Won't start when hot

My 92 xj 4.0 will not start when the temp is near the "red zone". I let it sit for 12-15 min and starts right up. What might be causing this?
Thanks

Why is the temp near the red zone ?

Fully define and explain "will not start". Does the starter not function due to excess heat, or is the starter not able to move the engine internal parts, or does everything seem to spin about normal, but the engine just won't go from starting to running ?

The latter is the case, engine spins freely...just does not start.
No even slight indication of starting.

It needs correct fuel pressure and spark to run, does it have both when it won't start ?

Are all the tune-up parts fresh ? Test the coil.

A weak or overheated Crankshaft Position Sensor (CPS) will cause a no-start, you should test the CPS.




The most likely cause of it cranks and cranks but won't start up is the Crankshaft Position Sensor (CPS) located on the transmission bell housing. Often this part is also referred to as the CranKshaft Position Sensor (CKP). CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason. Typical CPS lifespan is about 150-200,000 miles. Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again when it cools back down.

Typical CPS Symptoms, (not all symptoms may be present, or occur at the same time) -
- Random stalling
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have NoBus displayed on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.
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If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.
Diagnostic steps to confirm the CPS is the cause of your no-start:
-You should be able to verify a bad cps, by unplugging it, and turning the ignition key to ON. If the voltage gauge and/or the fuel gauge now displays correctly, replace the CPS.
-Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.
-Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate these relays as the cause of the no-start. Confirm that the fuel pump runs for 3-5 seconds when you turn the ignition key to ON.
-Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will lose communication.
-Check the ground connection for the PCM. Make sure it is clean, snug, and has no corrosion.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

You must also perform basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.

Crankshaft Position Sensor Connector (CPS/CKP)
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CPS Testing

TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

TESTING PROCEDURE for 1987 –1990 4.0 L engines

Test # 1 - Get a volt/ohmmeter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.
Test # 2 - You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. ( The engine can't fire up without the CPS connected but watch for moving parts just the same ! ) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.

The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector is on the passenger side, near or on top of the Transfer case, not as shown in the diagram below. Simply follow the wire from the sensor to the wire connector.
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Thanks for all the input, I replaced the CPS (just barely able to reach up there and get the top bolt started, but did it!) and starts fine at lower temps as before. Have not yet had the chance to test it at higher engine temperatures.