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Can Rain cause a car to run differently?

I have a 2002 Kia Sedona Mini van and it runs real well except for when it rains. i will get the dreaded squeeky belt from time to time while driving in the rain and the van just doesn’t seem to run as smoothly…is that possible?

Very. The rain makes the belt and pulley wet, which decreases the friction between the pulley and the belt. If the belt is already worn, or not properly tightened, that can tip the balance between squealing and not squealing. Depending on how long this has been happening, the belt might now be glazed and require replacement, but it certainly requires at minimum a tightening.

In addition, those 9 year old spark plug wires probably have cracks in the insulation that admit moisture when it is rainy and humid. That could certainly account for a rough-running engine.

If the belt is 9+ years old it’s time for a change.

Your spark plug wires are likely original and due for replacement.

The belt however won’t effect the running of vehicle.

Cars that don’t run well in very humid, moist, and/or heavy rain are mostly due to worn ignition parts. Dirt and dust on plug wires hold moisture so sometimes cleaning off the plugs wires can help. If the car has a distributor cap, rotor, and plug wires then replace them with new ones. Old spark plugs are next on the list. Old plugs can get rusty at the base and collect moisture and short out in humid conditions.

New plugs, wires, rotor, and cap should have the car running the same in wet or dry weather.