Yesterday while tinkering around I noticed a bit of goo inside my overflow (header) tank ('95 EF 4.0litre Fairmont auto), opened the cap to take a look, and saw it was slick with oil. I removed the tank and drained it into a clear plastic juice bottle - there's quite a lot of oil in there (couple of cm's, maybe a few cupfuls) on top of the bright green fresh coolant.

Or, could it be auto transmission fluid?? It looked kind of slightly reddish, before it all mixed up into a gooey grey-brown emulsion.

My main questions are:
Is there any way I can test/figure out whether it's engine oil, or ATF? And then, where do you start to look to find the source, or where/how it's getting in to the coolant? I drive this thing every day and I'm hoping to avoid some major mechanics repair bill.

Bit of background.

About 5,000km ago, I flushed the cooling system, and replaced the coolant with new fluid. I was worried then that I might have put too cold fluid in to a hot motor during the process. But hadn't observed any problems in the months afterwards.

About 1,000km ago I drained and replaced the auto trans fluid and filter.

There is no water in either the engine oil, or the auto trans fluid. Both (oil and ATF) are very clean.

I've refitted the overflow tank and topped up the coolant, and will see how much more oil has accumulated there after a decent run.

But meanwhile, any input much appreciated!
Thanks

Thanks - here's a pic of the 2litre juice bottle, with the contents from the header tank.

That's about one litre of coolant in there.

there is a common problem that the trans cooler (located in the cool side tank on your radiator) splits. now i'd be checking you auto trans fluid too. hopefully there is no coolant in it. usually the auto fluid is at a high pressure than the cooling system so it won't happen.

if thats the case you'll have to replace the radiator and give the engine a really good flush. through the heater core, block, everything.

What do you think of - instead of fitting a new radiator, just fitting an external trans cooler, and plugging off the ATF line ports into the existing radiator?

New radiators here seem to go for about $250-$300 kiwi $. That's just the rad, not fitted.

imo looking at that bottle i would be thinking engine oil . i kno if you mix tranny fluid with water it goes pink & milky . do a test with engine oil & coolant & mix the 2 together to see if it matches what you got in the bottle .

also be carefull what coolant you use in cars with aluminium heads/parts etc some of the aftermarket replacement coolant is highly corrosive.

That's a LOT of oil...has to be tranny fluid leaking into the rad. Replacing the radiator is a piece of piss...10 minute job. Bite the bullet and get a new one.

Picked up a brand new radiator today for NZD$180 - not bad.