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Old February 6th, 2013, 05:17 PM   #1
joshuaarutoff
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Default drivability expert needed near P.H.

Hey all. Need some help fixing my jeep. Its a 98 4.0 m/t. It started bucking/missing during cruise/ acceleration. Also hard start and rough idle. Code p1391 intermittent loss of cam or crank sensor is popping the mil.
I'm using a genesis scanner.

Crank sensor is all over.
Cam sensor gets some spikes once in a while.
Ignition adv. Seems way wrong
Up stream o2 is more rich than lean.

Checked..
powers and grounds good.
Swapped in extra can and crank sensors
New plugs,cap and rotor
Broke IAC cleaning,so replaced w/ new
Tried used CPU. No change.
Map,tps, vss and temp sensors check out good.

At this point I'm looking real hard at the CPU connectors.

Any ideas? Suggestions?
THIS HAS BEEN DRIVING ME NUTS!
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Old February 6th, 2013, 05:24 PM   #2
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just a thought, look at the wiring harness real close at the passenger rear of the head, tech notes all over the place on wires chaffing against the rear head stud, just a thought, good luck, david
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Old February 6th, 2013, 05:27 PM   #3
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Just went through exact same thing, twice. First time it was the downstream 02 sensor, second was the VSS, although speedo still worked. This was on a 97 4.0 five speed.
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Old February 6th, 2013, 05:47 PM   #4
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just a thought, look at the wiring harness real close at the passenger rear of the head, tech notes all over the place on wires chaffing against the rear head stud, just a thought, good luck, david
I saw those notes too. I have the whole engine harness unwrapped and have checked it out pretty good.
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Old February 6th, 2013, 05:54 PM   #5
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Just went through exact same thing, twice. First time it was the downstream 02 sensor, second was the VSS, although speedo still worked. This was on a 97 4.0 five speed.
I unplugged both and test drove with no changes. So I'm guessing their good. At this point I might change the rest of the old sensors. I miss my jeep.
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Old February 12th, 2013, 11:01 AM   #6
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Originally Posted by joshuaarutoff View Post
I unplugged both and test drove with no changes. So I'm guessing their good. At this point I might change the rest of the old sensors. I miss my jeep.
Um. If they are not working, and you unplug them they are still not working! That is not a valid test. The computer needs imputs to operate properly. If it isn't getting the right imput than it won't work.
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Old February 12th, 2013, 03:17 PM   #7
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did you replace the sensors that the scanner told you to replace?
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Old February 12th, 2013, 03:45 PM   #8
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Quote:
Originally Posted by joshuaarutoff View Post
Hey all. Need some help fixing my jeep. Its a 98 4.0 m/t. It started bucking/missing during cruise/ acceleration. Also hard start and rough idle. Code p1391 intermittent loss of cam or crank sensor is popping the mil.
I'm using a genesis scanner.

Crank sensor is all over.
Cam sensor gets some spikes once in a while.
Ignition adv. Seems way wrong
Up stream o2 is more rich than lean.

Checked..
powers and grounds good.
Swapped in extra can and crank sensorsNew plugs,cap and rotor
Broke IAC cleaning,so replaced w/ new
Tried used CPU. No change.
Map,tps, vss and temp sensors check out good.

At this point I'm looking real hard at the CPU connectors.

Any ideas? Suggestions?
THIS HAS BEEN DRIVING ME NUTS!
I think he did.
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Old February 12th, 2013, 05:41 PM   #9
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check for a cracked flexplate. Sometimes the crack where they bolt to the crank and they move around sometimes you can hear it sometimes you cant.
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Old February 14th, 2013, 04:55 PM   #10
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Quote:
Originally Posted by Route55 View Post
Um. If they are not working, and you unplug them they are still not working! That is not a valid test. The computer needs imputs to operate properly. If it isn't getting the right imput than it won't work.
My train of thought goes like this:

If the sensor has failed at its job, it will pop a code.
If a sensor has developed an internal short between 5v, ground or sig, unplugging shorted sensor will stop this short.
The vehicle will not run correctly but the symptoms will change enough to help pinpoint the problem.
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Old February 14th, 2013, 05:06 PM   #11
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Originally Posted by GearDrive View Post
I think he did.
You are correct. New cam and crank sensors. Also replaced connector and pins at pcm that control:powers,grounds,cam SIG,crank SIG, map sig, upstream o2 SIG.

Map, tps, iat and vss are now new.
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Old February 14th, 2013, 05:14 PM   #12
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check for a cracked flexplate. Sometimes the crack where they bolt to the crank and they move around sometimes you can hear it sometimes you cant.
I pulled the starter and barred the engine over while watching the tone wheel on the flywheel.all looked good. Do flywheels crack or just flex plates?
How do I check without pulling the trans?
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Old February 14th, 2013, 05:44 PM   #13
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Are you using a dual trace lab scope to check the sync? If the chain is stretched a bit your sensors can be working but not in sync.

On the other hand, if they are in sync I would tend to think it wasn't due to a cracked flywheel.
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Old February 14th, 2013, 06:09 PM   #14
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Are you using a dual trace lab scope to check the sync? If the chain is stretched a bit your sensors can be working but not in sync.

On the other hand, if they are in sync I would tend to think it wasn't due to a cracked flywheel.
No lab scope. No abilities tocheck sync with genysis scanner. You have one I can borrow?
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