When it rains my 1993 Previa shakes and spurts. It seems like it’s electrical but I’m not sure. How can rain get on the spark plugs etc. to make this happen?
It seems that everything stays dry under the hood but it does not. That part of the car is not sealed. Due to the heat of the engine, water will evaporate rapidly on most of what you see. The problem with you car may be more than just wet wires.
This sounds like spark plug wires that are old, cracked and high humidity and/or rain can make them fail. After the engine runs for a while the heat dries them out and they work OK, but the next time its rainy they do the same thing. Consider replacing the wires. You can buy a set online for about $50.00. Replace them one at a time so you don’t get them mixed up.
Yea, it is a typical example of what wet wires (sometimes cap) do. I recommend OEM or basic name brand wires. You don’t need to get them from the dealer (although the dealer wires are usually good) but your local auto parts store will usually have them for less. Avoid the expensive fancy wires. They are not better for your use!
Rain doesn’t have to get on the plug wires directly. As wentwest already pointed out, high humidity is enough. As you may know it would be difficult for liquid water to get onto the wires because of their position under the passenger seat and nearly impossible for water to get to the spark plugs unless you drove through some deep water because they are sealed. In fact you have to remove the passenger seat, support brackets, possibly the floor seat belt bolt, door sill, carpet and access panel to get to the spark plugs and wires. Access to the distributor cap is much easier with the panel removed though not absolutely necessary. It’s been about a year since I did this last so I’m foggy on the exact details.
Because of the hassle of getting to them, I recommend replacing spark plugs, plug wires, distributor cap and rotor at the same time. I’ll bet it hasn’t been done for awhile.