I stopped at the car wash today with the expectation of cleaning my engine bay. I bought some WD40 degreaser that was recommended to me at my local parts store.

I put a bag over the battery and fuse box, then went to put one on the alternator. It was warm/hot to the touch though, which made me think a plastic grocery bag would melt. Is it safe to put a plastic bag over the warm alternator? Obviously I’d use common sense and not spray water directly on it, but I wouldn’t want to create any issue should water be splashed on it.

 

As long as you are not blasting water into the alternator, you should be fine without a bag. I personally have never had any issues.

A plastic grocery bag is pretty thin plastic. You might think about a ziploc bag instead if you are concerned. They are thicker and more temperature resistant (designed for filling with leftovers that may be hot/warm).
Nice thing about them is that if you're really concerned, you could partially zip up the bag to reduce the exposed opening.

 

Ok, maybe I'll just try to avoid it. Thanks

 

Use aluminum foil, it will deflect the bulk of the moisture away from the alternator.

 

Great idea, I like it. Thank you sir!

 

In all honesty I've never had any problems spraying the alternator or battery with pressure washer and that's with all my cars. The way most electrical systems are set up on modern cars for water to not affecting it as long as there are no loose wires, holes in the wiring, ect.

 

I don't cover anything when I wash the engine bay. I don't use a pressure washer though. I use low pressure from a garden hose and I am careful not to directly spray water on the alternator or air box. Have never had a problem in 20 years of washing engine bays this way.

 

I wouldn't want to wash a hot or warm engine anyways, I only do it after the car has sat overnight.