1996 Ford Windstar - strange fluid in coolant reservoir

I asked my mechanic to pressure-test the cooling system, because it looks like there’s oil floating in the coolant reservoir. The thing has always consumed coolant fluid (let’s guess less than a gallon over the course of a year), and the rate of consumption has increased significantly this fall (let’s call it a gallon since September or so). The last coolant flush was at least 4-5 years ago. There have never been puddles on the ground, so I was guessing the coolant was going out the tailpipe and starting to get nervous about a possible serious head gasket failure in the near future.

However, the shop thinks it’s transmission fluid and recommended replacing the radiator, on the theory that there is a crack or small hole in the transmission lines that run inside the radiator. They don’t think it’s engine oil because there is no evidence of coolant residue on the dipstick or filler cap, and when I changed the oil a few weeks ago, it was the typical dark brown / nearly black color of used oil. The transmission was replaced in June (about 7,000 miles ago - current odometer almost 168K). The transmission fluid looks OK to me on the dipstick (typical light red color) but is over the “full” mark.

It’s hard to get a good look inside the reservoir, since it is opaque white. I took a small water bottle and used a long straw to extract a couple ounces of coolant, and it sure looks like oil floating on top to me.

Ever hear of a one-way leak from the engine into the reservoir? Theories? Any additional driveway diagnostic tests I might try to narrow this down? (I’m going to put some transmission fluid into another small container of coolant and compare with the first).

The heat exchange process for both the anti freeze and transmission fluid occurs in the radiator. You’ll notice the trans lines fasten to one of the tanks of the radiator.

You need a new radiator.

There’s internal corrosion allowing the fluids to mix…yes plural, you should check or change your trans fluid too.

Go ahead and do your experiment with transmission fluid and coolant. You may find it matches what you’re seeing in the coolant reservoir.

Or not.

I think it’s more likely there’s transmission fluid mixing with the coolant than oil.

You didn’t mention any oil loss.

Did you check the trans fluid with the engine running, in park, transmission warmed up, and parked on a level surface? If not, you’ll not get the correct dipstick readings.

If the trans fluid is nice and pink, but the oil in the reservoir is brown or black, I think it is a safe bet it is engine oil.

Most gasket failures will lead to coolant getting into the engine oil or combustion chamber. But, some rare ones can see the oil from the pressure side getting into the coolant. The real trouble is tracking down such a leak. I’ve seen this type of leak happen with a head gasket and with an engine oil cooler that was actually cooled with coolant. I’m not familiar with the engine used in this van, so I can’t comment on any other likely sources.

Yeah, the trans fluid looks a lot different when I put it in the coolant: much more red. I think the van has engine oil in the coolant reservoir. As for oil loss, I will guess I’ve added a quart in between oil changes less than half the time, or maybe 1 time out of 3, so that didn’t seem too bad. The engine on this thing is a 3.8L V6 and I have read that head gaskets are notorious on the Windstar. I don’t think it has a separate engine oil cooler but I can look that up to be sure.

A friend at work suggested squeezing the upper radiator hose when cool and then again after the engine has been running a few minutes. If the hose is a lot firmer, he says that indicates that there is pressure from the cylinders coming through the head gasket into the cooling system.

And here’s a truly ugly thought: what if there is both engine oil AND tranny fluid in there, and I need a radiator AND a head gasket? I think I would have to cut my losses at that point…

A blown head gasket almost always pumps combustion gasses into the radiator. These are easy to detect. Shops have “sniffer” tools for this purpose. A new radiator for your Windstar is fairly cheap and that’s the most likely cause. After the ATF has been mixed in the coolant, it no longer looks like ATF…It looks like brown sludge…ALL the coolant hoses will have to be changed and the cooling system flushed to remove all the oil. The ATF will quickly soften the rubber hoses…You can suck out a good sample with a turkey baster…

Ok, coolant does NOT look like brown sludge or chocolate milkshake. When I open the radiator cap, it’s nice and green. The black / dark brown stuff is floating in the reservoir in a thin layer, and there is black residue above the high coolant mark. Someone else suggested the possiblilty of the intake manifold gasket, but I need to look at the book - I don’t have a clear picture in my head about how the symptoms would be different than head gasket starting to go. I don’t understand how oil could get into the cooling system from there. I’m going to take it back to the shop and ask them to do some more tests.

P.S. Thanks all for your info and suggestions. I’ll keep you posted if there are any other lucky Windstar owners out there…