Ok this is a fairly common problem on most vehicles so I thought I would share a little knowledge on the subject. Having previous mechanics knowledge has its advantages....
Sometimes when you remove your oil filler cap on a car there is a gunky yellow/creamy coloured residue.
This is a sure sign of water getting into the oil system. Oil and water do not mix, in fact oil floats on top of water due to being less dense. In a car engine the heat and pressure caused by running the engine can force these two to mix and the yellow residue that you see is the result of this.
If there is just a little of this residue this could be caused by lots of short journeys in which the car engine gets hot while running and when switched off it will cool down and condensation can occur (usually at the top of the rocker cover). If this type of journey is common for you (just a few miles before switching off the engine) it is highly likely that this residue is just the result of condensation in the engine being forced to mix with the oil. It will usually only show on the inner lid of the oil filler cap. An easy diagnosis is to drive the car a good distance say 40-50 miles down the motorway and back in 5th gear at 70 miles an hour or if fuel isn't a concern in 4th gear is even better so as to keep the revs up. This will get the engine hot enough to burn off any minimal water deposits and hey presto your oil filler cap should now not show any sign of this residue.
If however the residue remains, or in fact gets worse, there is a more severe problem. Unfortunately a common issue is a blown head gasket. There is a small usually rubber but on older models cork gasket that seals the two main block parts of your engine. This gasket has holes in it for the cylinders but also smaller holes which allow oil through to lubricate the engine and water through to cool the engine. If this gasket is cracked in any place this can allow water to enter the oil system or visa versa. This can also be very costly and unless you have good knowledge of mechanics should visit a car garage to have this replaced before any further damage to the engine is caused.
If a head gasket is blown there are often signs of oil in the water system too such as the shiny bright coloured effect you get when water runs over a small amount of oil on the roads. This can be located in the header tank floating on top of the water so easily identified. If there is the yellow residue when you remove the oil dip stick from the car this usually means there is enough water in the oil for it not just to be condensation.
Another common symptom is white smoke from the exhaust when driving but make sure not to confuse this with the usual smoke/steam you will notice when first running a car on a cold morning. Basically, if you have white smoke still coming from the exhaust after driving till the engine is fully warmed up there is usually a problem.
It is a good idea to change your oil every 6000 miles or every 6 months as a standard rule or whichever of the two comes first. With any oil change it is also worth changing the oil filter as these get clogged up very quickly especially on older engines. I will upload simple details of how to change your oil and filter in another post soon so keep a look out. In lucky cases just changing this may also cure your yellow residue problem.
If you have any questions or extra information I may have left out feel free to comment.
Thanks and hope this has been of some use.
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