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Oil in radiator and reservoir

Hi Folks,

My 1998 Honda Civic has oil in the radiator. And even stranger, the oil winds up in the overflow coolant reservoir. What is the most likely cause of this?


There is a transmission oil cooler which is located in the bottom of your radiator… If that leaks, you can get transmission fluid in your coolant. To get engine oil in your radiator, someone could have accidentally poured it in. Do you have service station attendants check and top up your oil?

The caliber of help today in that areas leaves a lot to be desired.

You have a blown head gasket. The compression from the engine is pushing oil into the coolant passages and it winds up in the radiator. When the radiator has to expell pressure it pushes this coolant and oil mix into the overflow.

You should check the oil dip stick. If the oil looks milky…the oil is contaminated and the engine should not be started. It could cause more serious damage.



Check your oil now. This sounds like Head Gasket problems.


+1 to both Yosemite and Volvo V70.
If the OP is very lucky, it will turn out to be a trans cooler that is leaking into the radiator.
However, given that this model is prone to head gasket failure, I think it is much more likely that this is a breached head gasket.

So far there is no sign of milky foam on the dip stick.

Would a compression test prove the oil from the head gasket theory?

It’s my impression that replacing a head gasket isn’t that hard. I’ve replaced shocks, break pads, starter motors, etc. I would think if I can do that I can replace a head gasket. Yes, no, maybe?


Replacing the head gasket PROPERLY is not easy. It takes more than just replacing a gasket. Leave this to a professional and cough up the $900 or so.

1 Like

Do you have a decent torque wrench . . . NOT a beam-type

Do you have the torque specs and the proper sequence?

Do you have a precision straightedge . . . your head and/or block could be warped, and it’ll look fine to you

Head bolts are often 1-time use, meaning you might need new bolts, as well

Ever replaced a timing belt . . . your engine probably has one

If you do this on your own, be prepared to be without the car for a few days, ESPECIALLY if the head is warped and you need to get it “cleaned up” at a machine shop

Check your ATF just in case it’s the trans cooler. Oil floats on water/coolant so it would be the first to get pushed into the reservoir as the coolant warms up and expands.

ATF is generally dyed red

So that would look a little different, if it got into the coolant

“ATF is generlly dyed red”


But it can turn brown/black if ATF isn’t serviced.