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Hard Start when warm, easy start when cold

Pinocpao

NAXJA Forum User
Location
Arizona
Ok, so my 87 jeep cherokee has a hard time starting when it gets warm. new fuel pump and filter, within alldata spec. fuel psi reg with in spec., injectors have a slight leak no drain back from reg. coil is above min ohmns so with in spec. so my other option was to replace ecm, i did so and now wont start at all cold warm anything?? Any ideas this is my dd and now dead in the water.
 
Go back to the previous ECM. Check your ASD relay and make sure it is working. From there, I would check the IAC control. My thoughts are that the computer isn't setting the IAC to the startup position when the motor shuts down. Will it start with partial throttle?
 
Pretty common for the CPS to weaken as it gets warm. Possible the replacement ECU might have more trouble reading the weak signal than the your old one.

Check the CPS for ohms. then voltage output while cranking.

The quick check is to test for spark while cranking, no spark, eight times out of ten it is a CPS issue.

Cruiser54 has a nice writeup on CPS testing. http://cruiser54.com/?p=50
 
Yeah, 9 times out of 10 for my 89 not starting is the CPS connector.

My most recent battle has lead me to have the ECU unbolted and sort of resting down a little as to relieve the bending wiring harness at the connector.

And the Renix has never started quicker or more reliably then the last few weeks.

I was going to post a new thread on my revelation after a few more weeks of good behavior.

I would bet that your CPS plug is your culprit like 8Mud suggested.

Good luck let us know how you did.
 
"injectors have a slight leak" FIX the LEAK!!!! It floods the engine....

"87 jeep cherokee has a hard time starting when it gets warm" This could be a dirty of defective IAT Intake Air Temp sensor. Took me days to figure that one out once.

Fix the 100% no start first with the 2nd ECU. That ECU may be bad? Put the other ECU back in.

Check the battery while cranking, it must hold at least 10.0 volts while cranking or the ECU will not run. Test the cranking CPS voltage with the CPS disconnected, using the AC volts setting on your multimeter. If it reads below 0.50 volts follow Cruiser54s CPS modification(even if you replace the CPS), to move it closer to the flex plate or flywheel. You must have 0.50 volts AC at the CPS while cranking or it will not always start.
 
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