Confirm my suspicions - CPS (or CKPS, whatever you prefer)

I have a 94 ZJ with a junkyard 4.0L engine of unknown history. The engine has an intermittent hard starting issue, always cranks fine but when starting it will do one of a couple things: 1) start right away 2) crank for a couple seconds and start 3) crank for a while, cycle key and try again it starts right away 4) crank, cycle key, crank, cycle key, start.

Engine runs fine when its running although it can be a bit wonky at times. The issue does not relate to being warmed up or not, it happens from cold, happens after a quick stop, happens when its 20F out and happens when its 100F out. I suspect a questionable crankshaft position sensor, maybe a questionable connection or failing sensor. Thoughts? Im probably mostly just looking for confirmation that the CPS would be a likely cause.

fuel pump

wouldnt it have issues when running though? It runs fine 99% of the time. Ive tried turning the key on for a few seconds before cranking to give the pump time to prime, if needed, and it doesnt seem to make a difference.

The symptoms you describe are indicative of a failing check valve in the fuel pump.

He said priming doesn't help, doubt it's the check valve if that's the case. I think a leaking injector is more likely.

Next time it happens, disco/recon the CKP. If it starts, you have your definitive answer. But, signs lead to the CKP.

Johnnie Walker said:

Next time it happens, disco/recon the CKP. If it starts, you have your definitive answer. But, signs lead to the CKP.

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If it was the CKP/CPS it seems like it wouldn't start at all, let alone run fine 99% of the time. I'd put a gauge on the fuel rail and run it with it for a while to see if the FP fluctuates. Then I'd check the condition/quality of the spark and ohm out the CPS per Cruiser54's procedure.

That's what I would do...your method/results may vary.

ParadiseXJ said:

If it was the CKP/CPS it seems like it wouldn't start at all, let alone run fine 99% of the time. I'd put a gauge on the fuel rail and run it with it for a while to see if the FP fluctuates. Then I'd check the condition/quality of the spark and ohm out the CPS per Cruiser54's procedure.

That's what I would do...your method/results may vary.

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When the CPS failed on my 96 it was incremental. Occasional no start, sometimes it would start and have a terrible miss until I shut it down and restarted, then it would occasionally just die while driving, usually at moderate speeds. In other words, it got temperamental or my favorite word "iffy".

I eventually got a "no bus" condition. The "no bus" disappeared when I disconnected the CPS. I eventually did a voltage check on my CPS supply voltage with the CPS connected and was getting 50%-60% of what it should have been. I got full supply voltage when I disconnected the CPS. The short was between the 5 volt (some models/years it is 7 or 8 volts) sensor supply and the ground in the CPS, I had 0.2 ohms of resistance. I really should have ohm tested a good CPS for comparison but didn't. The short was between the supply voltage connector pin and ground. And not the normal Ohm test for short between the ground and the sensor output. And/or there are sub systems that use the same pulse signal from the CPS, like ignition, timing, fuel delivery at the injectors etc, that may act up before the PCM opens the ASD relay or in other words, the sub systems have some autonomy.

My guess is my PCM was getting a borderline intermittent usable pulse signal from the CPS, which was incrementally deteriorating with time and usage. My guess is there is programming to deal with the occasional missed pulse, but there is a point where too many missed pulses screws up the program and/or the PCM initiates shutdown (opens the ASD relay).

It seems mine was able to periodically (randomly) start or run crappy before the CPS went completely bad and PCM shut it completely down.

But like PradiseXJ said it could be fuel, some testing is needed or swap in a new CPS on a hunch and if it turns out to be fuel you have a CPS for the tool box. I favor testing, new isn't necessarily good, one out of three of my new CPS's have been defective, quality control isn't what it used to be.

I have a CPS from my XJ that I know is good so I can test using that one, just wanted to make sure I wasnt completely off base on that thought before swapping the sensor. Ill give that a try when I have a chance

You can pretty well rule out the check valve if instead of cranking every time, you turn the key on, wait a few seconds, off and immediately back on, repeat and on the third time, after waiting a few seconds, crank. Even my 95 with a high mileage and apparently no residual pressure at all would start right up after sitting a long time with three primes. If you still have intermittent no-starts, then I'd look elsewhere.

In the very first post you said

1) start right away 2) crank for a couple seconds and start 3) crank for a while, cycle key and try again it starts right away 4) crank, cycle key, crank, cycle key, start.

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Those are classic fuel pump symptoms.
Could it be something else? ... yes, but rule out the most obvious first

if it was a fuel pump or anti-drainback valve, allowing it to prime would make a difference, but it doesnt. If I turn the key on when I first get in the car and let it run for a couple seconds, it still rarely starts first time. Also, it doesnt make sense to rule out "the most obvious" first when ruling out another possibility is significantly easier, quicker, and cheaper to rule out. If the CPS swap has no effect then fuel pump will be the next stop.

Do it your way. good luck, I'm out.

Do you smell fuel when you are cranking?

Hoooper said:

if it was a fuel pump or anti-drainback valve, allowing it to prime would make a difference, but it doesnt. If I turn the key on when I first get in the car and let it run for a couple seconds, it still rarely starts first time. Also, it doesnt make sense to rule out "the most obvious" first when ruling out another possibility is significantly easier, quicker, and cheaper to rule out. If the CPS swap has no effect then fuel pump will be the next stop.

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That may be so, but from my own experience a well worn out system with a pump that's still good may not prime fully the first time. If you want a really definitive answer to 'is it the priming' you can cycle the pump a few times before you crank. It's free.

Thread jack..

'88 I6 auto 203k

I've noticed recently my jeep doesn't start on the first try but always starts on the second. I've tried priming 3 times before the first start and still nothing. I then had a hunch.
Turned the key over for a split second just long enough to hear it try to start. Then I turn the key again and it starts right up. To me it seems like the cps has loss the ability to read the position from a cold start and just needs to see the shaft move to "wake it up" and able to read position.

Does this sound correct and I need to replace my cps?

Thread jack over.