Trying to figure out what is likely the cause of it. We believe it could be a number of things: wet distributor cap, wet spark plugs, possibly the coil pack getting moisture in it, or the air intake.

It doesn't always happen when it is raining, sometimes it can be going through a large puddle, it won't typically doing it if it's just sprinkling outside.

We unplugged the wire that went from the coil pack to the distributor cap, and got the same result as though it had gotten wet, could be a correlation.
It's a '91 740 non turbo. Any advice would help, please.

 

Usually this is cause by moisture interfering with the plug wires and or distributor/cap. You can try coating the wires with hair spray (will actually provide some waterproofing) and see if it helps.

 

It's best to ask a question relating to your model car in it's area as you're more likely to get another 740 owner who might have had the problem.

General chat is just that, it's for Volvo chat but not really specific model problems.

 

first, a 740 doesn't have a 'coil pack', it just has a coil. 'coil packs' are for cars that have a coil for each spark plug, and don't have spark plug wires or a distributor.

disconnecting the HV wire from the coil to the distributor certainly will stop the engine from running, but its hard to draw any conclusions from that vis-a-vis your puddle-induced stalls.

is this 1991 740 Bosch or Regina/Rex ? bosch uses a round 'beer can' style coil, while Regina/Rex have a square coil that looks like a transformer.

How long has it been since this car got a full service, with new plugs, wires, distributor cap + rotor, and so forth?

in addition to ignition related problems, its remotely possible exposed wiring down by the main fuel pump under the car could cause the pump to stop running when its drenched, this will stall the car nearly immediately. If you have Regina, there's no fuel pump under the car (Bosch uses two pumps, one in the tank and one under the car, Regina has only an in-tank pump).

 

I had a problem with my 240 having hard starts when it rained. What I did to diagnose the problem is to take a spray bottle on a nice dry day and wet down different components in the engine compartment to identify which one was being affected by moisture. I was able to narrow it down to my distributer cap. Replaced and everything was fine.

All you need is a fine mist sprayer - don't go overboard and dump water all over the place.

 

The spray bottle is the mechanics best friend for finding bad wires and distributor caps.

Add a drop or two of soap to the water. The soap acts to allow the water to blanket what you're spraying where straight water tends to bead up and roll off.