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Unread 09-15-2013, 08:41 PM   #1
Registered User
1998 ZJ 
Join Date: Sep 2013
Location: Lafayette, LA
Posts: 57
98 ZJ dies during warm-up

1998 Grand Cherokee 4x4 5.2L

Jeep starts fine when engine cold. Runs great actually.
When operating temp reaches about 150F the Idle Air Control starts to open and the engine rpms start dropping until it dies. Will not restart when warm.
I have to wait until the engine cools completely to repeat the above cycle.
No codes are thrown.

This is what I know:
Fresh fuel, good pressure.
Good spark throughout, even to the point of dying.
Fuel injectors are audibly working.
If I choke off the intake hose with my hand, it wants to recover.
It doesn't miss or run rough, just dies slowly.

Throttle body and attached sensors cleaned with new TB gasket.

Coolant temp switch is new and resistance of sensor corresponds correctly to approximate temp when cold and warm. This sensor throws a code when disconnected. Wiring to PCM good.

Manifold air temp sensor resistance also corresponds correctly. This sensor throws a code when disconnected. Wiring to PCM good.

Upstream O2 does not throw a code when disconnected. This makes me believe the PCM is not making it to the OPEN/CLOSED loop switch.

PCM connectors in good condition. No loose pins, no corrosion.

Map sensor gets 5volts like it should.

I am using the factory service manual for all my checks.

I have not messed with the Crank Sensor. It appears to be working based on how smooth it runs when cold. Of all sensors or parts I have ever changed or tested on any vehicle, this has got to be the worst placement of anything I have ever seen.

Any help is appreciated. I don't want to throw parts at it until I have all testing exhausted.


Bullwinkle_98ZJ is offline   Reply With Quote
Unread 09-15-2013, 09:09 PM   #2
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1995 ZJ 
Join Date: May 2008
Location: , Alberta
Posts: 6,453
If it still doesn't start when the gas pedal is applied , it would rule out the IAC .

The 02 should definitely throw a code . First thought , does the light work when you turn the key to the on position ? Just because , track the 02 wiring on both sensors to make sure nothing is burned or scarred . Shorts in these areas can cause grief .

Get a code reader on it if not already done . CEL's (check engine lights) can run but they can't hide . 1998's have to be read by a scanner .

Agreed , V8 crank sensors are a PITFA . I just did one on my 5.9 . Advice , use the allen key in the bolt head , instead of the 13mm bolt head . Wobbled a little more but came out much better . Size for the key is 3/16 or 5mm . Tools used were in order from the sensor
3/16 allen key socket
3" extension
socket universal joint
approx 2' of extensions

Small mirror a must but you can get your hand to the back of the cylinder head to line it up . O yeah , sounds like fun till you have to do it . If you can , pressure wash the area , you have just enough room to get a nozzle in there .

Important you use Mopar sensors . Others are really hit and miss . Not worth the chance . Being on the forum , I have seen lots replace new aftermarket sensors and cure the issue .
newfieZJ is offline   Reply With Quote
Unread 09-26-2013, 07:10 PM   #3
Registered User
1998 ZJ 
Join Date: Sep 2013
Location: Lafayette, LA
Posts: 57
Fixed !!!!!!

Fuel pressure regulator causing the problem. Swapped out the whole assy with new pump and regulator now runs and runs and runs.

Found I was only getting 10PSI at the rail which was enough to run it cold, but not once it warmed up and the PCM started making changes for temp.

Now.....steady 50PSI, smooth idle and good response.
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