I'm planning to do a "deep clean" on my engine and part of that is to clean carbon / gunk from the piston faces and cylinder. I was planning to use WD40 and a slightly abrasive cloth like a green pan scourer, as well as some shop wipes. I was also planning to inspect and clean the coolant / oil channels and re-tap the bolt holes.

I will not be removing the cylinder block from the engine however, so this will be done whilst it is in the car, with the pistons still connected to the crankshaft.

My concern with this is mostly around WD40, coolant, metal particulates etc. getting into places they shouldn't. To mitigate this I will be taking the sump off and, once I'm finished up top with the pistons, inspecting the underside of the cylinder block and trying to blow any hidden contaminants out with compressed air.

Is this a good plan? What other precautions should I take?

  • Welcome to Motor Vehicle Maintenance & Repair! Jul 14, 2018 at 13:07
  • What engine is it???
    – Moab
    Jul 14, 2018 at 14:44
  • @Moab It is a 1.2L inline four.
    – user38986
    Jul 15, 2018 at 8:32
  • Make and year??
    – Moab
    Jul 15, 2018 at 16:33

2 Answers 2


No. Abrasive cloth = sand paper ( or emery or silicon carbide, etc) more or less. Abrasive particles will come off the cloth and get places you can not remove it. Then when you reassemble ,you will be circulating abrasives with the motor oil. That will cause wear. For example , when polishing metal samples for the microscope, various grades of these same abrasives are used. Between each grade these small ( one inch diameter) samples are washed in flowing soap and water. Then in the final stages they are submerged in an ultrasonic cleaner. These steps are require to clean off the abrasives. Take the engine out and clean it if you think it is necessary.


The WD40 will not work well. Use Marvel Mystery Oil instead. On a warm engine, allow it to soak for ten minutes. You can just pour an ounce or two through the spark plug holes, and spin the engine with the plugs removed, so you don't even have to disassemble anything. Then, spray carburetor cleaner onto the piston crowns. Crank the engine with the spark plugs removed and you will see the carbon just fly out! Keep adding the Marvel Mystery Oil so that you always maintain roughly a 50-50 mixture of MMO and carb. cleaner. You may have to repeat the procedure a couple of times on a very dirty engine.

Use MMO in the crankcase, substituting it for one quart of oil. Change the oil and filter after 130 miles, with another quart of MMO, and change the oil again after 300 miles. Repeat after 500, 1000, and finally two thousand miles. Your rings, valve train, and sump will be clean as a whistle! Any minor tapping noises, vibration, lose of power will disappear. If you happen to develop an oil leak, keep using it and it will also disappear after 1k miles.

Use MMO in the gas tank as well. I use it initially at a treat rate of 1.6 ounces per gallon of gas and it has worked magic for injectors, intake valves, compression chambers, EGR system, and the face of catalytic converters on multiple different cars, including expensive Mercedes-Benz. Run the engine hard and maintain 3,200 RPM for at least 20 minutes.

  • "Crank the engine with the spark plugs removed" - How much crud will this put into the catalytic converter? Not everything is going to come out of the spark plug holes. Even putting unburned fuel into the cat, apart from any other gunk, isn't a good idea - but I suppose you could disable the fuel injectors as well.
    – alephzero
    Jul 15, 2018 at 16:55
  • Next to nothing will get in the cat., since practically all of the liquid comes out the spark plug. The Marvel Mystery Oil is harmless to cats. Only a minute amount of fuel will come out of the injectors. I've read, however, that carbon particles in the cat can cause damage, especially when they are about the size of a spark plug gap. If that were to happen, continued use of MMO in the gas will eventually dissolve them.
    – Carguy
    Jul 15, 2018 at 20:12
  • @Carguy You should change your username to MarvelMysteryOilâ„¢. Sep 13, 2018 at 13:38

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