Yet another....crank no start

[zippy289]

My 01 with 179k on it has intermittently started no-starting. Cranks and cranks but won't turn over...the first time this happened a year ago, I replaced the CPS with an OEM unit and it worked fine for a year. Now recently its started doing it again. Here's what I've done:

• Replaced CPS with two different Napa replacements, no change
• Replaced CPS with junkyard unit, ran fine for about a week before showing current symptoms
• Verified continuity from Crank Position Sensor and Cam Position Sensor to the PCM
• Verified good resistance from replacement CPS
• Verified good fuel pressure
• ASD relay clicks when power is turned on

I'm really scratching my head with this one. My XJ doesn't want to start at all now despite me being able to crank it over and run it over to the garage. There are no codes stored. I did replace the CPS connector with a really nice junkyard pigtail since some of the insulation was exposed and I suspected a short at the connector, but again no change. My ONLY guess could maybe be a faulty PCM, but I don't have access to play with a spare unless I hike back to the junkyard. I find it hard to believe that the cause of this could be a faulty CPS (unless I have really bad luck).

No other symptoms besides not running. Everything else functions as normal and there aren't any warning lights...

Any idea what to try next? If I had access to an oscilloscope that may have been my next step.

 

[sandyeggoxj]

I just went through this process. The main difference for me is that my jeep sat for a decade. But I initially had a bad CPS. I replaced it with a new OEM unit. Still no start. I failed to verify that the fuel injectors were actually sending fuel. Turned out to be gummed up injectors. Cleaned them out manually with some brake clean and it fired. There was absolutely no difference in the cranking noise the engine made between no spark and no fuel. But I likely had clogged injectors the entire time.

You can test this with some starter fluid. Just spray it in the intake for a few seconds and see if it starts. Even if it doesn't run you know you have a fuel problem.

Some other things to check, physically swap the fuel pump and asd relay. Check your block and battery grounds for corrosion. Do you gauges work correctly when the key is in the run position? Have you checked spark? Measured the values on your coil? Probed the signal wire to the coil for pulses from the pcm? The pcm closes the ground on that coil signal to control the spark timing based on the cps. Fuel injector timing is referenced from the cam sensor.

 

[zippy289]

I just went through this process. The main difference for me is that my jeep sat for a decade. But I initially had a bad CPS. I replaced it with a new OEM unit. Still no start. I failed to verify that the fuel injectors were actually sending fuel. Turned out to be gummed up injectors. Cleaned them out manually with some brake clean and it fired. There was absolutely no difference in the cranking noise the engine made between no spark and no fuel. But I likely had clogged injectors the entire time.

You can test this with some starter fluid. Just spray it in the intake for a few seconds and see if it starts. Even if it doesn't run you know you have a fuel problem.

Some other things to check, physically swap the fuel pump and asd relay. Check your block and battery grounds for corrosion. Do you gauges work correctly when the key is in the run position? Have you checked spark? Measured the values on your coil? Probed the signal wire to the coil for pulses from the pcm? The pcm closes the ground on that coil signal to control the spark timing based on the cps. Fuel injector timing is referenced from the cam sensor.

Thanks for the reply! Ok, your reply made me remember that I did try spraying starting fluid into the intake in case the injectors weren't pulsing...no dice. I also verified no spark by pulling the coil rail and grounding a spark plug while cranking....no spark. Which to me sounds like the ASD is shutting down, which could lead to a faulty CPS. But again, I've replaced the darn thing 4 times with no change.

I did physically swap the ASD and fuel pump relay, no change. Last night I pulled all of the engine grounds (including battery), cleaned them and reinstalled...like everything else, no change.

I suppose I could try testing the cam position sensor. I know that it's been replaced by a previous owner so maybe it's finally let go? When I get home I'll verify 5v signal going to it and then measure resistance.

 

[sandyeggoxj]

On the 99 the pcm supplies ground for all the core sensors. I'm sure the 01 is the same. I pulled apart my harness and found 2 splices in the pcm sensor ground. I can't remember exactly but it was pin A4 or A7 on the pcm. I then checked for continuity to everywhere that went. Including the o2 sensors. Apparently they can short to ground and cause issues too.

With a test light you can check the signal into your coils from the PCM and see if it is flashing while cranking. You can also check that the ASD is sending 12v to the coil signal. You'll have to look up the pins on the relay because I can never remember what is coil and what is load.