2000 Octavia 1.9 TDI AHF engine is losing coolant, and actually boiled on a long uphill. There's no smoke from exhaust and not apparent leaks, but I'm suspecting the oil cooler is leaking since I have seen some red coolant on it one time, and it looks washed after ascents. When the oil gets hot, the oil cooler thermostat runs water through it causing water leak?

Edited 4 August, 201114 yr by RallyeSport

Anyone experienced a coolant leakage from the oil cooler?

It's not uncommon to see a bit of crystalised G12 where the pipe meets the oil cooler because this pipe is often removed to drain the coolant. If you wipe it off, does it come back?

If you remove the oil cap, is there 'mayo' under the cap?

You should have a block test done to see if it is the head gasket.

No oil under cap. Water is on hoses and cooler is not connected to drainage of coolant. I see oil coolers are available as new, but does it have a thermostat that regulates the water flow? If so, this can be connected to the increasing coolant loss during uphill ascents.

No oil under cap. Water is on hoses and cooler is not connected to drainage of coolant. I see oil coolers are available as new, but does it have a thermostat that regulates the water flow? If so, this can be connected to the increasing coolant loss during uphill ascents.

It doesn't have its own thermostat. I've seen a few oil coolers fail internally, but you would see mayo under the cap or dirty coolant.

If there isn't an obvious leak, then check...

for damp carpets inside (faulty heater matrix)

below the vent tube at the side of the reservoir (faulty cap/reservoir)

between the condenser and radiator (the a/c condenser can make a leak here difficult to see)

If you can't find a leak, then get a block test done to see if you have a bad head gasket. The test is not always conclusive. It is not uncommon for HGs to leak more at high temperature/load.

I found the leak. It was the oil cooler, but not the seals or hoses. A plastic clip which holds a part of the wiring harness to the coolant hose from the thermostat was the cause. The end where it hooks on the the coolant hose is flared and this edge was rubbing on the oil cooler just below the top egde on the front side. When the engine vibrates, it vibrates too. This was enough for the plastic edge to erode (with dirt/sand and spray from water on the road) into the aluminium making a 3 mm deep groove, just enough to get through to the water by a tiny hole where it dripped. I suspect the hole was so tiny that to cause a serious leak the coolant pressure needed to be high, which happens when the engine gets hotter. I cleaned it, sanded it and slapped on some chemical metal and now there is no more coolant loss.

have a look @ my post in this topic (with pictures)

www.briskoda.net/forums/topic/155136-slow-oil-leak-around-front-of-wheel-arch/

is it the same issue?

cheers

b

Edited 4 August, 201114 yr by berre

Yes, something like that. Meaningless external wear on the oil cooler.