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Author Topic: new Jeep starting issue...  (Read 946 times)

Offline workgoats

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new Jeep starting issue...
« on: August 22, 2015, 09:04:01 pm »
I have a 96 Jeep Grand Cherokee I have been building for my grand-daughter.  This is Courtney, not Georgia.  She's a high school senior.

The Jeep has a 4.0 engine with a Chrysler transmission.  This was bought from an impound auction.  It had enough radiator stop leak to stop a 2" crack in the plastic radiator.  Of course wasn't much room for water...

After all that was taken care of it takes spells of simply not starting after it has been sitting for a while.  Turns over but no start.  I thought it was the fuel pump but with a gauge on the fuel rail I get 40# with the 2 second prime and it runs consistently at 50#.

I thought it might be the coil and low and behold it had a coil identical to a Magnum coil attached.  It was spliced into the harness with scotch locks.  The terminal on the coil was so corroded I couldn't see how it fired at all.

Ran pretty good for a week or so with a real Jeep coil and now it is back to same old tricks.  A couple of times it would start immediately after I bled the pressure off the injector rail and other times it would start after a few minutes without doing anything.

It has cut out a couple of times and acted like it was trying to die, ending up with a backfire out the exhaust and starting running again.

To me, this is a decrease of battery power to the coil.  I had this happen once with a 318 in a small motor home when the alternator quit and it build up gasoline in the muffler.  In fact, it blew up the muffler.

Ernest had one acting similar.  It had some other issues and they decided to get rid of it.  There are NO codes.

Anyone got any suggestions?


Offline ProjectPW

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Re: new Jeep starting issue...
« Reply #1 on: August 23, 2015, 03:15:36 pm »
Cam or crank sensor...while cranking use scan tool to look at cam/crank sync. Does it have spark when it doesn't start? Do you have injector pulsewidth when not starting? I also find that the pcm is a very suspect item on the late model Chryslers... When it doesn't start wiggle the connectors firmly on the computer... Clean connections at PCm with electronics cleaner.... Do not buy a pcm from the parts stores, if pcm is suspect find a good used one on eBay for same year/model and same transmission/motor. Good luck this kind of problem is hard to track down.... Also swap the asd relay when the problem occurs with the horn relay
« Last Edit: August 23, 2015, 04:25:06 pm by ProjectPW »
1979 W150 "TOP HAND", and way to many other mopars!

Offline workgoats

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Re: new Jeep starting issue...
« Reply #2 on: August 23, 2015, 05:20:20 pm »
I don't have a scan tool.  Ernest does and I might get a chance to borrow it but the problem is that this is so random a problem that we don't seem to have tools or be in a place to do much testing while it is acting up.  It is not likely to play while we are watching...

I have already swapped the relays and I have a bunch of relays that I can swap in but they seem to me to be working.  I can change out the sensors from a parts car and will try to do that tonight or tomorrow.  The parts car is a 97 but there should be no difference between the 96 and the 97.

I can swap out the PCM also but it appears to be aftermarket.  I guess it's worth a try.

I have two of these Grand Cherokees, and the parts car.  They all have 250k to 295k but they are solid, no rust and just a little lifter noise.  We like the way they drive.  I just need the grand daughters to start all the time so she doesn't borrow mine.

Offline workgoats

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Re: new Jeep starting issue...
« Reply #3 on: September 29, 2015, 02:08:51 pm »
Here's the fix for this problem:

Lots of people have had this problem.  Nearly all were solved with the replacement of either the crank position sensor or the cam position sensor.  It appears that a lot of folks don't' know the difference between the two.

Anyway, I replaced the crank position sensor.  Not really a hard fix but year dependent.  I bought a new one from NAPA.  I drove the Jeep for a couple of days with no issues.  Gave it back to the grand-daughter and it started dying again after a couple of days. 

I replaced the cam position sensor (in the distributor) with the one out of the parts Jeep.  NAPA said that there were 4 different sensors and made me wonder so I pulled the used one and the Jeep has been running great since.  I have come to the conclusion that there are 4 different suppliers but that the Hall Effect sensors are all the same.  Might not be right but a used one from a 97 Jeep worked good in a 96 Jeep.

Cam position sensor is a real easy fix, two screws on the distributor cap, pull the rotor, pull the sensor and unplug from the harness.  A true 5 minute replacement.

So, apparently they can both cause the problem.  Book says the Crank Position sensor runs the ignition and the Cam Position Sensor runs the injectors.

Offline ProjectPW

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Re: new Jeep starting issue...
« Reply #4 on: September 30, 2015, 08:05:17 am »
When dealing with crank and cam sensors I always replace them as a pair.... as long as the hall effect has the correct plug on it they will fit nearly any chrysler distributor originally equipped with this type of pickup coil. Glad you got it fixed  :great:
1979 W150 "TOP HAND", and way to many other mopars!

 

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