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Pontiac Montana stumbles in the rain

I thought my 2001 Montana just did not like the rain until I turned the windshield wipers to intermittent and realized the engine stumble was timed with the wipers making a swipe. Every time the wipers move, with or without rain, the engine stutters just as if there is water in the gas. When the wipers stop, the engine runs smoothly. What’s happening?

Are your headlights on as well? Try turning off the lights, but leave the wipers on. Does it still stumble? I’m leaning to a bad alternator unable to keep up with electrical load, and letting the voltage drop when a lot of current is being drawn.

I agree with BustedKnuckles on insufficient electrical power reserve. There is a Technical Service Bulletin from the car makers about this. The alternator cannot keep up with the electrical demand when a lot of electrical stuff is turned on and the engine is at idle. You are advised, when you have a lot of electrical stuff turned on, to raise the engine speed above 1,000 rpm----and keep it there as long as that electrical load is turned on. Turn some stuff off !

Let an automotive parts store LOAD test the battery and alternator (free). If the battery is more than 5 years old, its capacity could be too low. Lowered capacity will make your problem worse. It could be time to change the battery.

DRLs are on in the daytime; headlights come on automatically at night. Engine stumbles in daylight and dark, at idle and at speed. The only variable is whether or not the wipers are moving.

That seems to point away from it being the alternator. Do you have a voltage meter? Get one and measure the battery voltage while the engine is stumbling. That way we can confirm it or move on to the next possible causes.

Here’s the answer to the problem - a plug wire was lying too close to the manifold and gradually getting its insulation burned off. The wiper motor took away enough power to cause that cylinder to misfire. New set of wires, laced up off the manifold, problem solved. Thanks to all who took the time to offer a diagnosis.