PapaBearXJ88

FINALLY got the issue straight that was causing 4.0 to run bad and skip (side note- BEWARE of ATP Automotive flex-plate part # Z-205), Jeep was running best it ever had -
- then sitting at a stop light, stalled. It started right back, but in the days that followed continues to happen more often and would not start back for several minutes. Seems to happen at idle (or when I let off the gas), but if I catch it before it shuts down I can sometimes keep it running by keeping accelerator pedal pressed -
- HOWEVER, yesterday as I pressed the accelerator to try to keep it running, it bogged down horribly before shutting down and then would not start again for several minutes. When it DOES start back, it will rev high for a few seconds, then level back out to idle.
Crank Sensor, TPS, O2 sensor, Cam Position Sensor/Distributor, Ignition Module, Coil, plugs, wires, fuel pump, injectors, fuel filter, ECU are all relatively new (less than 6 months old). Engine overhauled recently, everything clean (throttle body cleaned inside and out, fuel lines cleaned out with laquer thinner, evverything spotless). Ran great until just a few days ago when problem started, ran better than it ever has.
**By-the-way - only happens after the Jeep is warmed to operating temperature.**
Just replaced Fuel Pump Ballast Resistor, did not solve the issue, thought it might be the Idle Air Control Valve, replacing that in the next few days.
Trying to get out in front of this - just ended 10 months without my daily driver, trying not to go down again.
Any ideas?

CastorTroy

Fuel vapor lock?

cruiser54

Check the CPS output.



Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS itself as shown in Figure 2 as Connector A, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from NAPA or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8″ from the stock 5/16″, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Another little tip to save tons of aggravation is to stick a bit of electrical tape to your 11mm socket and then shove the bolt in after it. This reduces the chances of dropping that special bolt into the bell housing.

4.3L XJ

I agree. The Crank position sensor is in need of replacement

PapaBearXJ88

I found the issue, it was the Idle Air Control was bad. It would stall anytime that I took my foot off the gas. When I pulled it off and the pintle looked like it was fully extended
I think I can still use the CPS info and test because now it is doing something else.... but that'll be for another thread.

Thanks for all of the help.