How to diagnose bad ecm? Check engine light and fuel pump delay upon cold starts.

[Intellect]

1995 cherokee 4.0 i6 4wd. Tittle says it all. Usually only when cold starting.. ill turn the key to acc (you expect mr pump to whuuuuur) but nope. Sometimes it takes 20 seconds to 3 minutes. Sometimes itll whuuur right away.

Side note, was just chasing bleed off issues so i KNOW the pumps good. 110%. Good. Bosch and good.

I know it is an electrical issue because know how the check engine light comes on when you turn the key to acc? Well... Whenever the fuel pump DOESNT whuuuuur, the check engine light DOESNT come on either. They are both delayed. They will come on at the same time... Wether its a 25 second delay or 5 minute delay. Sure enough, whenever the whuuur finally happens, than i see mr check engine illuminate.

Odd part, when the delay happens, the radio, headlights, abs light, ebrake light, all work fine....

I have a seperate forum open that i was informed my above issue is the ignition switch, so i tried a new ignition switch. Identical problem... So im back here

(Abs light is on cause i clipped the lines when i got rid of my turdy 35, and brake light is on cause ebrake is up, ebrake also does nothing when i swapped axles i deleted it)

Side note, anyone ever hot wired a switch to the pump? Imagine its a terrible idea.

No ballest resistor.. 1995 return style...

New fuel pressure reg, pump, injectors, filter, etc. Bosch and name brands. Warrantied multiple parts multiple times to eliminate them from possibilities. (During my bleed off issue, which is FIXED)

Back to topic at hand... I researched it for hours. Here and other forums. No direct answers. Ecm,pcm,ignition switch etc are some of the things i was told could be it. I can use a multimeter if someone can help me. It sucks cause its an intermittent issue, USUALLY when cold. (Hasnt ran within 3 hours seems to be the cap off).

MAP/IAC/TPS/ GOOD GOOD GOOD. 125% GOOD. (Replaced, tested, multimetered, GOOD)

Cps.. 85% its good, only multimetered/tested. Via bleepin jeeps tutorial.

Also , ONLY WHEN COLD usually. Florida.."cold"... If its been running i can usually recrank within an hour - three no issues...

Let me paint a picture.

(Fuel gauge hooked to rail and within view)

Get in the jeep, half turn key, no check engine light or fuel pump whuuur.... Wait 20-400 seconds... Finally! Pressure builds some. Half turn it again, 50/50 change of the check engine light/pump instantly working or waiting 20-400 seconds again, lets say it works instantly, pressure builds more and to spec (49psi ish), i go to crank. (Has full pressure), starter works, engines turning, but acts as if no fuel or something because it doesnt want to start. At this point i can forcefully hold the gas down some and cause terrible idling like a car running with no gas/bad plug/ cant even describe it/misfire ish? , For about 2 minutes, than it be fine. Or i can crank it 3-12 times and wait for the one that actually turns over and stays without stalling... I can make a video if need be.

After this though, i can highway it fine.. offroad 6 hours fine... Stop for lunch and gas and brakes just fine, as long as it isnt off TOO long, i can hop in, crank it up like oem, and head off!

1995 4.0 Cherokee 4wd i6

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Excuse any double posting. Not my fault. Tap a talk app.. not sure if the apps glitching or my posts didnt post. :badpc:

 

[old_man]

Put a voltmeter on the power lead to the fuel pump, right at the tank. Verify when you think the pump should be running that there is 12v. or hook up a fuel pressure gauge at the same times.

 

[napavinetiger]

Did you figure this out my 92 xj has no light and no fuel pump priming at all, it'll crank but not start. i can jump the wires in the relay box and get the pump to run and if i supply power at the ecm the pump and check engine light will come on. I'm starting to think a bad ecm

 

[Tim_MN]

The primary suspect for when the starter cranks the engine but the engine won't start/run, is the Crankshaft Position Sensor. ECM failure should only be considered after a thorough and in-depth diagnostics and testing of the more common and more logical causes for a no-start.

You should test the CPS.

TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3. Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

 

[Patrick S.]

Had exactly the same problem. It's a bad ECM. Replace or repair by changing dielectric capacitors.

 

[Green Mesa XJ]

These Ecm usually do not go bad. I’ve bought spare ecms from junk yards just to test it out. Used to be pretty cheap to find the same part #.

I would do the swap to test it, prepared to do more testing.





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[Tim_MN]

Failure of the ECM/PCM should only be considered after completing a logical and in-depth diagnostics and trouble shooting process of the basics necessary for starting/running and of the more common and logical cause for the symptoms described. ECM/PCM failure can happen, but failure is very highly unlikely, it is unlikely to be the the root cause of a no-start situation.

The only DIY test is to swap in a known good ECM/PCM.