So after a fair bit of highway driving on a hot day, the car overheated slightly (went to 100c at about 1pm instead of 90c at 12pm where the temperature gauge normally sits) and the oil temp went up to about 100c (which is a bit higher than where it normally sits).

The problem resolved itself while driving. When checking the overflow tank after this trip, I found light brown sludge in the top of the overflow tank. Cleaned that out, topped up levels slightly, drove again, all good. Drove a 2nd time, same problem (which resolved itself while driving again).

What could it be? I'm guessing oil leaking into coolant system, either via oil cooler or head gasket?

 

it is more than likely coming from the oil cooler, mixing oil and coolant, under you oil cap is there similar sludge in the oil? Regardless if the oil is mixing in with coolant system or coolant mixing with oil start with the oil cooler as it will be a relatively cheap fix.

Regards,
George

 

There was no sludge under the oil cap. It appears to have resolved itself? It it possible to have a temporary oil leak into coolant from somewhere?

 

It’s better that coolant hasn’t come in with the oil verses oil coming into the coolant as it can be a bigger issue when coolant mixes with oil as it will affect lubrication and people now days don’t care for there car much hence some serious damage to engine.

I doubt the issue can resolve itself, it may have just stopped leaking due to changes in temperature in hoses fittings etc with expansion and shrinking. I would still start with the oil cooler as this is the most likely place where these can mix.

Regards,
George

 

Yeah, it was intermittent but seemed to resolve itself. But in any event, I got the oil cooler changed for piece of mind. I just didn't quite understand how it would come and go. I would've imagined that if the oil cooler failed (being a metal piece), it would just fail and mix the oil and coolant together.

 

So after a fair bit of driving, it happened once again. It usually happens on a pretty hot day if I'm stationary for a while, like an event triggered by some kind of heatsoak. There was a little bit of brown sludge in the overflow and no sludge under the oil cap. Car temperature and oil temp overheats above where it normally sits when this happens.

 

Unfortunately it sounds like a cracked head or head gasket issue if a new oil cooler didn’t fix it. Have you checked compression lately? If not, take all spark plugs out and do one. If you get low compression and hear gas moving from one cylinder to another, it will be a head gasket. Do a compression test and report back your findings.

 

Yep as stated above, if the oil cooler has not fixed it the next place to look is the head gasket, do a compression test and go from there.

 

It appears to happen when there is significant heat. Like middle of a hot day, driving up a hill with air con on, and then that will cause a slight overheat (from where the coolant temperature normally sits) and a slight rise in oil temperature, which then results in a little bit of sludge at the top of the overflow tank. It can also happen from sitting still for a while in hot conditions.

Would that indicate a slight problem, like a hairline crack in the head or the head gasket having a slight break?

 

Do a compression test and report back your results.