Yeah its best to leave the engine running. I don't cover anything up when I clean engines. But I haven't ran into anything with an open aftermarket air filter yet either.

 

Leaving the motor running is not what you do,all the electronic components charged,pulleys and belts turning very hot manifolds equal a recipe for trouble.Spray a cool motor with a lite cleaner like simple green let sit 10 minutes and power wash keeping the tip about 12 to 16" away,let drip dry 5 minutes start and run 5 mins then go for a ride.I have never heard of a diesel getting wet and not starting(theres no ignition) unless the air intake has been submerged.

 

You still might have achance of getting water in a vital connector or something and the computer won't get the signals it needs to fire. Which is still a pretty simple fix of pulling the connectors apart and spraying them with wd-40 and putting them back together.

 

Most any electrical connection that is going to cause the engine to no-start if water contaminated is going to cause the engine to stall if water contaminated. The OEM electrical connectors are as waterproof as any connector on the market; Framatomes, Deutsches, AMPs, WeatherPacks or MetriPacks depending on the signal and year.

Steam cleaning engine bays is a way of life in the OTR world. I've even washed down my engine compartment a few times...



Everything starts working again after it dries out. If I can get away with it you guys sure can.

 

I have been cleanining the **** out of my engine bay for about 4 weeks now. Just whenever I get 3-4 dollars worth of quarters I go and throw some tire/engine cleaner on it. Let it sit for a bit then wash her all off with high pressure soap. Then I go for the High pressure wash.

My engine was a mess of dirt, mud, and Oil but shes slowly getting shiny!

I've been doing it trying to track down where my oil leaks are coming from.

I have done it both with engine running and engine off. I have an open airfilter in a custom housing and I try to avoid directly hitting it but other than that its worked quite well never had a problem!