Start, died, no start - help needed
Thread Starter
Heel & Toe
Joined: Jan 2025
Posts: 16
Likes: 0
Start, died, no start - help needed
Good evening y'all, I have an LT4 in my C10 with an aftermarket PSI harness. I recently rebuilt the engine due to some oil consumption and cylinder scoring issues.
I got everything installed, cranked for 5 seconds and it fired right up. I was doing some checking around to ensure everything was okay (aka not leaking) and it turns out I completely forgot to seal the water pump bolts so it was pouring out of the coolant holes. Then all of a sudden, while idling, the truck died and refused to start again.
My first thought was water ingress to the optispark so I pulled it to check, looked good on the inside, but I replaced the cap & rotor while I was in there (Petris parts). I got everything reinstalled & it still won't start.
I have plenty of fuel pressure (direct wire aftermarket pump) but am not getting spark at either the plugs or the coil wire (pulled & grounded while cranking). I haven't checked if the injectors are firing, so still need to check that (tough since I'm working by myself). I checked continuity at the opti harness (both opti to connector, as well as connector to connector at the side of the intake) and all good there.
I don't want to throw parts at it arbitrarily, but not sure the best way to diagnose considering the aftermarket items not mirroring OEM wiring.
I got everything installed, cranked for 5 seconds and it fired right up. I was doing some checking around to ensure everything was okay (aka not leaking) and it turns out I completely forgot to seal the water pump bolts so it was pouring out of the coolant holes. Then all of a sudden, while idling, the truck died and refused to start again.
My first thought was water ingress to the optispark so I pulled it to check, looked good on the inside, but I replaced the cap & rotor while I was in there (Petris parts). I got everything reinstalled & it still won't start.
I have plenty of fuel pressure (direct wire aftermarket pump) but am not getting spark at either the plugs or the coil wire (pulled & grounded while cranking). I haven't checked if the injectors are firing, so still need to check that (tough since I'm working by myself). I checked continuity at the opti harness (both opti to connector, as well as connector to connector at the side of the intake) and all good there.
I don't want to throw parts at it arbitrarily, but not sure the best way to diagnose considering the aftermarket items not mirroring OEM wiring.
Advanced
Joined: Dec 2020
Posts: 80
Likes: 4
From: New York City
Good evening y'all, I have an LT4 in my C10 with an aftermarket PSI harness. I recently rebuilt the engine due to some oil consumption and cylinder scoring issues.
I got everything installed, cranked for 5 seconds and it fired right up. I was doing some checking around to ensure everything was okay (aka not leaking) and it turns out I completely forgot to seal the water pump bolts so it was pouring out of the coolant holes. Then all of a sudden, while idling, the truck died and refused to start again.
My first thought was water ingress to the optispark so I pulled it to check, looked good on the inside, but I replaced the cap & rotor while I was in there (Petris parts). I got everything reinstalled & it still won't start.
I have plenty of fuel pressure (direct wire aftermarket pump) but am not getting spark at either the plugs or the coil wire (pulled & grounded while cranking). I haven't checked if the injectors are firing, so still need to check that (tough since I'm working by myself). I checked continuity at the opti harness (both opti to connector, as well as connector to connector at the side of the intake) and all good there.
I don't want to throw parts at it arbitrarily, but not sure the best way to diagnose considering the aftermarket items not mirroring OEM wiring.
I got everything installed, cranked for 5 seconds and it fired right up. I was doing some checking around to ensure everything was okay (aka not leaking) and it turns out I completely forgot to seal the water pump bolts so it was pouring out of the coolant holes. Then all of a sudden, while idling, the truck died and refused to start again.
My first thought was water ingress to the optispark so I pulled it to check, looked good on the inside, but I replaced the cap & rotor while I was in there (Petris parts). I got everything reinstalled & it still won't start.
I have plenty of fuel pressure (direct wire aftermarket pump) but am not getting spark at either the plugs or the coil wire (pulled & grounded while cranking). I haven't checked if the injectors are firing, so still need to check that (tough since I'm working by myself). I checked continuity at the opti harness (both opti to connector, as well as connector to connector at the side of the intake) and all good there.
I don't want to throw parts at it arbitrarily, but not sure the best way to diagnose considering the aftermarket items not mirroring OEM wiring.
if youve got an h41 or 42 (icm circuit short/open) code that'll disable injectors and spark until the code is cleared and the issue is fixed. check your pins at the icm connector you should have 12v at pins A and D, pin B is your ground verify that its grounded. pin C is your signal wire. if you probe pin C and probe ground, then crank the engine you should be getting 1-4vAC. if you pass all those tests you should have spark at the coil. if you pass and dont have spark at the coil you either need a new icm or to fix the female pins on your icm connector. if you fail any of those tests report back what failed and well go from there
if youve got an h16 or h36 (high res/lowr res opti signal failure) that means theres an issue with the opti. most likely the optical sensor since you mentioned you replaced the cap n rotor. if thats the case i believe the only option is to replace the optical sensor but someone may chime in on that.
run all those tests for now n report back the results
Thread Starter
Heel & Toe
Joined: Jan 2025
Posts: 16
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I don't believe the harness/truck has the provisions to pull codes - the PSI harness backdates to OBD i & uses their own PCM, and I haven't seen a port anywhere for connecting to.
