Oil in my Coolant / oil Cooler air leak test

Crook90 · 2025-09-11 01:44:55

Hey all, so a little bit of back story. I’ve got oil in my expansion tank and have been trying to figure out where it’s coming from. So about 2 year ago I gave it a clean hoping that it’s just residual due to the age of the vehicle and that the previous owner didn’t look after it as well as I had hoped…. I know it’s a closed system and I shouldn’t have it in there, but you never know with previous owners. Whilst looking on forums and asking FB I found that people were telling me that it could be the oil cooler, so I checked…. And I found that the oil cooler had been by passed which referring back to my various sources learnt that it’s common place with these vehicles. So my thoughts was that maybe the previous owner bypassed it because the oil cooler was faulty. I’ve struggled for about 5 years to find a replacement and then had a moment of realisation that the bypassed oil cooler wouldn’t make a difference to the clean coolant I put in unless of course I didn’t clean it well enough. So without getting a new cooler I decided to test the oil cooler I had to rule it out as faulty. To do this I waited until I needed to carry out a service, so once I removed the oil from the sump tank and took of the oil filter I removed the cooler. With the cooler removed I gave it a good clean, snipped an old inner tube about 30cm’s each side of the inlet valve and jubilee clipped it to the pipes on the cooler. Submerged it in water and pumped up the inner tube up to about 4/5bar. I hoped that the positive air pressure would push the air through any holes I had in the cooler which would indicate it was faulty. This didn’t happen so I can assume it’s fine and the previous owner took it for face value that it was the cooler. The problem is, now I can’t blame the damn thing and I’ve got to look elsewhere. Having read and spoken to people in other MK1 group areas people are telling me it could be residual dirt, or from what the internet says a crack in my engine block or blown head gasket.

So I checked my oil cap first and the stuff that came out of the cap was just oil same in the sump tank, no milky residue to be seen anywhere. So I feel like I can loosely rule that out. I have a sniff test, stuff to clean the coolant system and steal seal for the engine block. Hopefully this will sort it.

The next step is to flush and clean the system again. Using dishwasher tablets to lift and remove the last of the grime/oil in the system. If that doesn’t work I’ll give it a sniff test to check that it’s not coming from a blown head gasket. If that fails then it’s steel seal time.

I hope this helps anyone in a similar position

kenbw2 · 2025-09-15 01:08:09

In my former van I had oil bubbling up to my expansion tank and in my case it was the oil cooler. I replaced it and it was fine since. But by god did it clog things up. I flushed out the coolant about 7 times and it still had residue.

My heater matrix never worked well since - only getting warm air if the engine was at decent revs

The only places the coolant gets close to the oil is like you say the oil cooler and the head gasket. Obviously we have to hope it's not the latter but without evidence of an issue with the oil cooler I can't think of another possibility

Are you losing oil over time?

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#1 2025-09-11 01:44:55

Oil in my Coolant / oil Cooler air leak test

#2 2025-09-15 01:08:09

Re: Oil in my Coolant / oil Cooler air leak test

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