94 3.1 - inconsistent spark

[Tinman238]

OK, I've been dealing with this problem for 3 weeks now.

First it died and would not restart. No codes.

good fuel pressure
good spark
new crank position sensor
new fuel filter - needed it
new plugs and wires - needed them
new coolant temp sensor
plugged off EGR
new O2 sensor - due
pulled ICM and had it checked - checked good

Still would not start

new ECM - Ah haaa!!!....it runs - badly - but it runs. Had to floor it to start it, pops, erratic idle, won't consistently stay running.

Hmmmmm.....checked for vacuum leaks, couple of questionable hoses, replaced them. Decided to pull the intake for two reasons: replace the o-ring on the oil pump drive and check for intake gasket leaks. Found no intake gasket leaks. While I had it apart, I replaced the PVC hose with a new one. Upon reassembly, it runs exactly the same - like crap. No codes other than 12.

Decided to hook my timing light up and, one at a time, put it on each plug wire. Timing flashes are inconsistent. When the engine is speeding up, in its errant speed fluctuations, the flashes are consistent. When the engine is slowing down, the flashes stop or are much less often. So...., this tells me that I do not have consistent spark. So I replaced the center coil and checked again. Same thing, even on the new coil. So I replaced the ICM (even though it checked good earlier). No change, no codes (except 12).

My thoughts are:
The crank position wiring has issues, or
the new crank position sensor is bad.

Any ideas out there?

[Bearwulf]

Try having the IAC valve reprogrammed. GM reccomendsdoing this every time the batt. is disconnected. This because it clears programs on these engines when power is cut off. This from GM service manual.
Check wiring and pulse from ckp sen. to IM to ECM as well.
Cannot be certain. Elec. on these years can be very tempermental @ times. I know i have four of them, the latest (May '07purchase) is givng me fits now. Think we have it down to inj. prob., harder to tell w/ported inj. as they have only 1 wire to pulse the sys. instead of 6 as w/SFI. Harder to isolate which inj.(s) are bad.

[john51md]

Originally Posted by Tinman238

OK, I've been dealing with this problem for 3 weeks now.

First it died and would not restart. No codes.

good fuel pressure
good spark
new crank position sensor
new fuel filter - needed it
new plugs and wires - needed them
new coolant temp sensor
plugged off EGR
new O2 sensor - due
pulled ICM and had it checked - checked good

Still would not start

new ECM - Ah haaa!!!....it runs - badly - but it runs. Had to floor it to start it, pops, erratic idle, won't consistently stay running.

Hmmmmm.....checked for vacuum leaks, couple of questionable hoses, replaced them. Decided to pull the intake for two reasons: replace the o-ring on the oil pump drive and check for intake gasket leaks. Found no intake gasket leaks. While I had it apart, I replaced the PVC hose with a new one. Upon reassembly, it runs exactly the same - like crap. No codes other than 12.

Decided to hook my timing light up and, one at a time, put it on each plug wire. Timing flashes are inconsistent. When the engine is speeding up, in its errant speed fluctuations, the flashes are consistent. When the engine is slowing down, the flashes stop or are much less often. So...., this tells me that I do not have consistent spark. So I replaced the center coil and checked again. Same thing, even on the new coil. So I replaced the ICM (even though it checked good earlier). No change, no codes (except 12).

My thoughts are:
The crank position wiring has issues, or
the new crank position sensor is bad.

Any ideas out there?

What about Throttle Position Sensor?

[Blue Bowtie]

I know that the location isn't very convenient, but if you backprobe the ICM connector and jumper a clean source of +12V to the pink wire, does it help?

[Tinman238]

Replaced the CPS again with a new one - no change - rules that out. Still no codes.

Don't think reprogramming the IAC will fix the spark issue, but I will reprogram it once I get good spark. Thanks. I need to check the pulse from the CPS at the ICM.

Don't think a TPS would affect spark.

Will check the pink wire and give it a clean 12VDC and will also check the blk/wht for a good ground. Thanks.