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I have oil leaking from a spark plug socket on my 1981 Volkswagen Rabbit pickup truck. Before you ask, it's swapped with the 1.6 gas motor instead of the 1.6 diesel.

The oil leak appears to only be coming from cylinder 1.

Here's a picture if that helps. Note that the engine is pretty filthy because it used to have a notorious valve cover gasket leak but that has been repaired. I just can't be bothered to clean it as I've tried a power washer and the stuff just wouldn't come off. Oil leak

I do not know the mileage on the engine nor how many miles have been put on it. The odometer is broken and the previous owner said he does not know how many miles were on the engine when it was put in.

It did idle pretty rough but I ran some seafoam through and that fixed it. It also has brand new E3 spark plugs.

If you're wondering 'where's the question' my question is what is likely causing this issue?

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  • Are you sure this is oil and not some other fluid (gas, coolant)? How is it you know for sure it's oil? Also, if you take some carb cleaner or engine gunk remover to it, I bet it would all come right off.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    May 30, 2014 at 14:18
  • I just did a oil change so the color of the oil is similar to that of gas. It didn't have a gasoline type smell to it.
    – Alec
    May 30, 2014 at 14:51
  • I don't quite understand why there would be coolant coming out but I'm definitely not a well seasoned mechanic. I should say I haven't noticed a coolant loss but I have had issues with my oil levels going down and sometimes when it's running I can faintly smell oil.
    – Alec
    May 30, 2014 at 14:59
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    It probably is oil, but I would highly doubt it is leaking from the sparkplug hole (or area), but rather probably from somewhere up above it, like the valve cover. Oil can be deceptive, especially new oil as to where it is coming from. Best I can tell you is clean it as best as you can, then look for the oil sheen. As far as cleaning it, carb cleaner can do wonders, as well as engine gunk cleaner.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    May 30, 2014 at 15:13
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    Sorry, I edited the above comment. Carb cleaner would work good. If that is not available (or procurable), you might try Dawn dish-washing liquid. It is very good at cleaning oil. It looks as though there is a lot of carbon buildup as well, which is way harder to clean. Carb or brake cleaner is by far better than anything I know at getting the carbon off, other than a stiff wire brush and some elbow grease.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    May 30, 2014 at 15:20

2 Answers 2

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Have you done a compression and leakdown test on that cylinder? If it is oil, and it is coming through the spark plug socket, you've got a couple of problems.

1) Oil is probably getting into the combustion chamber.

Remove the plug, check if it has oil on the electrode, or if it's clean. Check the hole for oil on the threads. Perform a compression and leakdown test. Check the plugs in the other cylinders as well. The rings or valve seals could be worn out.

2) The spark plug socket is not fully sealed.

Attach a leakdown/commpression test hose and fill the cylinder with air pressure. Place a little soap/water mix around the base of the test hose. Look for air bubbles coming out at the top of the socket. Perform the compression and leakdown tests as well. If the hole has become out of round enough to leak, you'll probably need to replace the head.

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  • If it were to have an out of round spark plug hole/threads, you can easily tap and heli-coil it to fix it. No need to replace the head. You can even do it with the head on the engine.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 5, 2014 at 2:01
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I can't believe that you're leaking fluid from a spark plug hole. Like Paulster2 above, I'd like to see what happens when that area of the motor is cleaned thoroughly so we can start looking at this thing fresh.

On the opposite end of the spark plug is a cylinder full of fire and very high pressure. If anything leaks into that cylinder (say during idle time or sporadically during intake strokes), it'll be introduced to that fire shortly. If it's flammable, it'll burn. Oil's flammable, which is why the upper end of the combustion chamber is so hard to keep lubricated - the stunningly thin layer of oil that gets deposited there on every piston upstroke gets burned away during the power stroke. LARGE amounts of oil - too much to burn - wind up very quickly fouling the spark plug, resulting in a misfire.

If liquid leaking into the combustion chamber is NOT flammable, it gets vaporized by the heat and pressure of combusion on every power stroke. The only qualifying liquid would be water-coolant mixture, and we know what leakage of water-coolant into the combustion chamber looks like - clouds of dense white smoke.

If ENOUGH nonflammable liquid reaches the combustion chamber to not be fully vaporized, again we wind up with a misfire, and we should expect that leakage to continue after the motor is shut off because the engine's still hot. Such a continued leakage into an idle cylinder would likely build up quite a charge of it in there, too much to compress on the next compression stroke. Result: hydrostatic lock, itself definitely leading to a no-start condition and possibly leading to catastrophic damage to the piston.

The seal between a spark plug and the head isn't a marvelous seal, but it's really pretty good. Always. It has to be, because it has to hold up against potentially thousands of pounds of pressure during every power stroke with no detectable leakage - if it leaked exhaust gases during power strokes, you'd be able to feel hot puffs of those exhaust gases around the back of the plug. If it carried liquid, it wouldn't seep, it's blow.

More likely oil than water-coolant, because oil can withstand engine operating temperature without vaporizing off quickly, while water-coolant can't. But... not leaking from the plug hole. Leaking from ABOVE the plug hole, from the valve cover or a crack in the head (also not likely, since that area of the head doesn't carry oil under pressure, it carries "waste" oil under no particular pressure back to the crankcase.

Let's get that area cleaned, let it run for a short while, and pull that plug for another close examination. A little white paper (toilet paper or paper towel works well) will point out the highest moist spot in that area.

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  • Oh my, I forgot to close this question. It turns out I did a poor job at adding oil and spilled it. It actually is leaking oil+coolant from the intake gasket which is the reason for the loss. Thank you for taking the time to write this detailed answer giving me full reason though, I learned a lot! I really appreciate that you are willing to dedicate that much time to help a random stranger.
    – Alec
    Aug 5, 2014 at 17:54

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