1990 4.0 Comanche running terribly intermittently

[Kligeti]

I’m chasing an intermittent issue on my 1990 Jeep Comanche4.0 Renix, and I’m running out of ideas. The main problem is that it never seems to enter closed loop. It runs extremely rich with a strong fuel smell, and every once in a while, it’ll suddenly run terribly, stalling repeatedly and backfiring. Between episodes, it can run relatively normally, but it’s always rich. So far I’ve replaced the TPS, IAC, CPS, coolant temp sensor, MAP sensor, O2 sensor, ignition coil, ignition control module, spark plugs, and the fuel pump. Fuel pressure is within spec. Vacuum lines all look good, including the MAP line, and I’ve checked the Renix sensor, and ECU grounds, which appear good. Using a Renix engine monitor hooked to a computer, all sensor data looks reasonable. It reaches normal temperature, but never actually transitions into closed loop. Instead, it reports a closed loop rich error and stays open loop. At this stage, I’m trying to figure out what’s left. Could this still be something like leaking injectors, a bad fuel pressure regulator even with correct pressure, an ECU injector driver issue, damaged wiring, or some other known Renix quirk? Has anyone fought this exact combo of symptoms? Any test or common failure points you’d suggest before I keep throwing parts at it? Thanks.

[ghetdjc320]

Out of curiosity, have you done cruisers tips? That’s typically the gold standard before firing the parts canon at seemingly mysterious renix issues

[Kligeti]

I’ve done a number of the tips mostly the cleaning of grounds and such

[Ωhm]

Make sure no fuel is in the fuel pressure regulator vacuum line. Leaky diaphragm.

[Kligeti]

On 7/13/2026 at 3:56 AM, Ωhm said:

Make sure no fuel is in the fuel pressure regulator vacuum line. Leaky diaphragm.

What is the check for this do I need to make some sort of catch can or something?

[Ωhm]

You'll know. Fuel drops in the hose. Should be wet.

[Kligeti]

On 7/14/2026 at 8:59 AM, Ωhm said:

You'll know. Fuel drops in the hose. Should be wet.

There’s no signs of fuel in this line, did some further diagnosis and it looks like I am also losing spark when it tries to die. I am leaning towards a bad PCM? I ordered a smoker to attempt to check more in depth for vaccum leaks this weekend.

[AnotherOldJeepGuy]

Hear are a couple things I have done as a result of suggestions here over time.

 

1.  Check that all your intake/exhaust bolts are tight.  I was able to spray water on the engine while running where these manifolds connect to the engine and heard a noticeable RPM drop.  After tightening these bolts a repeat of the water test no longer affected REMs.  These loose bolts are apparently not uncommon and can cause vacuum leaks as well as incorrect oxygen readings.

 

2.  I found a failing exhaust gasket (the "donut) at where the exhaust manifold connects to the next pipe by putting the output port of my shop vac into the tailpipe and spraying soapy water at various locations along the exhaust manifold looking for leaks.  This gasket is very close to the exhaust oxygen sensor and can also effect oxygen readings.

 

 

[Ωhm]

Wiggle Test your under-hood wiring harnesses. See if you can get problem to act up or go away.

[pizzaman09]

14 hours ago, AnotherOldJeepGuy said:

Hear are a couple things I have done as a result of suggestions here over time.

 

1.  Check that all your intake/exhaust bolts are tight.  I was able to spray water on the engine while running where these manifolds connect to the engine and heard a noticeable RPM drop.  After tightening these bolts a repeat of the water test no longer affected REMs.  These loose bolts are apparently not uncommon and can cause vacuum leaks as well as incorrect oxygen readings.

 

2.  I found a failing exhaust gasket (the "donut) at where the exhaust manifold connects to the next pipe by putting the output port of my shop vac into the tailpipe and spraying soapy water at various locations along the exhaust manifold looking for leaks.  This gasket is very close to the exhaust oxygen sensor and can also effect oxygen readings.

 

 

This.  Check the manifold bolts and check for exhaust leaks before the O2 sensor.

Also what brand O2 sensor did you use?

[Kligeti]

Wiggle tested, smoked vaccum(no leaks visible), manifolds look good. Currently have Bosch O2 but picked up ntk, planning on install tomorrow. Thank you all for the help so far. Also going to pick up an extra ecu off someone for cheap tomorrow, figured not a bad idea to have a spare and I can always sell it again.

[Kligeti]

O2 sensor unfortunately didn’t fix it. I am starting to think I am loosing spark intermittently. A light between coil and distributor cuts out when stalling. Coil and icm are new. Cps and distrubutor new. Any ideas?

[Ωhm]

Put a DVOM on D2_4. Connect GND back to battery negative (-). KEY ON or Engine running look for voltage dropping out when stalling.

 

[Kligeti]

1 hour ago, Ωhm said:

Put a DVOM on D2_4. Connect GND back to battery negative (-). KEY ON or Engine running look for voltage dropping out when stalling.

 

Just had a buddy help. Voltage went from 14.3 to 12.1 when stalling

[Ωhm]

Drop in voltage seems to be alternator voltage. I'd call that normal during a stall. 

 

 

Using a voltmeter or 12vdc testlight (preferred) check for Battery_Voltage on the following pins (use battery_negative terminal for ground):
D1_5:  Battery_Voltage (Hot at all times)
D1_6:  At KEY ON, (Hot for 2-3 seconds). During CRANK and Engine Running, (Hot at all times).
 

[Kligeti]

17 minutes ago, Ωhm said:

Drop in voltage seems to be alternator voltage. I'd call that normal during a stall. 

 

 

Using a voltmeter or 12vdc testlight (preferred) check for Battery_Voltage on the following pins (use battery_negative terminal for ground):
D1_5:  Battery_Voltage (Hot at all times)
D1_6:  At KEY ON, (Hot for 2-3 seconds). During CRANK and Engine Running, (Hot at all times).
 

Same story there, 14 running 12 dying. Swapped ecu plugs over to new ecu and that didn’t help.

[Ωhm]

Key points (splices) in vehicle wiring harness seem good. Bummer on that ECU swap. If equipped with TACH, disconnect it at Instrument Cluster (C203). Just take connector out. PITA. Retry.

[Kligeti]

19 minutes ago, Ωhm said:

Key points (splices) in vehicle wiring harness seem good. Bummer on that ECU swap. If equipped with TACH, disconnect it at Instrument Cluster (C203). Just take connector out. PITA. Retry.

Pulled the instrument cluster. No change In Behaviour. 

[Ωhm]

On 7/12/2026 at 9:45 PM, Kligeti said:

Could this still be something like leaking injectors

 

Wiring seems good. Did you ever prove this out?

[Kligeti]

1 hour ago, Ωhm said:

 

Wiring seems good. Did you ever prove this out?

I’m going to pull the rail now, and inspect

[Kligeti]

I want to say I’ve got a decent lead now! After removing the rail anll the injectors look good but the diagram in the regulator is visibly leaking! Which in this case might be a good thing haha. Going to replace it now. Not sure why it was dry last time I checked. Perhaps it was too long since it ran last or something.

[Kligeti]

This unfortunately did not fix it😔

[Ωhm]

One last thing from me. ECU needs RELEARN time for any and all repairs. Looking at 40 STARTS and DRIVE CYCLES. Hope this helps, you've put a lot of work into this.

 

88 FSM Computer Relearn Procedures.pdf

[Kligeti]

Temp closeout on this thread. Going away from home for work. Will be back in late August to hopefully solve the problem.