BMW 535 E34 randomly overheating

[bluetester]

I recently bought a 530 BMW with a 3.5 inline 6 cylinder engine transplant. I drove it to the MoT place that took 40 mins, stop starting and top speed of 40mph. Half way through the MoT the engine started to overheat slightly and coolant started to come out. I had to pay £53 for an MoT which wasn't even a full test, but i noticed that the engine temp went back to running temp and the leaking stopped. Aparantly it was during the emissions test it did this. Any ideas why or what it could be? any help?

[bob1179]

Has the engine been rebuilt or was it just a swap?

If the coolant hasn't been changed regularly in the M30 engine, the engine can fur up and reduce the amount of coolant flowing through it. If the channels are clear, I'd check the thermostat and radiator to make sure they are working as they should.

[E30M3SE]

VC Fan, themostat, waterpump all need checking.

[bluetester]

Swap inclusing gearbox, the radiator fluid is brown-ish. Would this cause the problem and create leaks?

[TheEnd]

Brown like chocolate milkshake?
That's not a good sign. Sometime they will be orangey from rust (thats actually not a good sign either, as it's usually from topping up with water and no coolant...)

Take a closer look at the water/coolant as it does sound like oil and water mixing. It's not terminal though, as it can be from a combined oil and water radiator (if fitted) in some cases.

The rest of the symptoms do sound more like the viscous fan being weak. This is characterised by overheating when it isn't moving as the fan clutch isn't strong enough to pull enough air through the radiator and cool the engine.