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A volvo S70 mechanic stumper

our 1998 volvo S70 has been stumping us and our mechanics for nearly a year. when it is rainy, there is a 70%+ chance that it will stall, perhaps repeatedly, and there is no guarantee of it restarting within a 1/2 hour, though it USUALLY does restart. it’s been towed at least 3 times in the last 6 months due to this. otherwise it runs fine at its fine vintage of 188,000+ miles. i’ve brought it to the garage minutes after it stalled, and still they weren’t able to recreate the behavior. they’ve wet down the distributor and it was fine. we have great mechanics, and they are incredibly frustrated to not have solved this thus far. it never happens when it’s dry out. today it rained and was fine, but we’re looking at buying a new car because my wife has been stranded in the rain by this car too many times. is this a common problem? any thoughts? we’re stumped.

This generally points to a problem with the secondary ignition. (dist., coil, plugs, etc.)
First off, make sure the plugs are new or lightly used. (Misfiring plugs carry over into the other areas)

Inspect the coil wire where it plugs into the coil and the dist. cap. The terminals should all be clean and free of corrosion.
If everything is ok in these areas try spraying down the coil, dist. cap, and plug wires with WD-40 to see if the problem goes away.
Heat attracts moisture and WD-40 will repel moisture so hopefully this will solve the problem.

“any thoughts?”

Wait for a sunny day and try to sell it.

thanks. i’m fairly certain we’ve taken those steps (though the wd-40 is a different approach). the more i look around, i’m seeing some others like it. this sounds close to ours, though much worse:

The wet conditions lead you to ignition issues. Volvo recommends changing plugs at 30K miles on this motor (5 cyl turbo) and they also recommend new plug wires and cap, rotor, every 30K miles. Therefore 1st item is when did you last get all new plugs, new wires, etc.?

If they are new and you still have a problem, I had similar issues with a '98 V70XC (same motor) and it was the Mass Airflow Sensor. It can get dirty and moisture collects more readily on dirty parts. That’s what I’d go to next on your car.

I had an RX7 that wouldnt start when it rained or was damp, and it left me stranded many times too. The issue with this car was the plugs and distributor cap. Moisture in and around this location was causing the problem. I eventually sold the car, I chalked it up to bad design, and poor engineering. I wish I had known about the WD40 trick back then!

It would be good to know if the ignition system is working when the trouble happens. I would try spraying some starter fluid into the intake and see if the engine tries to fire then. If not then the ignition system most likely is at fault. If it does fire then a fuel delivery problem is the cause. I would check out the relays as a possible cause of this trouble. One that controls the power to the ECU or fuel system.

I had a similar problem with my '99 S70 and it got worse over time.

It turned out to be a dirty throttle body. Humidity turned the dirt into a pasty sludge.