An oily engine does not just look bad. Under a layer of oil and dust it is hard to spot leaks, overheating or damaged hoses, and the dirt traps heat and can speed up the ageing of rubber and wiring.
Only wash an engine when it really needs it: when the engine bay is heavily oiled, when it is caked with salt and sand from Ukrainian roads after winter, before selling the car, or before a repair so you can see where it is leaking. You can do it yourself if you follow a few simple safety rules.
What you will need
Important: Only wash the engine where it is permitted, for example at a self-service car wash with a suitable bay. The engine should be warm, not hot: cleaners dry quickly on very hot metal and can leave marks. Always wear eye protection.
Step 1
Prepare the engine bay
Remove everything you can lift out by hand from under the bonnet: leaves, twigs, grass, bits of paper. This debris rots, holds moisture and can cause problems over time, such as blocking the drains below the windscreen.
Look over the engine bay and note the dirtiest areas so you can give them more attention later.
Step 2
Protect the electrics
Water and chemicals must not reach electrical components. Disconnect the battery terminals and, if possible, remove the battery and clean it separately.
Cover the alternator, fuse box, engine control unit, sensor connectors, ignition coils and air filter intake tightly with plastic sheeting and tape. If you are unsure about a component, cover it.
Step 3
Warm up the engine
Grease and oil come off more easily when warm. Run the engine for about five minutes, then switch it off and let it cool slightly so the metal is warm but does not burn your hand.
Never start work while the manifolds or turbocharger are hot.
Step 4
Apply the foaming degreaser
Spray WD-40 Specialist Foaming Degreaser evenly onto the dirty engine surfaces, working from the bottom up. The thick foam clings even to vertical surfaces and gradually dissolves oil, baked-on deposits and road grime.
For spot stains, bolted joints and small parts, the trigger spray WD-40 Specialist Degreaser is handy.
Keep the product off the painted bodywork. If a drop lands on a wing or the bonnet, rinse it off with water straight away.
Step 5
Let it work, then scrub
Leave the product for 3-5 minutes. The dirtier the engine, the longer it needs, but do not let the foam dry out.
Then go over the most stubborn areas with a brush or sponge: the valve cover, the sump and the auxiliary mountings.
Step 6
Rinse and dry the engine
Carefully rinse off the dissolved dirt with a gentle stream of water. Do not aim a strong jet at connectors or seals, even if they are covered.
Wipe away remaining moisture with microfibre and blow out hard-to-reach spots with compressed air. Remove the plastic, reconnect the battery and let the engine run for a few minutes so it dries completely.
Tip: Test the product on an inconspicuous area first, especially on plastic covers and older rubber parts. Rubber seals and plastic under the bonnet stay looking cared for longer if you avoid harsh solvents.
What next
A full engine wash once a year is enough, ideally in spring when the winter salt needs to come out of the engine bay. Between washes, local dirt is easy to remove with WD-40 Specialist Degreaser, and a clean engine lets you spot any leak in good time.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
More guides in «How to use».