A clean engine compartment makes a used car look newer and better maintained. Some shops that perform engine cleaning services claim a clean engine even runs a little cooler because the gunk formed by dirt, oil and grease seals in some heat.
Appearances aside, does an engine that sparkles like new run any better than one that's grimy from normal use? Probably not.
Though it is true that oily, greasy dirt that accumulates on an engine can trap some heat, it's doubtful it would be enough to cause an engine to overheat or even run hotter than normal. If an engine overheats, the probable cause is more likely in the cooling system, or because the vehicle is towing or carrying too much weight or some other issue unrelated to the cleanliness of the engine bay.
However, there are benefits to cleaning a dirty engine. First, it can help determine where oil or grease is coming from, such as a leaky valve-cover gasket, so that a small problem can be isolated and repaired before it becomes a big one. In addition, because oil and grease can accelerate the wear of rubber hoses and plastic parts, cleaning off the gunk may save on repairs. And if you're the type who likes to do some minor maintenance, such as checking the oil level or accessory belts, a clean engine means your hands and clothes are less likely to get dirty.
Many mechanics and detail shops recommend using a degreaser and tools such as a vacuum with a small nozzle, compressed air or a toothbrush to get at hard-to-reach spots. Those are preferable to blasting an engine with a high-pressure hose or steam cleaning an engine, because moisture can damage electrical parts.
The cosmetic appeal of a clean engine will probably be the biggest payback. Most used cars are thoroughly detailed before being put up for sale, including many sold by private owners, so buyers have come to expect to see a clean engine when they shop. A dirty engine on a used car will hurt its curb appeal.
