Jeep Wrangler PCM and module repair and replacement

Down on power, up on heat?

k1n3k

k1n3k

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Utah
So weird thing. I've been driving my 04 Rubicon with a manual and 4.slow to work for a year now. Its always been able to pull the large hill I have to climb at a solid 55-60mph in 4th. The temp needle always stayed right above 210. Never moved, never thought twice about it.
Well, the last couple weeks its been extra warm. Not overheated by any means but actually hitting probably 220-225. Not really worried about that, but what making me concerned is I also can't hold a full 55mph anymore. I can't even hold 4th. I need to drop to 3rd now. This is a fairly sudden change from normal. That change concerns me.

So, I'm wondering what I should be checking. Right now the game plan is a compression test, and depending on the findings a leakdown test. I have no CEL or any pending codes. Coolant is topped up. Reservoir is filled to the line and the rad is totally full.

Before someone says "its your AC!" the AC has been off for the last week climbing the hill to check that. No change. def can feel the compressor kick on if its on, but even with it off, still hitting 3rd and getting warm.

Thoughts or ideas on what else I can check?
 
Sounds like a bad cat issue.
badcat (2017_11_20 00_38_12 UTC).webp
 
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I have been chasing this same issue but mine includes a P0138 issue. My loss of power usually includes some backfiring through the intake from a lean condition. If you don't have any CEL issues I agree the cats are worth a look. I haven't pulled mine because of the intermittent CEL that come and goes. When the light comes on I have all the issues when it goes out it runs much better with most of the same symptoms you are having.
 
k1n3k said:
So weird thing. I've been driving my 04 Rubicon with a manual and 4.slow to work for a year now. Its always been able to pull the large hill I have to climb at a solid 55-60mph in 4th. The temp needle always stayed right above 210. Never moved, never thought twice about it.
Well, the last couple weeks its been extra warm. Not overheated by any means but actually hitting probably 220-225. Not really worried about that, but what making me concerned is I also can't hold a full 55mph anymore. I can't even hold 4th. I need to drop to 3rd now. This is a fairly sudden change from normal. That change concerns me.


Thoughts or ideas on what else I can check?
Click to expand...

The timing pulls back when ECT actual and IAT actual cross certain thresholds which your notes indicate you are crossing.

But you can also unscrew you upper O2 sensor and stick a boroscope camera in there to check the condition of the precats.
 
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GoldenGorilla said:
The timing pulls back when ECT actual and IAT actual cross certain thresholds.
Click to expand...

You have to be running REALLY hot for the PCM to not let you do 55mph due to ECT or IAT inputs.

I didn’t notice significant power loss when my ECTs were much higher than OP’s numbers on the highway.

I could tell when high IATs caused the PCM to pull the timing and making it feel like I was pulling a trailer. I’d be surprised if OP was in that 190-250* range though.

GoldenGorilla said:
But you can also unscrew you upper O2 sensor and stick a boroscope camera in there to check the condition of the precats.
Click to expand...

Just a heads up, mine looked good until one rotated sideways after shaking the assembly.
 
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tr21triton said:
You have a 4 cylinder Rubicon, another rare one !
But seriously like others have said you have plugged cats. I went through the same thing on my 05 6 cylinder Rubi.
Click to expand...

I would NOT jump to that assumption without inspection and if you don't have leaking exhaust manifolds, I wouldn't unbolt them to check. It's been exceptionally hot in the western states and the pullback of timing between 1 and 5 degrees would produce specifically the effect he is seeing. As would extremely hot fuel, poor performing injectors and some other issues.

You can also see the failed cat in the O2 BnS2 traces.
 
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MikeE024 said:
You have to be running REALLY hot for the PCM to not let you do 55mph due to ECT or IAT inputs.
Click to expand...
Depends on how steep your hills are and the altitude. Where I live, I couldn't hold 55 in 4th gear at a part of I70 with the stock setup when I bought the Jeep brand new. I also have an 04 Rubicon 5 speed. The factory timing on the 04s specifically is a bit low and the whole PCM map is SIGNIFICANTLY different from the 05/06.

I've also recently had a failed precat. Also been down this road.
 
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GoldenGorilla said:
Depends on how steep your hills are and the altitude. Where I live, I couldn't hold 55 in 4th gear at a part of I70 with the stock setup when I bought the Jeep brand new.

I've also recently had a failed precat. Also been down this road.
Click to expand...

Were you tracking your temps to know what they were at that time?

We know that 220-225* ECTs don’t cause this problem.

I have real-time digital gauges on my A-pillar to go with the experiences I listed above, and it was pretty easy to see/feel what temps did and didn’t matter.

