Crank, spark, no start

[movietvet]

Please post close up pics of the plugs in cylinder order. Make sure that if you reinstall the same plugs, that they go back in the same cylinder they came out of.

 

[Schurkey]

'95 c1500 350tbi

it gave some grief starting. Had to hold the throttle down slightly to maintain a smooth idle.

Verify that the IAC is working, and the coolant temp sensor is providing valid data all the way to the computer.

-Attempted to start the truck the next day, no dice.

I assume that it cranks, but doesn't run. Previous day's poor running may have fouled the plugs.

-checked the voltage at the fuel pump harness, pump has 11v with the key on. Good spray from the injectors when cranking.

11V on the supply side. Nobody remembers to check for voltage on the ground side.

But at least the pump runs, and the fuel filter isn't totally plugged.

-checked spark by pulling #1 and #3 plugs, good arc across the gap. Measured the gap at .035 as per my manual.

Not a waste of time, but not all that helpful, either. A .035 gap in atmospheric pressure is no test of an ignition system's voltage capacity. The ignition system has to fire a .035 or larger gap, with the gap under compression pressure many times higher than atmospheric. Simulating this requires an enormous gap.

Re-do this test with a real spark tester, calibrated for HEI. The spark will jump a gap of more than 3/8 inch--ten times or more than a normal plug gap.
There's many styles, and some that have a damned worthless light-bulb inside instead of an actual real, live, gap for the spark to jump.
My favorite kind:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

Plug is carbon fouled, but nothing extreme.

CLEAN THEM, or replace them. "Fouled" is no good. Carbon conducts electricity.

-Cap, rotor, plug wires, fuel pump, spark plugs, starter, ignition coil, all less than 2 years old and with less than 10km on them.

SOMETHING is wrong if the plugs get fouled in 10km. Oil burning? FAULTY O2 SENSOR(s)? How old is the O2 sensor? Kinda thinking there is only one on this vehicle.

Verify ALL the sensors and computer outputs. Again, scan tool makes this a snap. Lack of a scan tool makes this a weekend project and "good luck".

Many aftermarket ignition coils have a moderately-horrible reputation for long-term service. The OEM coil you removed may be better than the expensive boutique-brand coil that gets installed.

-No scan tool to read obd1 data on hand

Probably want to get one.

no codes showing with the jumper wire/CEL.

"Codes" can be helpful. The real diagnostic power is in the data stream, and the bidirectional control...and that takes a scan tool.

 

[movietvet]

Theoretically, if you have good compression, air, fuel, spark....should fire off.

 

[Schurkey]

Theoretically, if you have good compression, air, fuel, spark....should fire off.

Define "good".

I'd have said he needs:
1. A burnable air/fuel mixture
2. Compression pressure between 100 psi, up to perhaps 200 psi.
3. Spark across the plug gap, at the right time
4. A reasonably free-flowing exhaust

 

[Canadian Rust Bucket]

Define "good".

I'd have said he needs:
1. A burnable air/fuel mixture
2. Compression pressure between 100 psi, up to perhaps 200 psi.
3. Spark across the plug gap, at the right time
4. A reasonably free-flowing exhaust

Thank you for your input! Lots of things for me to check. I've ordered a compression tester and the spark tester you linked, I've seen you mention it before.
The o2 sensor was installed August 2022, and I haven't put too many miles on it since.
The exhaust is out of the equation at the moment; to make sure it wasn't a factor after my first few failed attempts, I disconnected it between the y-pipe and the manifolds.
I will look up the IAC and try to test it. Coolant temp sensor is a few years old, but I will still test it. Not sure the temp is an issue, because I have the same no-start in -10° and +15° outside temp.
I appreciate you taking your time to help me learn stuff and figure this out!

 

[movietvet]

Define "good".

I'd have said he needs:
1. A burnable air/fuel mixture
2. Compression pressure between 100 psi, up to perhaps 200 psi.
3. Spark across the plug gap, at the right time
4. A reasonably free-flowing exhaust

Ok, compression that is close to even with all 8 cylinders. Not 100 on a couple and several at 150 and a couple at 200. IMO, the good to go compression would be 150 to 160 and all close to that range. The free flowing exhaust is great but will not hinder it from firing off and starting. Depending on the lack of free flow, it could idle fine and starve for power on acceleration, or start and if completely blocked, start to strain and falter asap and then die.

 

[Canadian Rust Bucket]

Long story short, got her running again!

I checked the IAC and coolant temp sensors, both are good. Resistances correct as per the FSM, etc.
Pulled all the plugs as @movietvet suggested (thank you!) and found an obvious issue! Finally, a lead!

You must be registered for see images attach

I can't figure out how to zoom the picture in, but the #2 and #6 plugs were particularly nasty. And, as @Schurkey explained (thanks a lot, your contributions are very educational), the fouled plugs might not be jumping the gap under pressure but will at atmospheric pressure.
After cleaning all the plugs, I held the gas pedal down while starting to clear any potential flood from my earlier failed attempts and she fired up and idled around 400 rpm.
I'll change the plugs shortly, but I'm still stuck wondering why they got so fouled in the first place.

After more reading in the service manual and on the forum, I have discovered the ICM. After the test procedure, my error is "IC Bypass circuit open, faulty connections or faulty DI control module."
Obviously this is an issue to be fixed, but I'm not 100% sure of the effect this would have on the truck.
Could this potentially be at the root of my problems, or am I barking up the wrong tree?

 

[Schurkey]

Long story short, got her running again!

the #2 and #6 plugs were particularly nasty... ...the fouled plugs might not be jumping the gap under pressure... ...After cleaning all the plugs, I held the gas pedal down while starting to clear any potential flood from my earlier failed attempts and she fired up and idled around 400 rpm.

Fouled plugs are the "silent killer" of engines that won't start.

I'm still stuck wondering why they got so fouled in the first place.

After more reading in the service manual and on the forum, I have discovered the ICM. After the test procedure, my error is "IC Bypass circuit open, faulty connections or faulty DI control module."
Obviously this is an issue to be fixed, but I'm not 100% sure of the effect this would have on the truck.
Could this potentially be at the root of my problems, or am I barking up the wrong tree?

Absolutely could be the root cause. There's many different kinds of problems that a faulty ignition module, or faulty communication between the computer and the ignition module could cause.

Weak spark, or lack of computer-controlled timing advance being just two that would be totally critical.