I have a 1999 Honda Accord EX which recently had been overheating. So upon checking the antifreeze levels I found what I thought was oil in the radiator reservoir. I knew this was a very bad sign of a huge problem but I figured I'd flush the system out first before I jumped to conclusions especially since I hadn't been having any issues before the car started to run hot the last few days.

So flushed the radiator out and everything coming out of it looked way cleaner that I thought it would; no sludge. I took off the reservoir which had had the oily residue and dumped it out and cleaned it good and replaced it. Checked the oil while I was under the hood and it wasn't low. Then checked the transmission fluid ... and it was indeed low. So now I'm thinking it was transmission fluid that I saw in the overflow reservoir. Btw I test drove the car for over an hour and everything was fine after I flushed the cooling system.

But my question is what would cause that to happen and how the heck would it get all the way through the radiator into the reservoir without mucking up the radiator itself? I'm not a mechanic by any means so any helpful thoughts on what's going on and what my next step should be in fixing the problem??

 

You popped a headgasket.

 

wow, jump-to-conclusions jones...

OP: it's true the headgasket leaking oil into the coolant passages in the engine is the more common scenario. but, if you truely think this could be transmission fluid (in which case i would expect it to be red to reddish-brown) it can easily get into the radiator through the internal transmission cooler in the lower tank.

you can look down from above the radiator and see two 3/4in. hoses running from the transmission to the radiator. they route through a single pipe in the bottom of the radiator.. if this has a leak... you know the rest.

if the radiator is cheaply made or you see additional leaks around the seams, you could be on to something.

 

Overheating with oil in the coolant...seems pretty obvious to me.

If there's trans fluid in the coolant, then there will be coolant in trans fluid.

Drain the oil and take a pictures, it's probably milky from coolant.

 

so your car is auto right? if it is, then your radiator is bad.

 

At one of the shops I worked at I once put like half a quart of ATF in someones radiator. (The gallon jug of ATF just happened to be sitting next to the gallon jug of coolant and they were both black jugs of the generic wholesale variety) Thing is oil is lighter then water right? I just put a hose in the top and kept it flowing till the water was clear. I think that's how it ended up in your reservoir while the rest of your coolant appeared uncontaminated.

If you blew a headgasket you might not know for a sure until the leak gets a lot worse and you see white smoke coming out of the tail pipe. If your coolant level seems to be dropping faster then usual it might also be a sign that its leaking into your engine.

However, I agree with speedooo first thing to do is check your ATF. If its not milky then you might need to worry about a head-gasket. I'd make sure you check all the other cheaper possible causes of this before you pay someone for a head gasket though.

Good luck hope this helps

 

My friend has a 2007 Honda Civic.
I am looking at the radiator and the transmission cooler. By all accounts these seem to be independent systems. So there really should not be anyway radiator fluid could be able to get into the transmission and vice versa.
Therefore, any "oil" in the radiator would almost have to be from the engine, and therefore indicate a head gasket. Especially when the added problem of overheating comes into play.
Am I wrong? Are these not independent?

 

If you look closely you'll see there are transmission hoses that go to the bottom of the radiator as the radiator helps to cool the transmission fluid. It's possible for it to leak between the two chambers within the radiator. It's not common but it does happen.