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.