water on engine

So someone at my flight school decided to clean one of the planes and decided to throw some water over the engine to 'clean' it. idk if that is a good idea or not but today another instructor took the plane and during the engine runup, the engine sounded rough. Also one magneto was fine but the other one had the engine running even more rough. How bad of a problem is this? Does this mean that water has messed up one of the magnetos? Or will the problem go with time as the water dries away?

Get an air compressor and a nozzle to spray some of that water outta there. ( Carefully ) In fact, have an A & P do it. There may be some water getting in the fuel system.

What aircraft type is it?

Is it fuel injected or carburetor?

it is fuel injected cessna 172

What about airplanes flying in the rain????

Not a big deal, though typically something like solvent or avgas works better for cleaning an engine because water won't do jack to the oil and grease on it. The rough mag thing is unrelated, if you have a bad mag drop/rough engine on a fuel injected 172 it's probably a fouled plug from taxiing too rich. Those things are known for that, even when properly set up

That's why in the R models that I fly, as soon as you get it started it says on the checklist to lean mixture. I find it really annoying to be third in line and the 2 planes in front of me have been running their engines at high RPMs for the past 10 minutes...

The procedure in the POH says to run it up 1200RPM and lean to max RPM and then keep it leaned out for ground ops. Works pretty well for me.

Something like that generally works pretty well. Cherokee 6 likes to foul too if you run it full rich on the ground all the time.

If the engine was hot, I would think splashing cold water on it would really harm the metal.

Anyway, like someone said above, water won't do much to clean a greasy, oily engine. You really need some fuel or a solvent to scrub it clean.

Exactly. Back when I used to get to wrench on airplanes, our shop had a pneumatic spray gun that ran on compressed air and siphoned mineral spirits out of a drum on the floor and we'd hose down the engine with it. Works great, but leaves a giant puddle of dirty solvent behind. Definitely something you have to do on a flat shop floor and not on a slope or over a storm drain.

As for throwing cold water on a hot engine, I would only worry about the possibility of thermal shock. We always hear about "shock cooling" when it comes to making quick power adjustments, but I would be way more worried about pouring cold water on the block! I wouldn't be too worried about water getting into the fuel system, because unless all the fuel lines and injectors are water-tight, you'd get fuel leaks anyway! Fuel + hot engine = Fire. Fire bad...

Spark plug fouling, especially on the bottom plugs, is the culprit. If it's only a little bit fouled you can taxi out to the runup area, throttle all the way up to 2000 RPM (or more), and gradually lean the mixture WAY out. Run it for a minute or so and watch for your peak RPM to increase and listen/feel for the engine to smooth out. The point is to lean the mixture enough so that you're burning all the unburnt fuel out of the inside of the plugs (perhaps softening and taking some of the lead fouling with it) rather than adding it (which you do when running the engine full rich while taxiing). Throttle back to 1000 RPM, mixture Rich, throttle 1800 RPM and re-check the mags. This procedure works great for small amounts of fouling that are "burn off-able," but if enough people have had the airplane before you and sufficiently fouled the plugs, sometimes the only way to get it all out (especially the lead fouling) is to have an A&P remove and sandblast the plugs.

I would be weary of that procedure on larger high performance engines, which require specific ROP/LOP temperatures, but the small block non-high performance Lycomings found on C152/C172s are pretty bulletproof. Please feel free to correct the above since it's been a while since I've had to do it.

It got wet? Only one solution, firewall forward overhaul.

Informative post, inigo!

I would definitely be most concerning about cracking the block. Hot engines do NOT like cold water being thrown on them. They crack. And they may not even crack in an obvious way... the thermal shock might just cause a stress fracture that will eventually fail at the worst possible time, like when the engine is REALLY working for a living (think max RPM, a few hundred feet off the runway).

This applies for any hot engine... car, plane, whatever. Don't toss water on a hot engine. You will almost certainly abbreviate that block's lifespan.

That's why I don't like going through the car wash where they wash the underside of your car. At least there's some protective plating on the bottom of my car. Even still...

There is no way you will crack a case by throwing water on it. If that were the case, engines would fail every time you flew through a rainstorm at climb power. Case cracks happen from fatigue, from re-using weld-repaired cases, and from improper fastener torque.

Gotta disagree with you on this one. If you have a big enough temperature delta over a short period of time, you will crack the block. It's absolutely no different than the warnings we have against shock cooling in a descent. Cool an engine block down too quickly (whether by air or water), and there will be significant thermal stresses that could affect the structural integrity of the engine.

You are right... the chances you'll crack the block in climb during a rain storm are highly unlikely, but that is only because you've already been throwing water on the block in the first place (therefore, not as big of a temperature delta), and the amount of water is relatively limited in the form of spray - the cooling takes place over a longer period of time, allowing the block to adjust gradually. This compared to opening the cowling on a hot summer day after getting the engine good and toasty, then tossing a garden hose of gushing cold water all over the entire block.

Guess it depends on how much the engine cooled during descent and how much it heated back up during taxi. For instance, the 337s are known to overheat the rear engine on longer taxis during hot days. Toss a bucket of cold water on that rear engine after shutdown, and you are looking for a lot of expensive trouble.

Now you've done it...

I agree with Roger on this one. Flying through some thick cumulous formations in the summer time, even if no echo is showing up on radar, can be a bit like flying through a power washer at full blast.

I'm a straight up troublemaker.

Crankcase really doesn't get all that hot though. Cylinder heads...eh, maybe I could buy shock cooling there...though I think overheating them has more to do with cracking than the rapid cooling, something about the aluminum alloys used losing x% of their strength above 400*F (x being a fairly significant number).