Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Don't cross the streams. Antifreezes don't mix

My 2000 Ford Expedition got very hot (briefly) because of insufficient coolant. I went into the store who’s parking lot I was in front of & bought a gal of 50/50 antifreeze & dumped it in. Ran fine the rest of the way home. What I found when I opened the hood is hard to describe. There was a mustard colored greasy-looking foam coming out of the cap on the overflow tank. Buddy said it was from mixing green 50/50 with Ford OEM yellow coolant. Flushed it and everything seems OK. Have about 2000 miles on it since refilling the radiator. Oil level is steady and oil looks clear, but is now getting dark (as expected - 3500 since last change). Loosing a little coolant (something 1 qt every 2 weeks). Suspecting a perforated hose or other leak. Question is this: Have I done anything elst to the cooling system that will reduce its reliability? Water pump, etc?

"Question is this: Have I done anything elst to the cooling system that will reduce its reliability? Water pump, etc?

No, the fresh antifreeze did not hurt anything…But the overheat and the undiscovered reason for the original coolant loss is more of a problem…The mess in the coolant reservoir is the result of never changing the coolant or coolant that is contaminated with oil or ATF…

If you flushed it really throughly you should not have anymore problems caused by mixing the coolants. But you buddy was right, some of the long life coolants (mostly the HOAT* ones) shouuld not be mixed. What i usually see as a result is a brown sludge mess as a result of mixing coolants.

*Hybrid Organic Acid Technology

I think there is a REMOTE chance mixing the coolants damaged the head gasket. Get the cooling system pressure tested. This will not only rule out a head gasket problem, it will identify your mystery leak, if you have one.

I guess I should have been more clear. I’ve always used the speedy greasy lubey places & relied on them to top up the coolant. I reallized after the fact that they never removed the cap & looked into the tank (it’s a “view-through” plastic tank). Tank was badly rust stained & looked like coolant was OK when it was really getting low. Low fluid = hot engine.

You can mix conventional antifreeze with the long life OAT/HOAT antifreeze. However, once this is done it comprimises the anticorrosive additives in the OAT/HOAT antifreeze. So the antifreeze should be serviced at the interval for conventional antifreeze.

Here’s what the boy’s have to say on the subject.


How’s the transmission fluid level though? It doesn’t take much ATF leakage through a leaking fluid cooler to create a real mess in the cooling system.

I’m assuming that the tranny fluid level is OK, based on the fact that there’s no more gunk once I got everything cleaned up. But I’ll check the ATF level.