Hot, hot day, engine quit in bumper to bumper traffic. '81 AT

[iflights]

As suggested in the post title, it was a scorching hot 90 degree day, full sun, AC on max, 45 minutes into a drive home with varying highway and local street traffic.

We hit a patch of highway bumper to bumper traffic and the engine suddenly quit. While still rolling, I restarted, engine fired back up with no hesitation or extra cranking, and we got back up to speed. Hit the next traffic slowdown, and it happened again.

Got off the highway onto side streets, and at every traffic signal, while idling, the engine wanted to die. Kept the RPMs higher than idle, but not much more, by shifting into neutral and keeping the accelerator lightly pressed as we hit traffic lights after that.

Took it out the next day for a drive on an equally hot day for 30-40 minutes, but could not re-create the problem.

Was thinking about possible causes, but for these symptoms, it could be a number of things. Up to this point, the car has run perfectly for 5 years. The only thing touched recently was a battery replacement.

That's where my mind went...did I have a situation where during idle, the voltage dropped low enough for an ECU to cut out? I had also installed a voltage data logger across my battery terminals during my battery replacement. The notated interval drop on the graph shows the point where we first encountered the engine issue. Is this this a low enough voltage to cause an ECU to quit?

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[Helirich]

Not low enough on the graph, but maybe your losing more in the wiring. I don't know about the Delorean in perticular, but most ECU's won't quit untill about 9 volts. I had issues with my alternator and the ran fine at 11 volts. But again, wiring can knock it down. If you can keep it running by revving, it sounds like power, but could be clogged injectors I guess.

[Michael]

Voltage never got low enough to cause issues. If you were to start that engine and just let it idle with the ac on, I suspect it will get just as warm as if you were driving in traffic. Try to replicate the issue and just keep the car in your neighborhood driving. If it does happen again, turn off the AC completely and see if that helps(which would zero you in on voltage issues). Injectors do not all clog at once, and even if you do get one dirty, it will not kill the engine.

On a very hot day, the OE fuel pumps are a disaster but they usually give a lot of warning in the form of a noise or cut out at higher flow demands. Possibly a loose or corroded terminal/connection on the ignition side but I think that would show up at all times and not allow for a restart. Maybe visually inspect for a vacuum hose pulled loose especially around the WUR area and check the voltage at the fuel pump.

If by any chance you recently added fuel, you could run this tank very low before refueling. Possibly some water or contamination in the gas.

[DaraSue]

I actually did have intermittent stalling issues due to a corroded negative battery cable once, so corrosion somewhere in the system is a possibility. (Mine stalled during this idle for a smog check, ran fine for 5-10 miles, died and wouldn't start in a parking lot, started with starting fluid, then ran another 5 miles on the freeway but died at the next 2 stoplights at which point I gave up and called a tow truck.) I assume you checked the battery cables when you replaced it but there could be an issue somewhere else in the ignition system as Michael suggested.

[Ron]

I'm thinking a bad connection somewhere too -- Since the battery re-cranked the engine, there was way more power available than the ignition needs...it just isn't getting there.

Could be the fuel filter stops up, then frees when it dies. But that would more dependent on time than temp...unlike ignition components, like the pick up coil, etc.

Trying to replicate and gather more clues seems to be the best option/suggestion at this point.

[iflights]

Voltage never got low enough to cause issues. If you were to start that engine and just let it idle with the ac on, I suspect it will get just as warm as if you were driving in traffic. Try to replicate the issue and just keep the car in your neighborhood driving. If it does happen again, turn off the AC completely and see if that helps(which would zero you in on voltage issues). Injectors do not all clog at once, and even if you do get one dirty, it will not kill the engine.

On a very hot day, the OE fuel pumps are a disaster but they usually give a lot of warning in the form of a noise or cut out at higher flow demands. Possibly a loose or corroded terminal/connection on the ignition side but I think that would show up at all times and not allow for a restart. Maybe visually inspect for a vacuum hose pulled loose especially around the WUR area and check the voltage at the fuel pump.

If by any chance you recently added fuel, you could run this tank very low before refueling. Possibly some water or contamination in the gas.

Thanks. Good actionable ideas.

Ran the engine on idle (it idles just under ~800) with AC on for 40 minutes yesterday and engine experienced no hesitation.

Could not see any hoses out of alignment.

Pushed on ballast wires while running hoping to find a loose connection, no fault found there.

No abnormal noises with fuel pump. Going to check voltage at pump next.

[iflights]

I'm thinking a bad connection somewhere too -- Since the battery re-cranked the engine, there was way more power available than the ignition needs...it just isn't getting there.

Could be the fuel filter stops up, then frees when it dies. But that would more dependent on time than temp...unlike ignition components, like the pick up coil, etc.

Trying to replicate and gather more clues seems to be the best option/suggestion at this point.

Just ordered some sealing washers and fuel filter. It has been a few years since that was changed.