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Discussion Starter · #1 ·
Hello! New to the forum, not to Jeep.

I recently bought a 99 WJ that has a crank no start issue. PO stated it just died driving one day.
PO told me that the PCM, Crank Sensor, Cam Sensor, and Coil Pack have been replaced.
I confirmed all of this.
Poking around I can see that the trans is a newer Jasper reman, and it looks like the engine has either been
replaced or rebuilt.
It backfired and had a weird crank sound when i cranked it.
Pulled the coil pack and it cranks normally.
I figured timing issue, so I would start by replacing the timing set. When I opened the cover up, it was fine and
correctly indexed. So I just replaced anyways as I had it open.
checked fuel pressure 50lbs, holds pressure.
I have checked the Cam Synchronizer and that is set correctly on the compression stroke.
I tested the ASD relay and that checks out.
Checked the 5v and ground to the cam and crank sensors at the sensor and at the pcm
Checked the PCM power, ground, and fuse
After getting nowhere I picked up a scope and this is what I am seeing.
Cam waveform looks fantastic, crank not so much
I have replaced the crank sensor with a mopar oem sensor.
I'm at a wall, I'm thinking flywheel? But I removed the inspection cover and don't see anything wrong.
Any help is appreciated.
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The PO said it failed out of no where which could be a big clue.
The original problem could of been the CPS or something else simpler.
But with the parts cannon loaded and fired its now a free for all.

If you're certain all the parts mentioned including that the cam sensor was installed correctly, sycn'd and apparently pulsing correctly, fuel psi is ok then the only things i can think of is:
Since it back fires when cranking it does sound like a timing issue, extremely clogged CAT or a leaky fuel injector issue.

Unfortunately the PCM was also changed which might add a level of diag complexity if it wasn't flashed per VIN.
Have you actually cleaned the grounds especially the one nearest the PCM?
After all those parts were replaced have you performed a PCM reset?
Are there any CEL codes?

Myself i would double check the engine timing and then perform a fuel rail leak down test to eliminate leaky injectors then go from there.
 

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Discussion Starter · #5 ·
Thanks for the reply!

I should have mentioned that I reinstalled the original PCM that was provided, and the symptoms are the same.
I have cleaned all the grounds, performed a PCM reset, no prior codes most likely lost when PO replaced PCM or reset codes.

What might cause the crank signal to act the way shown in the waveforms? Shouldn't there be 3 set of 4 between and inside of each cam signal?
The waveforms are on cranking.

Thanks again
 

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OK a PCM replacement issue is likely eliminated.
Where did you take that CPS waveform from? The sensor connectors, PCM, etc.?
Both sensor pulses are there but are they getting to the PCM?

Something doesn't look right with the CPS pulses in that 2nd waveform.
It could just be the Oscope you're using isn't fast enough to capture the pulses.
Whats missing from those wave forms are voltage amplitudes to see if they're are within spec.

Before diving any deeper into this i'd triple check both the cam and CPS connectors for spread pins, dirt or corrosion.
Any little thing like stray capacitance or grease with the connectors can affect rise times and amplitudes of fast pulses.

Not sure about how many CPS pulses there are per cam sensor pulses.

Far as the timing relationships between the cam sensor and CPS can depend on how the Oscope is triggered.
Try different scope settings like triggering CH 1 on the rising edge of the cam sensor signal, etc.

Since the PO said this happened out of no where while driving that rules out a wrong part flywheel?

I gotta say this is a tough one considering all the parts replaced so far which can point to a wiring, ground or connector issue.
 

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Discussion Starter · #7 ·
Thanks for the reply, I picked up the signals at the devices, I will probe up at the PCM and play with the OScope. I will post the results this Saturday. I will
Also check the sensor pins again, but I’ve checked that once already.
 

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Thanks for the reply, I picked up the signals at the devices, I will probe up at the PCM and play with the OScope. I will post the results this Saturday. I will
Also check the sensor pins again, but I’ve checked that once already.
Yeah both pulses seem to be there but are they getting to the PCM is the remaining question.
At some point those pulses would be good enough for me to go on to the next trouble shooting diag.

Also if not already check both male and female pins on the PCM connectors since it been changed once already.
Who knows what the PO did or said to sell the Jeep.
 

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Checked signals at PCM, they are both there and a little cleaner. Checked fuel pressure again and it dips quick with key off. Pulled fuel rail and found 2 injectors that dump fuel. Going to replace them both and see what happens. I will update.
Excellent, hope thats the problem.
Forgot to mention the method i use for easily diaging spark or fuel is immediately after cranking check the plugs for either a dry condition or a fuel soaked condition.

Keep us updated.
 
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