Oil in radiator 03 Subaru Forester with only 64K miles

Noticed the smell of radiator fluid after a quick 2 mile drive. Investigated and noticed oily radiator fluid in the overflow. Had the oil changed but mechanic did not say anything about radiator fluid in the oil. There is oil in the cooling system.

Is it a head gasket or transmission oil leaking into the cooling system?

This could be due to either one if the transmission is an automatic. Check the condition of the transmission fluid to see if it’s low/contaminated and checking the engine oil for contamination is a bit after the fact.

Subarus are known more for leaky head gaskets than leaking transmission coolers.


Take a sample of the oil in your overflow jug and send it to someone who does oil analisis and include a letter with your question.

How Full Was The Overflow ? Overflowing ? Has The Cooling System Been Flushed And Refilled During The Past 7 Or 8 Years ? How Long Ago ?

Oil in the coolant or what appears to be oil in the coolant can be quite normal if the cooling system is left to stew with old coolant in it. It may not indicate an engine or transmission / transmission cooler issue, but those are possibilities.

However, this does not explain the coolant smell. Since the engine oil has just been changed, I’d check it now and very frequently for contamination and level. A little coolant in the oil will do serious engine damage. Carefully check the coolant level and monitor it, too.

I’d think that if what you’re smelling actually is coolant then it’s likely either leaking onto hot engine parts, and vaporizing, coolant escaping the overflow (overheating or over-pressurized), or the coolant is leaking inside the passenger compartment from a leaky heater core.


If the coolant hasn’t been flushed out in a few years; I’d drain all the current coolant and look for evidence of oil contamination. Clean out the reservoir too and refill with fresh fluid. Then keep an eye on it.

Someone might have poured something improper in the overflow tank. But you could have a bad radiator that is allowing auto trans fluid into the coolant. This is bad, but easier and cheaper to deal with compared to a blown head gasket which is another possible source of the oil contamination.

Thanks for the input.

Over flow was three quarters full (hot) when checked. system was flushed a couple of years ago.
will check the transmission level today. Think that I will use a local mechanic (San Diego area) to help with the investigation and possible repair options. Will use one with a recommendation from this site. If anyone is from the San Diego area and knows of a mechanic that has experience with Subarus would appreciate contact info.

Thanks again for the thread.