Renix gurus, need help

foreman1063

foreman1063

Well-Known Member
Joined
Jul 11, 2005
Location
Raleigh
I have an 88 Comanche, 4.0l with 230,000 miles on the clock.

The thing runs like a top until it gets hot. When the ambient temp is high and the motor is at normal operating temp, it runs really rough at idle. Also, when you shut it off and try to restart, it can be very difficult. It cranks and cranks until it finally starts to fire with some pedal finesse. Again runs really rough at first until revved for a few seconds. At this point it still idles like crap until it cools down. Almost acts like it flooding when warm.

What I've done so far: New timing chain, new distributor, new plugs, rebuilt fuel injectors, Fuel pressure check appears to be normal, replaced cps, MAP sensor and coil with units from another MJ and condition remained the same, Adjusted TPS.

Any ideas would be appreciated as this truck runs great otherwise and would make a good daily driver if I could resolve this issue. Someone mentioned replacing the 02 sensor but I'd rather not start throwing parts at the problem without justification.

John
 
could you possibly have an injector stuck on? It sits over night or after it cools the fuel has evaporated away but after it runs the injector is stuck on making it run rich and when you shut it off it leaks he fuel out of the fuel rail into the intake making it real rich when you try to start it back up. I only mention this because I have seen it a couple times on both gas and diesel engines causing either this type of problem or not starting at all.
 
Hmm, sounds feasible. I guess I could pull injector plugs while it's running rough and see if one of the injectors doesn't cause a change in the condition when unplugged. Thanks for the input.
 
It sounds like the injection system is constantly operating at a cold start setting..meaning when the vehicle is cold everything operates normal but when warm it's dumping too much fuel into the system which causes, rough idle, slow hot starts that require holding the throttle did open allowing enough air into the system to burn off the addition fuel, as well as crappy milage, black exhaust smoke..etc. I'd look at what controls the fuel system dealing with warm up. There will be a temp sensor for the engine control unit, an oxygen sensor, wiring between the two going to the ECU and the ECU itself. The temp sensor does just that, senses engine temp and sends it to the the ECU to adjust the mixture. The O2 sensor takes a reading of from the exhaust system as the vehicle warms and sends a signal to the ECU to adjust mixture, so if any of these have a problem you will have the issues you're describing.
 
Good info there. IIRC their is a coolant temp sensor and air temp sensor. I'll find how to test those as well as the 02 sensor. Hopefully it's a sensor and not the ecu.

Thanks
 
Fuel vaporizing ?
 
Back in my dealer days, we used to bypass the bulkhead connector and hardwire the crank sensor directly to the PCM for this very issue. I'll try to dig up the old TSB when I get time.
Basically, the bulkhead connector causes poor connection in the CPS circuit.
 
That would be great Jody, seems like I've heard of that some where before.

Vapor lock may be an issue as well, maybe in my troubleshooting I can insulated the fuel lines around the motor, never know.
 
http://www.lunghd.com/Downloads_and_Links/CPS-87-90-update.pdf

I would also verify fuel pressure with the engine running and also monitor it after cutting off. Another "gotcha" is to make sure the injector connectors are on the appropriate injector. That can cause poor running/stalling/hard start as well. Unlike the newer H.O. 4.0s, they aren't labeled and routed in an easy to distinguish manner.
 
Thanks for the link jody, I'll look into it. I think my fp is good but I'll verify again. I think fuel injectors are hooked up correctly as the Jeep runs fine most of the time. Drove around today and it ran like a brand new vehicle, OAT was about 68.
 
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