Volvo 940's engine seemingly randomly stalls while driving

Question

Our Volvo 940 (1998/Classic)'s engine randomly stalls while driving. There is little to no pattern to it; following the first time it happened an entire year passed before it happened again. Sometimes it happens multiple times while driving relatively short distances, others it can drive for hours with no stalls. I've not recognised any pattern in terms of speed or such, it has happened both when driving in 120km/h and sub 30km/h and various speeds in between.
When this happens, it usually comes back on if you turn the engine off for a minute and then back on. Attempting to immediately turn it back on after it stalls generally results in it not starting up. Other notable things are that the dashboard lights light up and that pressing the accelerator has no effect whatsoever. Everything else apart from the engine apparently keeps going (radio keeps playing music, fans stay on, and so on). The car uses a physical key which you turn, and the keyring only has 3 other keys attached to it; it isn't very heavy.
We've tried the following with no success:

Using a different key (as suggested by an acquaintance)
Cleaned battery contacts and ground
Switched fuel filter
Diagnosis via the OBDII/OBD2/OBD connector ­— no failures reported

What could be done to fix this, alternatively further diagnose it?

GdD · Answer

Usually when your engine stalls in a modern car there will be an OBD code giving some indication of the reason, for instance ignition problems, fuel injector issues and the like. If there are no codes then it points to a few possibilities:

Idle speed controller problems: this doesn't sound likely as it wouldn't explain why your car is hard to start afterwards
Vacuum leaks: again doesn't explain not starting up again right away
Fuel pump: you could have an intermittent problem with your fuel pump, which is electrical rather than mechanical. Overheating could be a possible reason for failure, and it may need to cool down before it works again
Fuel pressure regulator: also could be intermittently failing
Vacuum buildup in fuel tank: Your fuel tank has a vent to equalize pressure, if that vent gets plugged vacuum can build up and stop fuel flow. Many cars have a vent built in the gas cap, others will have a separate venting system, I'm not sure about your car. Often the pressure will equalize after a few moments, and then things work normally again until vacuum builds up again, so it could cause intermittent stalls. Try taking your fuel tank cap off right after a stall, if it starts right back up this is likely your problem. You'd probably hear a pop when you open the cap if there's a vacuum in the tank. In fact, I'd inspect the cap and give it a good cleaning to see if that fixes things
Electrical problem: you could have a short or loose connection in your ignition system somewhere, it could be in the ignition barrel housing, in the steering column, or somewhere forward of the firewall in the engine bay, it's hard to say for sure. With the car idling try rattling the key and pushing the barrel up and down, side to side, in and out to see if that causes the engine to cut out. Then I'd tap the steering column firmly with a mallet to see if I can replicate it that way. Get a powerful flashlight and start looking for frayed cables near the steering column and under the hood

Volvo 940's engine seemingly randomly stalls while driving

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