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?

 

You replaced the spark plugs, but did you verify that they actually have a hot spark?

 

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.

 

Yes, I've got good spark.
I actually replaced the ignition coil pack as well.

 

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.

 

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.

 

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.

 

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?

 

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.

 

I've swapped the CMS and another CPS same problems still

Need the pin out for a 99 Grand Cherokee PCM connector

 

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

 

went to the J/Y today and pulled some new plugs in anticipation of replacing some

 

Wiring Harness Checked out Good.

Going to get yet another CrankShaft Sensor

 

Did you check the PCM wiring plugs? Your codes are for intermittent signals from the CPS and cam sensor.