Random Loss of Spark - Please Help

[Alexf2002]

Long time follower of the forum... first time posting. Very glad this forum exists. I’ve gotten a lot of info from here but I’m at a loss of ideas for what’s wrong.

Truck started having an issue with randomly shutting down while at idle or while driving. When it first happened it shut off and never started again until I replaced the ICM and ignition coil. Once those were replaced it started and ran fine for a few weeks. Truck started having the same shut off then no start problem so I took out the new coil and ICM and got them replaced under warranty. Truck started and ran fine again for a few weeks. The random shut off issue started up again and I decided to do a basic ignition tune up and replace a few sensors that I had been told could cause shut off issues. After installing all new plugs, wires, cap, rotor, fuel filter, crank sensor, and camshaft sensor the truck ran better than ever for a few days, but eventually the truck shut off while driving again and now here we are....


I cannot consistently get the truck to replicate the issue. Every so often it will have a bad day where it shuts off while driving and takes a few minutes before it’ll start again, but it could be minutes, hours, days, or weeks before the issue will ever happen again. It can shut off at both idle and while driving.

I bought a fuel pressure gauge and inline spark checker when the issue started happening. Fuel pressure is good, but during the period where the engine shuts off and refuses to start for a few minutes there is no spark at the cylinder.

I’m at a complete loss for what could be wrong. If anyone has had an issue like this before or has suggestions as to what could be causing my random, inconsistent loss of spark please leave me some suggestions. Thank you guys for all your help in my build.

 

[stutaeng]

Ok, first thing's first. Is your truck a TBI or Vortec? Do you have any codes? If you don't have spark on the cylinder and fuel pressure is 55 psi (for Vortec,) then it is seemingly electrical. Is there no spark on any cylinder? Do you have spark at the ignition coil? I believe the camshaft position sensor is not required for the engine to start and run, just runs rough if it not working. Also, does the fuel pressure hold steady for about 15 minutes?

Oh, don't forget to test by spraying the throttle body with carb cleaner to see if it starts momentarily.

If you don't have signal at the ICM, then check for signal at Crankshaft Position Sensor (CPS) near the crank pulley.

I always find these articles handy on these engines: https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-troubleshoot-a-no-start-1
A lot of diagnostics tests can be done with a digital voltmeter and test light.

 

[Pinger]

Don't mean to be argumentitive with someone who knows more about these engines than I do but....

Ok, first thing's first. Is your truck a TBI or Vortec? Do you have any codes? If you don't have spark on the cylinder and fuel pressure is 55 psi (for Vortec,) then it is seemingly electrical. Is there no spark on any cylinder? Do you have spark at the ignition coil? I believe the camshaft position sensor is not required for the engine to start and run, just runs rough if it not working. Also, does the fuel pressure hold steady for about 15 minutes?

.....but, it's my understanding that the cam sensor is required for starting - its only purpose as otherwise from the CPS, the ECU can't tell TDC firing from TDC gas exchange.
The only exception I've encountered is my smart which can start from the CPS alone. I'll leave you to guess how that's done!

If you don't have signal at the ICM, then check for signal at Crankshaft Position Sensor (CPS) near the crank pulley.

CPS symptoms when they go bad are usually stalling when coming to a halt with a hot engine (and hot CPS). On cooling they will normally allow restart - until they get hot again and the process repeats. They don't usually stop a driving engine - usually an idling engine.

 

[Alexf2002]

Ok, first thing's first. Is your truck a TBI or Vortec? Do you have any codes? If you don't have spark on the cylinder and fuel pressure is 55 psi (for Vortec,) then it is seemingly electrical. Is there no spark on any cylinder? Do you have spark at the ignition coil? I believe the camshaft position sensor is not required for the engine to start and run, just runs rough if it not working. Also, does the fuel pressure hold steady for about 15 minutes?

Oh, don't forget to test by spraying the throttle body with carb cleaner to see if it starts momentarily.

If you don't have signal at the ICM, then check for signal at Crankshaft Position Sensor (CPS) near the crank pulley.

I always find these articles handy on these engines: https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-troubleshoot-a-no-start-1
A lot of diagnostics tests can be done with a digital voltmeter and test light.

Engine is the standard vortec 350. From the testing I've done, when it shuts off there is no spark at all for any cylinder. From this I would assume that there is no spark at the coil either. Would I just take a test light to the output of the coil and see if it has power on crank to test this? When I messed with the truck a bit today it started and ran fine. I inspected the harness as best I could to look for any bad wiring going to the coil and ICM. Didn't see any issues with the harness. The fuel pressure gauge I had is a bit messed up and leaks fuel but from the testing I did with it the pressure does not seem to hold. I know this is a symptom of bad fuel pressure regulator but I wouldn't think that is causing the intermittent shut down as well as spark loss that I'm having.

 

[stutaeng]

Don't mean to be argumentitive with someone who knows more about these engines than I do but....



