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Oil in Coolant reservoir not in radiator

I have a 99 Honda civic LX 4 cyl. There is an oily residue in the coolant reservoir an on the fill tube but when the radiator is opened the coolant is clean. The car is using oil but not running rough or hot. There is no tailpipe smoke or evidence of leakage. I am planning to do a compression check and look at the plugs. Any suggestions would be appreciated.

I would remove and clean the fill tube and coolant reservoir then watch it for oil contamination.

A compression check can yield some viable results. If the leak is very minor it may difficult to determine if there is a leak with a compression test alone. I suggest doing a cyliner leak down test in a quiet area to see if you can hear any hissing or bubbling.

If your reservoir is the highest point for coolant in the system, the oil may be collecting there. You can also drain the coolant to do an inspection of it. I would check the plugs first and do a compression test. If neither give you any results, I would do a cylinder leak down test to determine whether or not you may just have some build up in your tank or if you rreally have a serious head gasket leak.

If there was a slight amount of oil in the coolant, it wouldn’t be surprising it all ended up in the coolant reservoir eventually. The reason is that oil floats on the coolant, so it would tend to wind up at the top of the radiator, and whatever’s on top in the radiator, that’s the first thing that goes into the coolant reservoir. It then gets trapped there, as what goes back into the radiator first is probably from the bottom of the coolant reservoir.

At some point some oil-contaminated coolant might have been added is all. Maybe it was poured in using a funnel that has previously been used to poor oil. Probably best to take a wait and see. Maybe remove as much of the film as you can, then watch to see if any re-appears. Best done in conjunction with a coolant change, if that is due soon.

Edit: One other idea, if you are concerned about a head gasket leak, you could purchase a test kit which does a chemical test to check for exhaust gasses in the coolant. I think they are relatively inexpensive.

It may not be oil you see in the coolant reservoir. But instead the cooling system hoses breaking down internally.

Cooling system hoses will start to break down to a point where small bits of rubber along with the carbon black used in the manufacturing of the hose will start to collect in the coolant reservoir. And this can look like an oil substance.

If there’s no oil in the radiator, then you might want to look into replacing the cooling system hoses if it’s never been done.


If it’s an automatic, the source of the oil can also be the the transmission cooler which is built into the radiator…