I can still check the pinouts for power & ground, though, so will do that after work today.
I'm wondering if it's the optical sensor for two reasons - 1: the aforementioned coolant leak all over the front of the engine and 2: it's an aftermarket Accel optispark w/out the Mitsubishi opti. Any reasonable ways to test that?
Either way, I'll run the ICM tests this afternoon. Thanks for the help.
I can still check the pinouts for power & ground, though, so will do that after work today.
I'm wondering if it's the optical sensor for two reasons - 1: the aforementioned coolant leak all over the front of the engine and 2: it's an aftermarket Accel optispark w/out the Mitsubishi opti. Any reasonable ways to test that?
Either way, I'll run the ICM tests this afternoon. Thanks for the help.
Thread Starter
Heel & Toe
Joined: Jan 2025
Posts: 16
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I ran the electrical tests on the ICM connector. I have 12v at the two outside pins (they're not labeled A-D as on OEM), ground is good, but getting no voltage when jumping ground to signal. Also, no spark at coil.
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
Advanced
Joined: Dec 2020
Posts: 80
Likes: 4
From: New York City
I ran the electrical tests on the ICM connector. I have 12v at the two outside pins (they're not labeled A-D as on OEM), ground is good, but getting no voltage when jumping ground to signal. Also, no spark at coil.
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
If the opti was fine then no signal to the icm would indicate a bad pcm or pcm harness which is unlikely. that leads me to thinking its the optical sensor but its tough to fully confirm that without codes or an oscilloscope
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Joined: Dec 2020
Posts: 80
Likes: 4
From: New York City
I ran the electrical tests on the ICM connector. I have 12v at the two outside pins (they're not labeled A-D as on OEM), ground is good, but getting no voltage when jumping ground to signal. Also, no spark at coil.
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
Based on this, I'm guessing that the optical sensor went bad after presumably getting coolant all over it. If it were the ICM I'd still get signal at the wires, I would think.
Thoughts?
Thread Starter
Heel & Toe
Joined: Jan 2025
Posts: 16
Likes: 0
another test you can try is using a vampire tap on the A and B wires of the opti and using a volt meter test for a fluctuating signal while cranking. not as accurate as an o meter but if you see absolute 0 voltage flowing through its a pretty good sign the optical sensor is shot
Considering the opti has 0v with the key off & 12v with the key on, I would rule out PCM or wiring harness issues. Sounds like a new optical sensor/full optispark is in my future. Just didn't want to throw away $500 if I didn't have to.
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Joined: Dec 2020
Posts: 80
Likes: 4
From: New York City
You are correct - I looked for voltage at the signal wire while cranking, but saw 0v. I just checked the power wire at the opti with the key ON & got battery voltage.
Considering the opti has 0v with the key off & 12v with the key on, I would rule out PCM or wiring harness issues. Sounds like a new optical sensor/full optispark is in my future. Just didn't want to throw away $500 if I didn't have to.
Considering the opti has 0v with the key off & 12v with the key on, I would rule out PCM or wiring harness issues. Sounds like a new optical sensor/full optispark is in my future. Just didn't want to throw away $500 if I didn't have to.
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Thread Starter
Heel & Toe
Joined: Jan 2025
Posts: 16
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yea it most likely is gonna need a new opti. just one last test disconnect the opti harness and test pins a and b while cranking for voltage AC. if the meter bounces around for both then you may have a different problem. if you get nothing on either one thats 99.999% gonna be a bad optical sensor
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Joined: Dec 2020
Posts: 80
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From: New York City
yea thats a near for sure sign its broken. an oscope would be able to show the actual wave of the signal but if the voltmeter was bouncing around thatd be good enough. no movement means no signal. no signal means pcm doesnt send a trigger signal to the icm which is why you got 0vAC when cranking at the icm harness. if you want to get an oscope to confirm you can find some cheap ones online under 50$ or maybe a rent a tool program near you has one. if not id say get that opti out and begin the rebuild. make extra sure to seal that water pump well
Thread Starter
Heel & Toe
Joined: Jan 2025
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Alright, got my new Petris optispark installed & that fixed the no start issue. TPS needs to recalibrate, but that's not a biggie (needs a bit of throttle to start/run).
My next self induced issue is my belt is falling off the front of the harmonic balancer. I removed the reluctor wheel because it was interfering with my timing chain set, which seems like a common mod that didn't need anything further. But, it makes sense that things aren't perfectly in line since I removed that thickness.
What is the best way to shim that back out? I can always put shims between the hub & balancer, but that...doesn't seem right. Any insight would be greatly appreciated.
Edit: the difference is .100" which is just enough to spin the belt off.
My next self induced issue is my belt is falling off the front of the harmonic balancer. I removed the reluctor wheel because it was interfering with my timing chain set, which seems like a common mod that didn't need anything further. But, it makes sense that things aren't perfectly in line since I removed that thickness.
What is the best way to shim that back out? I can always put shims between the hub & balancer, but that...doesn't seem right. Any insight would be greatly appreciated.
Edit: the difference is .100" which is just enough to spin the belt off.
Last edited by F/51 LRS; Aug 22, 2025 at 08:05 PM.