Call to arms! Any GM techs here?

LS3C6

LS3C6

Forum Member
Working on a 1999 Grand Prix GTP / L67 car, 142k miles... intermittent no spark at crank for the first 5-8 cranks, then fires up, chugs on dumped fuel and runs PERFECTLY afterward with no drivability problems whatsoever, fuel trims are near 0%. Fuel pressure always 50psi at crank, no stored codes, no freeze frame incidents, 3X/18X values from CKP match above/below 1200rpm as they should, it simply just doesn't fire the coils, repeatable if let to sit 10 mins - 3hrs after it's been driven, MOST of the time it doesn't do this cold and fires up on half crank, but always an issue warm.

replaced:

-ICM
-coils
-CTS
-IGN switch
-Crank sensor
-Cam sensor
-cleaned all grounds and power junctions
-PCM
-MAF
-OEM wires, autolite 104 plugs @ .050"

Don't have a spark tester, but with 1-2 wires unplugged (and alternating which ones are unplugged), there's no spark, soon as spark is seen it starts. About to dig into wiring harnesses next, this is bizarre.
 
Make sure your fuel pump is priming when you first turn on the key. Some GMs with bad fuel pump relays will automatically kick on once they see oil pressure, but not before.
 
cjmatt said:
Make sure your fuel pump is priming when you first turn on the key. Some GMs with bad fuel pump relays will automatically kick on once they see oil pressure, but not before.
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huh? the pump primes fine as indicated that i'm getting 50psi at the rail and 38psi with the FPR connected @ idle, fuel delivery's perfect, it's spark that's the problem... outside of what i've tried controls spark?
 
Isn't there a relay fr the icm. I don;t have time to dig through schematics but that sounds like a cam/crank sync issue or a bad ground to the ICm.
 
Sean said:
Isn't there a relay fr the icm. I don;t have time to dig through schematics but that sounds like a cam/crank sync issue or a bad ground to the ICm.
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there is an IGN relay i believe that i can try swapping (maybe it's lazy?), but on the scanner the 3x/18x CKP values match each other and they're consistant, I don't believe it's a sync problem... also checked all chassis grounds as well as checked the grounds in the ICM connector.

Also inspected wiring harnesses, interior fusebox (has sunroof, sometimes water leaks down teh a-pillar), everything's fine there... 0 live misfires once running, no pending codes, no nothing.... runs/drives like a brand new one once you have it running, just has a long delay while it pulses the injectors, fills the cylinders up with fuel and then the coils finally decide to fire and it starts fine, running poorly for 2-3 seconds while it gets rid of the dumped fuel
 
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where did you get the replacement ICM. Also have you checked the timming chain for slack ? is the balancer wobbly ?
 
Sean said:
where did you get the replacement ICM. Also have you checked the timming chain for slack ? is the balancer wobbly ?
Click to expand...

tried two different ICM's from very low mileage L67s that were in running cars, symptoms haven't changed a bit in anything we've done.

no wobble in balancer, also inspected magnet behind timing cover.
 
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