Her 1995 4Runner starts, runs, and quits sometimes when the weather is damp even if left in a damp garage, but it won’t quit in pouring rain if she’s been driving for a while.
It’s always on the first start up of the day. Never does it once the engine is warm except for the time she drove through a huge puddle.
It’s not plugs, coil, or plug wires.
After it cuts out like this it won’t start for some time, then after it sits for a while, sometimes as short a time as 15 minutes, whatever it is that gets damp dries out and the truck starts right away and runs great.
I think this is an odd problem because the truck doesn’t run rough or sputter and then die; the engine just shuts down completely. Could it be a sensor of some type?
I have no idea where to look.
Does it have a distributor cap? if so that would be my next step.
Distributor looks good, can’t see any problems.
Thanks.
How old are the distributor cap & rotor? I’d try this if they’re old.
It’s not just how it looks on the outside, there also might be some slight water condensation on the inside, combined with maybe some carbon tracking. If you haven’t replaced the cap, do so.
Do you live in Seattle or Portland ? Moist and humid weather can leave moisture in your distributor cap. RTV high temp silicon around the cap edge works well to seal out the moisture, as well as silicone spray on the wires. But check if you have spark first. Last resort - have someone check the ignitor…Mike
If it won’t start it shouldn’t be that hard to diagnose. You’ll have to establish what it’s missing though. Start carrying a can of starting fluid around with the car. Next time it won’t run spray a LITTLE sparingly into the throttle body and see if the engine will start. If it starts then dies it isn’t getting fuel. If it doesn’t start check to see if it has spark.
Yep. It’s often not possible to see the crack on the distributor cap. I once discovered one when it was slightly wet, and I had someone cranking the engine so I could diagnose spark. Accidentally brushed my hand over the cap and got a heckuva shock off of it.
I do not recommend this diagnostic procedure 
as an item to look at, when the engine quits, have you immediately pulled a plug, and checked for spark while cranking? and smelled for gas in the cylinder? that would be a HUGE indicator of where the problem lies.
are you troubleshooting this yourself? with the intention to repair it yourself? if so, then start with the basics.
moisture related cut outs, are mostly due to a ‘short’ in the ignition system. as described, the rotor and cap are suspect. You mention the plugs, coil and wires. have they been replaced? how have you determined they are OK? If you didn’t replaced the wires, but just anointed them as ok, start there. The cap and rotor are next, followed by the plugs. OOC, when was the air filter last changed? a clogged air filter, can be exacerbated by moisture., OR a loose lid on an air cleaner, will let too much unfiltered moisture in, and could mess up the carb too.
How is the AC system, the heater, the blend door, and all the vents functioning? is all the air coming out of the correct vents, at the time it is supposed to? If not, then there mnay be a vacuum problem, with it sucking water in on a wet day too.
has this been happening since, or closely related to an engine maintenance? or repair in the past?