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?