I use avgas or petrol on a soft cloth offcut to clear the oil / exhaust residue on the belly every so often.
With regards to the engine, I’m reasonably convinced that wash downs with compressed air and avgas lead to a marginal spade connection contact giving me erratic tach readings which took some time to track down.
no never wash
i just spray some aviation degreaser like extreme simple green or whatever on a cloth and just wipe it.
This stuff is great for the plane belly:
https://www.frasersaerospace.com/product/wash-wax-all-degreaser/
…and the blue stuff is great for bugs on leading edge etc and general cleaning.
I would wash an engine with avgas and a brush, but never by spraying water over it. It won’t do the ignition any good, for a start.
Re the belly of the plane, avgas is quite popular too 
But there are “less hazardous” solvents.
DeeCee57 wrote:
why do you wanna wash your engine, genuine question?
Because my A&P asked me to do so. He said that on a washed engine you can trace any new oil/dirt better to its origin. But I insisted that I do not think that it’s a good idea, that there’s more to destroy than to learn from that. This is where I was looking for some backing up, to “test” if I’m right or not.
But of course there are things that get dirty in an engine. There’s mud from the tire, with a wide open wheel well (retractable); There are lots of things to be greased right around the engine, and the grease distributes to quite about everywhere; There are lots of mechanical connections to be oiled, according to the manual. And then there is removal of the oil filter, where at least some drops inevitably find their way along some tubes, if all goes well.
So the answer is: “coz it’s dirty” 
Udo, to come back to your original question… why do you wanna wash your engine, genuine question? Your answer might be “coz it’s dirty”
Should an engine get dirty, determine the reason, and clean as, and if necessary. Non exhaustive list of causes:
Generally speaking, an aircraft engine should not accumulate any crud, if properly maintained, and flown regularly (and more than once a year). Engines normally only get fully washed at overhaul
There are several different products available for cleaning the underside of aircraft. I’ve never used any of them, so can’t comment on them very usefully. My plane with 50 years since the engine was assembled does not blow anything onto the belly except lead oxides from the fuel – which I clean occasionally with solvent, mineral spirits or whatever comes to hand.
A related question: how do you clean the bottom of the aircraft behind the engine and exhaust? What I did:
This is the first time I did it since owning the aeroplane, and it was a very messy job. Is there an easier and cleaner way to do it? I don’t want to use a pressure washer.
O.K. so I’m not the only one who is a bit reserved regarding engine washing.
Downside is, that it’s quite some work involved cleaning the engine manually. But it’s another opportunity to really see a lot, to check all hoses for leaks and smallest cracks and so on.
One interesting and harmless way to clean easily accessible surfaces of the engine, cowls, landing gear etc. is to spend some time flying in a dense cloud. As counterintuitive as it is, a stream of fog can wash off the oil even without detergents.
