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How-To: Creamy Gunk (mayo) Under Your Filler Cap?

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  • [All Models] How-To: Creamy Gunk (mayo) Under Your Filler Cap?

    What do you do if you are checking you oil and find creamy gunk in your filler cap? This is a common question, so I hope these few words may be helpful.

    First of all - don't panic. In most cases this is NOT a serious problem. But in a relatively smaller number of cases it MAY indicate a problem with your head gasket.

    You need to know what the gunk is and why it may be there to understand the situation. The gunk (sometimes called mayonnaise) is an emulsion of oil and water. One byproduct of combustion is water, you will often see water dripping from the exhaust pipe of the car in front of you, which is why your exhaust pipe rots.

    Some of that water finds its way into the oil where it is meant to evaporate off and leave the engine via the oil breather pipe that runs from your camshaft cover to the inlet system. However, things don't always go to plan!

    In cars which only do short journies, particularly during cold weather, the water vapour condenses on the coldest part of the engine, around the filler cap. When this happens you get the creamy gunk mess forming. If this isn't regularly removed it can build up and eventually block the oil breather pipe, which of course makes the problem even worse.

    IN MOST CASES THIS IS ALL IT IS - and providing you clean it off regularly it is not a major problem.

    What can you do?
    1. Try not to use the car for short journies if possible.
    2. Consider increasing the frequency of oil changes (I always change mine every 5,000 anyway.)
    3. Keep cleaning the gunk away frequently.
    4. Keep a careful eye on your oil and coolant levels (see below).

    The worst case scenario is that the gunk is caused by water mixing with oil as a result of the headgasket failing. Most people jump to this conclusion as soon as they see the first signs of gunk. A few times they will be right.

    If you are seeing this gunk forming and your oil and/or coolant levels are dropping then take extra care. It may simply be that you have a leak somewhere. But check carefully. If you are losing oil and/or coolant, and you cannot find a leak, then the gunk may indicate a problem with the head gasket.

    In summary - if you find this gunk do not jump to the worst conclusion. It is probably a very minor issue. Only if you are also losing oil and/or coolant may it indicate a more serious problem.
    Last edited by Dave; 09-07-2013, 04:23 PM.
    Originally posted by Vegas
    When I hit boost me fuel runs out

  • #2
    Great guide. When I got my last B, 1.4 Hi-Torq it was like this as the old lady before me only did to the shops and back in the car, once I cleaned it away and pressed it into a 60 mile a day commute it never returned!