See my comment above regarding why you might want to unbolt and shake the cats. I’ve been down this road as well and would have been led astray had I not shook them.

TJ4Jim said:
I think he meant a 4.0
Click to expand...

I think he was joking. Took me a second to catch it.
 
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macleanflood said:
I'd add a fuel pressure test and monitoring live ODB data...watch your O2 voltages and short term and long term fuel trims.

-Mac
Click to expand...

I did and when it fails it flat lines at 0 volt and sometimes at 5 volt. Either way when it fails it flat lines. Wasn't watching fuel pressure but I am thinking I might get the rail someone posted in here for 120 bucks with a port for fuel pressure gauge. I just got the pinout of the circuit and plan on trying some tests in the coming weeks. Too hot right now 105 degrees outside.
 
MikeE024 said:
Were you tracking your temps to know what they were at that time?

We know that 220-225* ECTs don’t cause this problem.

I have real-time digital gauges on my A-pillar to go with the experiences I listed above, and it was pretty easy to see/feel what temps did and didn’t matter.

See my comment above regarding why you might want to unbolt and shake the cats. I’ve been down this road as well and would have been led astray had I not shook them.



I think he was joking. Took me a second to catch it.
Click to expand...

We DO NOT HAVE THE SAME PCM. I have also tuned an 06 6 speed. The way the 04 JTEC+ pulls timing is really different from an 05/06 NGC3.
 
So what I'm gathering:
1. take a run, do some data logging to track fuel trim and o2 voltages. EZPZ.
2. check cats/precats for being plugged or melted or other obstruction. I'll probably replace the o2 sensors while i'm at it.
3. check fuel pressure... somehow. No valve on my rail so i'll work on that one


I had an issue recently just before this that might have something to do with it. I had a tank of fuel that just sucked. It vapor locked like crazy, seemed like it burned off fairly fast and generally ran like crap. issues went away with a new tank of fuel so I wrote it off and didn't think about it. Bringing up the possibility of damaged cats does make me wonder about that....

anyway, I'll do some checks this weekend and see what I can kick up.
 
Intimnasc said:
I did and when it fails it flat lines at 0 volt and sometimes at 5 volt. Either way when it fails it flat lines. Wasn't watching fuel pressure but I am thinking I might get the rail someone posted in here for 120 bucks with a port for fuel pressure gauge. I just got the pinout of the circuit and plan on trying some tests in the coming weeks. Too hot right now 105 degrees outside.
Click to expand...

Voltage for O2 sensors should range up to .9v. I've seen mid 1s but not 5v. Perhaps you're talking about the heater circuit?

Glowshift makes a 5/16 QD to 1/8" npt adapter for $20. You would need a gauge or a Schrader valve to go in that port.

https://www.glowshiftdirect.com/5-16-quick-connect-efi-fuel-line-adapter/

If you do replace the entire rail make sure you replace all of your injector o-rings and lubricate with motor oil (not silicone based...bad for the O2 sensors.) Also consider the DEI heat shield for the injectors.

Screenshot_2025-06-30-18-16-12-82_b9c66f921be6b9ee18febc2ed6906538.webp

IMG20260728174530.webp


-Mac
 
GoldenGorilla said:
We DO NOT HAVE THE SAME PCM. I have also tuned an 06 6 speed. The way the 04 JTEC+ pulls timing is really different from an 05/06 NGC3.
Click to expand...

The NGC PCM in my rig is known to be super aggressive with pulling timing.

And it doesn’t matter since 220-225* ECTs will not prevent your rig, my rig, or his rig from doing 55mph. Could you imagine how many posts there would be on this subject and how poor the 4.0’s reputation would be if that was the case.
 
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macleanflood said:
Voltage for O2 sensors should range up to .9v. I've seen mid 1s but not 5v. Perhaps you're talking about the heater circuit?

Glowshift makes a 5/16 QD to 1/8" npt adapter for $20. You would need a gauge or a Schrader valve to go in that port.

https://www.glowshiftdirect.com/5-16-quick-connect-efi-fuel-line-adapter/

If you do replace the entire rail make sure you replace all of your injector o-rings and lubricate with motor oil (not silicone based...bad for the O2 sensors.) Also consider the DEI heat shield for the injectors.

View attachment 705266
View attachment 705272

-Mac
Click to expand...

Correct me if I am wrong but the reference voltage is 5 volts and drops as the O2 sensor heats up right? Ill look at the data again it was a couple of weeks ago and I may be misquoting. I've been chasing this for a couple of months now.
 

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