Strange renix warm start issue.

I have a 90 4.0 that I having a tough time starting after it's warm. Starts cold, starts hot. The strangeness is if I try to start when warm, or after it has been sitting in the sun on a hot day, it cranks and fires then as soon as I move the key from start to on position, it dies. If I leave the starter engaged after it starts, it keeps running (I hate putting the starter and flywheel teeth through that kind of cycling). I get lucky and it will stay running 'sometimes' after keeping the starter engaged for about 15 seconds after the engine is running. Once in a while it dies out while driving in the warming up stage. So here is what I've done/tested thus far, chime in renix gurus please:

1. Pulled and cleaned out all the hard plastic tubes to the tb and airbox (that whole grouping of tubes) with mass air flow sensor cleaner.

2. Sprayed out tb and cleaned the stepper motor on the tb (no carbon on the pintle part is 1-2 years old).

3. Put on a new crank position sensor.

4. new coolant reservoir and cap, I could hear mine hissing and noticed fracturing had to fill the old one once. I had to sand the new reservoir opening with 600 grit on a block of wood to get the casting/molding imperfections out, got a tight seal with the cap now. What is it with those reservoirs made in Spain?

5. checked to see if I had an air pocket under the coolant temp sensor (the one for the gauge on top of the block)

6. Checked with voltmeter the tps was in spec

7. Fuel pressure 39 psi and it jumped 10 with vacuum line removed

8. Pulled cleaned and tested IAT sensor.

9. Tested the MAP, in spec.

10. Recent oil change.

11. New air filter.

lawrence said:

7. Fuel pressure 39 psi and it jumped 10 with vacuum line removed

Click to expand...

Renix XJ 4.0 fuel pressure 31 psi and 39 psi w/vacuum disconnected.

Am I reading you wrong or are you getting 39 and 49 psi?

Opps. I was backwards. It was reading in spec.

I just changed out the fuel filter, and put on a new combination ignition switch on the steering column. Still no luck.

Check the adjustment of the ignition switch at the foot of the steering column. The remote rod from the key cylinder may be moving it too far out of range and shutting off the power.

You said dying out, like running out of gas versus like someone killed the ignition?

If your problem was consistent, I'd wonder if it was the ballast resistor. Still wouldn't hurt to bypass it to check. It's the 1/2"x1/2"x 3" white ceramic thing on the front drivers fender. Just unhook it and connect the wires together. If that fixes it, then it can be left that was permanently.

A few other things to check would be the ground strap at the back of the valve cove to the firewall - again usually not a temp dependent problem but easy enough to inspect. You checked the coolant temp sensor? Could be a bad coil or ign module, but that would be odd to be a problem only when warm.

I C U changed out the CPS but it may have been the connector. They are prone to go bad.

This would be the connector to the engine side of the harness.

Twas the fuel pump relay!!!!!!!!!!!!!!!!!

Jebus. I should have been listening more carefully. I can hear the fuel pump cycle/prime now, quite a nuanced sound over the abs module priming up when keying on, I just didn't pay close enough attention. At least I got all the column components greased while I had the steering wheel apart, the turn signal now gives a soft click just like when it rolled off the assembly line, feels like a brand new jeep :clap:

Kinda makes sense that it would only do it while warm. If the car is cool the gas in the fuel rail is more condensed not needing the addition priming by the fuel pump.? When warm the fuel is less dense and or vaporized in the rail, thus needing the pump to cycle.?