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CPS causing had starting?

Aversnell

NAXJA Forum User
Location
FL
Chasing hard starting issue on 96 XJ 4.0 2wd auto with in-tank fuel pressure regulator. Takes multiple cranks, hot or cold. Have already....

Replaced fuel pump and pressure regulator
Replaced tps
Replaced IAC
Cleaned throttle body
Battery, dist. cap, rotor, plugs and wires all replaced 10 months ago.

Ordered an intake air temp. sensor to try next. No CEL during all of this. Just hard starting.

From what I’ve read here, another possible culprit is the CPS. But I have NO other issues... once she starts, she runs and idles smooth and strong. Is it possible for a faulty CPS to cause hard starting and no other symptoms?
 
Could also be an injector leaking after shutdown. That won't throw a code. My starting problem was solved by replacing the injectors.
 
Could also be an injector leaking after shutdown. That won't throw a code. My starting problem was solved by replacing the injectors.

Yeah, I’ve thought about that... but if I had a leaky injector, wouldn’t I have perforrnance problems beyond hard starting?
 
Before throwing expensive parts at the problem. Do some troubleshooting. Before starting, cycle the key from off to on several times, allowing the fuel pump to spin up and build fuel pressure.

If it starts quickly with that, then you most likely have a bad back flow valve in the fuel pump. There are tons of threads on here about that and you can insert a one way valve in the line to fix the problem without having to replace the pump.
 
Before throwing expensive parts at the problem. Do some troubleshooting. Before starting, cycle the key from off to on several times, allowing the fuel pump to spin up and build fuel pressure.

If it starts quickly with that, then you most likely have a bad back flow valve in the fuel pump. There are tons of threads on here about that and you can insert a one way valve in the line to fix the problem without having to replace the pump.

Already replaced the in-tank regulator (check valve) and pump (Bosch pump). Regulator part is no longer made for 96 in-tank but used crown model that worked for another poster. Can’t do external check valve—returnless system, I understand that’s a no no.

Cycling the key about 10 times will get a start, but cycling two or three times with foot on pedal works too. Don’t want to assume I put faulty parts in the tank yet... hoping to eliminate other possible causes first.

Does anybody know the answer to my question: is bad CPS worth a try?
 
Diagnose first, then buy parts.

A faulty CPS typically causes random stalling and/or a no-start. The CPS and all the other sensors can be tested. A faulty CPS does not typically cause hard starting. Having the issues occur hot and cold somewhat suggests fuel problems rather than sensor problems.

Test the fuel pressure with the engine running. Leave the tester installed overnight, and check pressure in the morning before attempting to start. Watch the fuel pressure while your helper cranks the starter.

If you have to feather the gas pedal to start, suspect low voltage or interrupted voltage, either/both of which delete the idle settings from the PCM.
 
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Check the auto shut down relay. It keeps the power to the ecu for a few seconds after the ignition is shut off, allowing it time to reset the TPS to be ready for a restart. If this doesn't happen, you will get the results you describe. If cycling the key several times fixes it, it is not the CPS.
 
Diagnose first, then buy parts.

A faulty CPS typically causes random stalling and/or a no-start. The CPS and all the other sensors can be tested. A faulty CPS does not typically cause hard starting. Having the issues occur hot and cold somewhat suggests fuel problems rather than sensor problems.

Test the fuel pressure with the engine running. Leave the tester installed overnight, and check pressure in the morning before attempting to start. Watch the fuel pressure while your helper cranks the starter.

If you have to feather the gas pedal to start, suspect low voltage or interrupted voltage, either/both of which delete the idle settings from the PCM.


^^^

This is what I was going to advise.

Finding out what is happening with your fuel pressure is a simple place to start.
 
If you are having to hold the pedal down double check your tps adjustment

There is no TPS adjustment for OBD-I or OBD-II Cherokees, only for pre 1991 Renix Cherokees.

It could also be the Di-Lithium Crystals, or perhaps even the Flux Capacitor, but using some common sense, and some basic trouble shooting, one should start with simple diagnostics of the most common and the most likely causes of hard starting.

Which is missing, movement of the starter, fuel, or spark ?






.
 
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Okay, got a fuel pressure tester. Here’s what I know (again, 96 4.0 auto with in-tank regulator):

Warm engine, key to ON: pressure reads about 52 psi.
Engine running: needle shakes, but center of where it’s shaking reads about 55.
Engine off. Pressure starts dropping immediately
After 10 minutes: 35
After 20 minutes: 25

Restarted and let it idle a few minutes. Then shut off and immediately clamped the rubber fuel line between the tank and the filter. Weirdly, pressure dropped off even faster:

10 minutes: 25
20 minutes: 18

I suppose the clamp may not be completely stopping flow, but still this suggests to me that the problem is not in the tank, but elsewhere. I can not see or smell any gas anywhere along the lines where I can see. Does this mean the most likely culprit is leaky injectors? I’m praying everything is okay in the tank, as having already replaced the pump once and the regulator twice, I’m not sure what else to do in there.
 
Check the auto shut down relay. It keeps the power to the ecu for a few seconds after the ignition is shut off, allowing it time to reset the TPS to be ready for a restart. If this doesn't happen, you will get the results you describe. If cycling the key several times fixes it, it is not the CPS.

Swapped the auto shut down relay with the ac clutch relay (ac works) since it’s the same part. No change.
 
With the rubber fuel line clamped, if the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.
 
With the rubber fuel line clamped, if the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

Thanks. I’m hoping that’s it. I’ll do the injectors this weekend. I already had a set ready to do an upgrade. Fingers crossed. Looks fairly likely that the injectors I have now. are the originals, with 150k on them. They’re due.
 
Dud the injector. Jeep starts and runs fine. Don’t know yet about the pressure, and I’m a little worried because the replacement injectors did not work with the clips that came off the old ones, so I didn’t use the clips. We’ll see.
 
Does your starter sound healthy when it engages?
I had an issue recently where wheb it was cold and/or didn't drive it for a couple of days it would barely crank or not at all.
At first I equated it to the terminals being covered in oil. But after I cleaned it up it didn't take long after that to do the same.
The last time I jumped the starter solenoid with a pry bar, I realizd that there was play in the solenoid housing.
Replaced the starter, and good as new. The old starter was at least 10 years old.
Not to say this is your exact issue, but if your fuel pressure and evefything elses looks good. Consider testing your starter.
 
Dud the injector. Jeep starts and runs fine. Don’t know yet about the pressure, and I’m a little worried because the replacement injectors did not work with the clips that came off the old ones, so I didn’t use the clips. We’ll see.

Are we talking about clips to hold the plugs on the top of the injectors (wiring) or clips to hold the injectors into the fuel rail (and prevent fuel leaks)?
 
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