Q: Oil cap is milky, but the dipstick isn't and the coolant isn't bubbling. Please advise.

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I checked my oil today, and the cap is milky white, which would lead me to believe it's the head gasket. When checking the dipstick, it is normal oil colored and when taking the radiator cap off to check for bubbles, it is normal coolant colored with no bubbles. Please advise.

Thank you,

My car has 160000 miles.
My car has a manual transmission.

The discoloration likely represents the effects of entrained moisture in the oil on the cap surface. There can normally be a certain amount of condensation in an engine, for example as weather conditions change or if the engine sits for a while unused. If the area of discoloration is limited to the cap and you are not experiencing ANY coolant loss or white steam in the exhaust once the engine has warmed up, then there is no issue to look into. However, if you are losing coolant, particularly if an internal leak is suspected, then the matter should be looked into with, for example, a coolant system pressure test. If you do suspect a leak the recommended service is a cooling system leak diagnostic.

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