I changed the spark plugs for the first time on my 2008 Mazda3, ~118k miles. Three of the four plugs had oil on the threads, on the top of the plug (where the wrench sits), and on the electrode. The only clean plug was the one on the far passenger side.

Some research I did indicated that this could be due to bad gaskets, valves, O-rings, pistons - or possibly just a sloppy oil change. I just had the oil changed at a local shop a few weeks prior. How can I (as an amateur DIYer, if that wasn't obvious!) determine what caused oil to get on the spark plugs?

As far as I can tell, there's no blue or oily exhaust, and the engine idles and runs OK.

  • Welcome to Motor Vehicle Maintenance & Repair! ;-) You could also check your coolant -- sometimes oil will also get in there if you have head gasket issues.
    – Cullub
    Commented Aug 21, 2018 at 17:31
  • More than likely the spark plug well seals are leaking on the valve cover. Replace the valve cover gasket and seals.
    – Moab
    Commented Aug 21, 2018 at 22:12
  • @Moab can i determine if the gasket and seals are the problem without replacing them, or is replacing them the check?
    – mmathis
    Commented Aug 23, 2018 at 2:38
  • It is the main cause of your problem, if there is oil in the spark plug wells, the seals are leaking.
    – Moab
    Commented Aug 23, 2018 at 16:42

2 Answers 2


If there was oil in the tubes before you pulled the plugs out, you most likely have leaking tube seals in the valve cover. I don't think you would get that much oil in the tubes with a sloppy oil fill, as the coils tend to seal the tubes when installed.

Depending on how much oil is in the tubes, you would have had oil showing on the spark plug boots as well.

In the future, I would recommend cleaning out the tubes with some solvent and blowing them out with an air gun to prevent oil and dirt from getting into the cylinders when you remove the plugs.

  • 1
    What would be the fix for leaky tube seals? Would you have to pull the engine? Welcome to Motor Vehicle Maintenance & Repair, by the way!
    – Cullub
    Commented Aug 21, 2018 at 17:38
  • 1
    The fix for leaky tube seals is to pull the cam cover off and replace them. Not too big of a job.
    – WeakMech
    Commented Aug 21, 2018 at 21:43

I bet you have bad seals on the plug boot access adapters. inside the valve cover is tubes that are supposed to be sealed from the engine oil that your plug assemblies drop into and I have had to replace those O ring seals many times. When those seals fail oil works its way down and surrounds your spark plug boot and plug mount. If your going to fix just one might as well do all 4 .

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