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Old 11-22-2015, 05:51 PM
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Well I purchased a 99 Grand Cherokee with a 4.0L on Friday.
It'll crank all day but won't start.

Started by replacing the fuel filter(has built in pressure regulator).
Next Used a fuel pressure gauge to verify pressure, 49 PSI on the rail.
Drops to 30ish when cranking?

Moved on to checking for spark, replaced all spark plugs and did a compression test.

130-140psi across the board between the cylinders.


Not sure what to do next?
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Old 11-22-2015, 06:17 PM
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You replaced the spark plugs, but did you verify that they actually have a hot spark?
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Old 11-22-2015, 06:45 PM
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You need a strong, BLUE, snapping spark. Yellow/orange/white indicates a weak spark, which is usually related to a failing ignition coil.

What color is your spark? Ignition coils can be tested for primary and secondary resistances with a manual and a meter.
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Old 11-23-2015, 06:58 AM
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Yes, I've got good spark.
I actually replaced the ignition coil pack as well.
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Old 11-23-2015, 08:37 AM
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At this point I'm thinking the pressure drop was too high on the fuel rail.
I've ordered a new pump module and also a set of 784 injectors.
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Old 11-23-2015, 04:52 PM
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Are you getting electrical signals to the injectors? Not power, on-cycle pulses? If you're getting spark, that's about the only thing left. If the CPS was shot, you wouldn't be getting spark.
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Old 11-23-2015, 05:18 PM
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Yeah, actually the PO thought it was the CPS so that's brand new.
I unplugged it to verify along the way.

Also pulled the fuel injector fuse at one point to verify its function.
But I have not individually checked each injector for a signal.
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Old 11-29-2015, 11:42 AM
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Replaced, the fuel pump and injectors

Still same issue...

Threw a P0320 code...
But again just replaced the CPS and the coil pack is firing


Tps, map, iac?

Cam shaft position sensor?
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Old 11-30-2015, 08:26 AM
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Posted up the issues on the Cherokee Forum FB page.

Got some great feedback that it's likely an intermittent CPS failure (defective sensor) or the PCM is throwing the wrong code and its really a CMS failure (common on 1997+ computers)

I'll be swapping both and see if it resolves.
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Old 12-06-2015, 09:12 PM
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I've swapped the CMS and another CPS same problems still

Need the pin out for a 99 Grand Cherokee PCM connector
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Old 12-06-2015, 10:05 PM
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try to prime it with some gas and see if it will run off the prime
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Old 12-07-2015, 07:47 AM
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Originally Posted by azdave35 View Post
try to prime it with some gas and see if it will run off the prime
In the process of all this I've replaced the whole Fuel system. It will run for about a minute at a time before throwing the P0320 Code & some times a P1391
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Old 12-07-2015, 11:58 AM
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went to the J/Y today and pulled some new plugs in anticipation of replacing some
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Old 12-12-2015, 06:20 AM
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Wiring Harness Checked out Good.

Going to get yet another CrankShaft Sensor
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Old 12-12-2015, 06:37 AM
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Did you check the PCM wiring plugs? Your codes are for intermittent signals from the CPS and cam sensor.
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