Automatic Transamission Fluid In Engine Coolant... WTF?

[TheExcession]

Here's a strange one. Toyota Land Cruiser - Automatic, 1999, 89,000 miles.

Driving today and noticed that the engine rpm was rising a little too much with a judder from the automatic transmission when setting off. Classic case of low AT fluid so I went to investigate.

Turns out that the engine coolant has turned from a lovely green into a dirty brown. Also, the expansion tank is full and overflowing with this dirty liquid. The engine oil is perfectly normal (so no blown head gasket), but how is the transmission fluid leaking into the coolant?

A few people I've talked to so far have taken the same attitude that I immediately thought... "IMPOSSIBLE..."

More interestingly, judging by the AT Fluid dip stick there doesn't appear to be any water in the AT fluid - so hopefully the gear box is still ok.

So I'm left wondering, either the coolant and AT fluid share a pump and there is a seal gone, or there is a leak in the (shared???) radiator that is allowing higher pressure AT fluid into lower pressure(?) engine coolant.

Anybody got a bit more experiance on these engines that might point me in the right direction.

So far my workshop manual is providing nothing to me.

best
Ex

[tank slapper]

My dad's audi had a very similar problem. It was the ATF heat exchanger that was leaking fluid into the coolant. There should be no water in the gearbox, as the fluid pressure is higher than the coolant pressure.

[That Daddy]

The radiator/oil heat exchanger for the gearbox as cracked internally,you need a new rad,you will see pipes from the gearbox going to the rad,you need to flush out the cooling system,and service the transmission oil also(just in case)coolant will wreck an autobox in no time(very expensive)

[TheExcession]

Feck... that was quick... cheers fellas.

As I'm suspecting... I need a new radiator.

The rad is split for engine and AT cooling. It's leaking between the two. I need to flush out the entire engine coolant circuit and just top up the AT fluid. Hopefully no water in the AT fluid means I've missed out on a gearbox overhaul.

[That Daddy]

If the gearbox was juddering then it was low on fluid,a sure sign its also overheated the fluid,i would strongly recommend changing the fluid for this very reason,if the gearbox fails in 6 months time you will kick yourself,changing transmission fluid is such an overlooked job with owners,that fluid is the gearboxes lifeblood,ignore at your peril,its not expensive or hard

[Fixedwheelnut]

Running a mix of washing up liquid to flush out the oil in the cooling system works

[TheExcession]

That Daddy said:

If the gearbox was juddering then it was low on fluid,a sure sign its also overheated the fluid,i would strongly recommend changing the fluid for this very reason,if the gearbox fails in 6 months time you will kick yourself,changing transmission fluid is such an overlooked job with owners,that fluid is the gearboxes lifeblood,ignore at your peril,its not expensive or hard

It wasn't low on fluid until this litle episode. No doubt I'll get it topped up and flushed at the same time.

I'd do it myself but really haven't got the time at the moment, this will be a case of getting a garage to come pick up the car on a flat bed, take it away and bring it back fixed. I just want to ensure that I know what I'm expecting them to do.

Thanks for your help.