Hey everyone, thought I’d give this forum a shot before I lose my mind…

I have an automatic 86 C4, with 133,000km’s (83k miles) that I bought in 2021.

Engine is stock with an air pump & EGR delete.

The car has been a headache since I bought it, with several issues over a short period of time. Due to these issues with intermittent stalling, and a complete no start last year. My original mechanic changed the ECM, hei distributor terminal, new spark plugs, wires, cap, coil, and the dreaded ICM. He also changed the IAC. The intermittent problems went away in 2022, however the car completely died and wouldn’t restart in August 2023. I took it to a reputable and recommended mechanic outside Toronto (Stevensons) and he changed out the ICM, and the problem was solved, and the car fired up.

Fast forward to 2024, and from May -date, the car has randomly stalled on me 3 times. Each time the car was coasting at a slow speed of 30-50KM per hour, with closed throttle. I changed the MAF and both relays myself, as I thought that might solve the issue (from reading forums) , however after about a month of apprehensive and timid driving, the car shut off on me today. Each time it dies, it’s as if the key was simply turned off, however lights and the dash are fine, so it’s not a battery issue. When the car dies I noticed the RPM’s go from 900, to 100 in about a second, and then zero. The car has started right back up each time so far.


I checked the fuel pressure a month ago, and it sits at about 34PSI, which I believe is ok.

At this point I’m looking to sell the car and replace it with a 94-96 C4, or C5…

Any thoughts or suggestions? Could my ECM or ICM have failed in such a short amount of time, or perhaps the wiring harness or connections are the issue, or a clogged fuel filter, or bad injectors?

Note that the car does idles funny when it’s warm, and in open loop. Usually when it gets to 600-700rpm, it seems to dip(or rpm hunt?) to 500-600 and sounds like it wants to die, but it doesn’t.

 

Anyone!!?

BTW- this C4 has an aftermarket security system, that randomly goes off with no one around. Sounds crazy, but could that somehow affect the “power” and cause a stall?

 

That would be the 1st thing I'd eliminate. Espically if its 'tapped' electrically to wires under dash. Could have caused bad and or broken connections.

 

I appreciate your response.

The car is with a local corvette specialist, as he’s trying to troubleshoot the issue. He brought up the alarm as a possibility as it went off twice in his lot today.

would poor “installation” cause a random/intermittent power loss, as if someone just turned the key to the off position?

the car immediately restarts whenever it’s stalled, so whatever it is, it’s likely electrical I would think.

 

Your tachometer indications suggest loss of reference pulses or ignition pulses. (The tach is different than reference pulses, but both come from the ICM which responds to pulses from the pick-up coil. )

Has the pick-up coil in the base of the distributor been replaced? Unlike the main coil that makes sparks, pick-up coils do go bad. I've replaced several over the years that had high resistance readings and were causing stalling over a period of weeks until it failed hard enough that it wouldn't start at all and I could find the problem.

I see lots of ICM replacements on here, but seldom see a pick-up coil replacement at the same time. To replace the pick up coil requires distributor removal and disassembly, its kind of fussy, so probably why it is ignored. The pick up coil can be checked with an ohm meter.

 

I mentioned the pickup coil, as I’ve changed everything else within the distributor except that.

hopefully I get a resolution shortly

 

first thing I'd do is tear that out

 

Yeah I’ve asked my mechanic to remove it. I would be over the moon if this was the cause of the random stalls.

 

Ok the alarm system has been removed. Mechanic tells me that they had the car running for 60 minutes and it didn’t stall/lose power. I’m not entirely convinced that this was the issue, but time will tell once I get the car back.

Has anything like this happened to anyone before? I’m having a hard time finding anything online where similar stall symptoms were caused by an aftermarket alarm…

 

If the guy that removed it was familiar with the car's wiring, and paid attention to what car wires he worked with during the removal, he should know whether the "alarm" was interfaced in such a way as to have been the cause of the stalling or wasn't. I doubt he knew what the car wire functions are, paid any attention to how the alarm was interfaced, and whether it was the cause or not.

I agree with you. It probably was not the "alarm". Stay close to home until you again trust it to stay running.

Aftermarket alarm systems generally are not interfaced into the car wiring in a way that when once running, they can cause a stall by shutting something off. Aftermarket alarm systems can (and many probably do) have an interface that when armed, disables the starter, and they won't crank. While it is possible to interface an aftermarket alarm system into the car wiring in such a way as to prevent running (ie fuel injectors, ECM turn-on, ICM power, tach wire grounded, etc) among professional installers on the12volt.com the practice is discouraged because of safety concerns regarding a malfunction or unintended button-push on the FOB causing a running engine to quit. I've removed a few systems installed by others and found starter-interrupts, but never a "won't run when armed" interface.

Way back in the day (80s-90s), there was a company called Jacobs Electronics. They had all kinds of devices, and marketed them convincingly. One of them was an "Anti car-jack" device that interfaced into the ignition system. When you started the car, you had about 15 seconds to push a hidden button to dis-arm the protection. If you didn't push the button the engine would shut off. If the engine was running and a door was opened (ie a car-jack) the button had to be pushed, or after the same 15 second delay, it quit running. I had such a device on my 93 Blazer interfaced into the ECM turn-on wire. Eventually, I found the button push to be annoying, and left the bypass switch in bypass for years. One day the truck began to stall randomly. It took a few days, but I eventually traced it to the Jacobs that I forgot it had, and removed it.
This device was not an "alarm", nor IIRC ,was it interfaced into the horn. It was very small and only had 5 wires. I doubt your device was the same one, but these were out there when your 86 was new enough to worry about car-jacking.

Cheers

 

I have two suggestions 1 clean the throttle plates and check minimum idle setting l.
Next and I hope it's not the problem, back in some 85/86ish might have made it to 87 years it wasn't uncommon for there to be excessive flashing from molding the transmission case
It was a ridge of Aluminum like a saw blade running up the center of the transmission, and we saw a few cases of the main harness behind Distributor get nicks/cuts causing all sorts of problems, so you might want to take a look.