troubleshot the wiring , fixed the wire, reman pcm, fixed.

Do PCMs fail for no reason? Sure, but not as often as people are led to believe.
I doubted his was bad. The description sounded like the SKIM module and or key went bad somehow and then after enough failed starts, bricked the PCM like it is supposed to do. When we put it in the rig, there was no signal out to the starter relay and a few other areas were non responsive. Found a replacement PCM, tested it in the same rig and all was well.
 

astjp2

TJ Addict
Aug 22, 2018
2,082
Utah and Alaska
I think this is how shitty mechanics without troubleshooting skills like do do stuff.... "Well, I can't figure it out so I think that there computer thingy is bad "

I have personally seen 1 bad pcm on a chrysler. It was like that when I bought my jeep. I had a wire chafe through and shorted out on the firewall. Caused the IAC motor to only increase idle, not decrease. Troubleshot the wiring , fixed the wire, reman pcm, fixed.

Do PCMs fail for no reason? Sure, but not as often as people are led to believe.
Most PCM's fail due to capacitors failing...
 
OP
mrblaine

mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
5,349
Quail Valley, CA