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Thread: Oil on spark plug threads

  1. #1
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    Oil on spark plug threads

    Hi All,

    I'm struggling to understand what could be going on with my engine. Two spark plugs on the cylinders closest to the firewall (L & R) have a lot of oil on the threads but not on the electrodes. I only changed the plugs <100 miles ago and already there is oil on the threads again....but only on two cylinders. Even stranger is that I get no smoke on startup or driving.

    I'm going to do a compression test this week to see if maybe the piston rings are worn and maybe slinging excess oil up to the spark plugs and its creeping up into the threads, but I would assume that I would see some smoke...?

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    It may just be a valve cover gasket leak.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
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    Quote Originally Posted by dmccanny View Post
    Hi All,

    I'm struggling to understand what could be going on with my engine. Two spark plugs on the cylinders closest to the firewall (L & R) have a lot of oil on the threads but not on the electrodes. I only changed the plugs <100 miles ago and already there is oil on the threads again....but only on two cylinders. Even stranger is that I get no smoke on startup or driving.

    I'm going to do a compression test this week to see if maybe the piston rings are worn and maybe slinging excess oil up to the spark plugs and its creeping up into the threads, but I would assume that I would see some smoke...?
    Are you sure it's oil and not carbon fouling from leaking injectors? If the injectors are leaking causing a rich mixture in those cylinders, it will look like oil on the threads and some black dry soot on the electrodes. What color are the tips of your plugs?

  4. #4
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    Quote Originally Posted by dmcman73 View Post
    Are you sure it's oil and not carbon fouling from leaking injectors? If the injectors are leaking causing a rich mixture in those cylinders, it will look like oil on the threads and some black dry soot on the electrodes. What color are the tips of your plugs?
    Admittedly my car is definitely running rich as the plugs have carbon build-up. However, I converted my car to carburettor some time ago so I would have thought that the spark plugs would be the same across all cylinders in this case?

  5. #5
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    Quote Originally Posted by Bitsyncmaster View Post
    It may just be a valve cover gasket leak.
    Ah Ok, I didn't realise that leaks from the valve covers would make their way into the spark plug holes. I'll go check that out.

  6. #6
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    Quote Originally Posted by dmccanny View Post
    Ah Ok, I didn't realise that leaks from the valve covers would make their way into the spark plug holes. I'll go check that out.
    Nothing is supposed to get into the spark plug wells, there is a big rubber seal that is supposed to prevent water (and oil) from getting in there. If the seal is old or not seated properly, oil or water could get past it. Or maybe someone replaced the ignition wires and the replacement wires do not have the big boot to seal the well. Check the oil level, do NOT overfill. Make sure you have the right oil in the motor, if you don't know then it should be changed. If you think the rings are worn do a compression test. Oil can also get into the cylinders from bad valve seals too but you would see the electrodes getting oily and black, it would not get onto the threads. Typically you put Never Seize or something like it on the spark plug threads but maybe a previous tech just used oil? Doesn't sound like you are burning oil.
    David Teitelbaum

  7. #7
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    Quote Originally Posted by David T View Post
    Nothing is supposed to get into the spark plug wells, there is a big rubber seal that is supposed to prevent water (and oil) from getting in there. If the seal is old or not seated properly, oil or water could get past it. Or maybe someone replaced the ignition wires and the replacement wires do not have the big boot to seal the well. Check the oil level, do NOT overfill. Make sure you have the right oil in the motor, if you don't know then it should be changed. If you think the rings are worn do a compression test. Oil can also get into the cylinders from bad valve seals too but you would see the electrodes getting oily and black, it would not get onto the threads. Typically you put Never Seize or something like it on the spark plug threads but maybe a previous tech just used oil? Doesn't sound like you are burning oil.

    I just gave the engine a look over and remembered that I replaced the valve covers recently - and even smeared a little rtv on them too.

    I also replaced the spark plugs and wires recently, but used ones that have the big rubber seal on them.

    Oil level is just below the highest notch on the dip stick and when I changed the oil last year I used the oil sold by deloreango.

    I will do a compression test to be sure, but it cant be that bad if theres no smoke? I'm also running without cats which should technically reveal even lesser levels of smoke.

  8. #8
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by dmccanny View Post
    I just gave the engine a look over and remembered that I replaced the valve covers recently - and even smeared a little rtv on them too.

    I also replaced the spark plugs and wires recently, but used ones that have the big rubber seal on them.

    Oil level is just below the highest notch on the dip stick and when I changed the oil last year I used the oil sold by deloreango.

    I will do a compression test to be sure, but it cant be that bad if theres no smoke? I'm also running without cats which should technically reveal even lesser levels of smoke.
    Hard to say, man. If there is a valve cover leak, that should be totally obvious just looking at the valve cover and where it sits on the cylinder head. If it's dry, the oil you see on the threads could be liquified never seize (copper). Wipe them off and re-insert, check again in a couple weeks and see if there's any change.

    The compression test will reveal any issues with rings. Good solid compression with throttle wide open and all plugs removed ( also disable fuel pump at the inertia switch and unplug the coil wire from the ignition coil), testing one cylinder at a time for about ten cranks should be 140 to 155 PSI. All six should be relatively close (within 15%) to each other.

    My most recent compression test showed I had 162 - 152 - 155 - 156 - 150 - 150
    Andy Lien

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  9. #9
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    Quote Originally Posted by dmccanny View Post
    Admittedly my car is definitely running rich as the plugs have carbon build-up. However, I converted my car to carburettor some time ago so I would have thought that the spark plugs would be the same across all cylinders in this case?
    Not necessarily. Since those two plugs in the back are the two that are difficult to get to, I would verify a few things:

    1. Verify that the wires are actually pushed on far enough to "click" onto the plugs. I've seen the wires on these plugs just sitting on the plug and not snapped on because of how challenging it can be to get your hand in those areas to push the wires on properly causing those cylinder to run rich/missfire. I pull the boot of the plug all the way up the wire to expose the metal connector, place the connector on the plug and us a flat head screw driver on the side of the connector to push it down to clip in place, then push the boot back down over it.

    2. Check the plug gaps and make sure they are between 26 and 28mm gapped.

    3. Make sure you have no vacuum leaks and that your carb is tuned to a proper mixture. Since the engine is sloped towards the front of the car (transmission side of the engine is more down then the water pump side) that would cause excess fuel from a rich mixture to drip down to the rear intakes to the two rear cylinders.

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