.....but, it's my understanding that the cam sensor is required for starting - its only purpose as otherwise from the CPS, the ECU can't tell TDC firing from TDC gas exchange.
The only exception I've encountered is my smart which can start from the CPS alone. I'll leave you to guess how that's done!



CPS symptoms when they go bad are usually stalling when coming to a halt with a hot engine (and hot CPS). On cooling they will normally allow restart - until they get hot again and the process repeats. They don't usually stop a driving engine - usually an idling engine.

Not an expert myself, LOL. I had an issue with my camshaft sensor on my 4.3 Vortec a few months ago. The sensor would act up and truck acted like it was about to die, then kept running. At the same time my tachometer would drop to zero. Would happen idling or driving. I had a camshaft code also. I put a new one in and issue was solved. I read somewhere it was not required to start, but may be wrong...

Engine is the standard vortec 350. From the testing I've done, when it shuts off there is no spark at all for any cylinder. From this I would assume that there is no spark at the coil either. Would I just take a test light to the output of the coil and see if it has power on crank to test this? When I messed with the truck a bit today it started and ran fine. I inspected the harness as best I could to look for any bad wiring going to the coil and ICM. Didn't see any issues with the harness. The fuel pressure gauge I had is a bit messed up and leaks fuel but from the testing I did with it the pressure does not seem to hold. I know this is a symptom of bad fuel pressure regulator but I wouldn't think that is causing the intermittent shut down as well as spark loss that I'm having.

Yeah, can't assume things like coil pack spark. Check coil with in-line spark tester. The coil may be producing spark, but a faulty distributor could be the problem. Some members here have stated the distributors have some plastic parts that are prone to breaking. If coil is not producing spark, check for signal wire. Those wire-piercing probes come in really handy for this. Find the signal wire and while cranking, your test light will flicker. Check for constant voltage and ground on all senors you check also.

 

[Alexf2002]

Yeah, can't assume things like coil pack spark. Check coil with in-line spark tester. The coil may be producing spark, but a faulty distributor could be the problem. Some members here have stated the distributors have some plastic parts that are prone to breaking. If coil is not producing spark, check for signal wire. Those wire-piercing probes come in really handy for this. Find the signal wire and while cranking, your test light will flicker. Check for constant voltage and ground on all senors you check also.

A few questions from this: I have replaced the cap and rotor on the distributor, but could the plastic bits you're referencing on the distributor cause the shut off no start issue to be intermittent like it is in my case? I figured if this was the case the problem would be continually replicated. The truck still runs and starts a majority of the time so the coil and ICM are getting switching signal from the PCM at least a majority of the time. Like I mentioned earlier I inspected the harness wires going to the ignition system and couldn't find any shorts or breaks in the wire that would cause it to randomly lose signal. Could the intermittent shut offs and loss of spark be a symptom of an ECM or PCM that is slowly failing?

 

[Pinger]

Generally, failures in the HT side of the ignition system manifest themselves as mis-fires under load (when it's harder to introduce the spark into the cylinder due to the higher pressure) and invariably insulation breakdown is the cause. For an actual break (ie discontinuity of electrical flow) as suggested, it would be very random as to how it appeared. When such a failure occurs, often it's the king lead (from coil to dizzy) as it is the hardest worked lead (x 8 for a V8) that is at fault.

 

[Alexf2002]

Generally, failures in the HT side of the ignition system manifest themselves as mis-fires under load (when it's harder to introduce the spark into the cylinder due to the higher pressure) and invariably insulation breakdown is the cause. For an actual break (ie discontinuity of electrical flow) as suggested, it would be very random as to how it appeared. When such a failure occurs, often it's the king lead (from coil to dizzy) as it is the hardest worked lead (x 8 for a V8) that is at fault.

I’ve just recently replaced all HT leads as well as the plugs, cap, and rotor and it still didn’t solve the issue. All parts were Delco so I like to think I wasn’t sold a faulty part or anything.

 

[98 Nitro]

Alot of Delco parts these days are made in China so do not count out the cap is already toast. Also you have to be careful replacing the cap the dizzys housing is plastic I think Dorman makes a part to fix it, or try a zip tie.

 

[Pinger]

I’ve just recently replaced all HT leads as well as the plugs, cap, and rotor and it still didn’t solve the issue. All parts were Delco so I like to think I wasn’t sold a faulty part or anything.

Given the comments made about possible distributor fault, I'd have a look inside it for any signs of tracking. As best I know, The HT comes in from the coil then to the appropriate cylinder. All of this is confined to the upper part of the distributor and if there's been any tracking, there may be tell-tale marks. I'd also try and check that the HT is getting to the dizzy (via the king lead). All of this merely to eliminate the dizzy as culprit.

Re-reading your first post - every time you've worked on it, it got better for a while then deteriorated again. Is there a bad connection somewhere that when you work on the truck you somehow manage to make more complete and thereafter it loses its ability to conduct until messed with again? Or a heat path?