Sludge in the coolant header tank

Q There is an oily sludge in the coolant expansion tank of my wife’s 62-plate Vauxhall Astra J 1.6 16v. The car doesn’t appear to have used any coolant and the oil on the dipstick and under the filler cap has no signs of coolant contamination. The car has done 60,000 miles and has been main dealer serviced. I am planning to flush the coolant system, and to remove and clean the expansion tank, refilling it with fresh antifreeze.

I am assuming this is a sign of a more serious issue with the oil cooler or head gasket – what would you suggest?

John Tyrell

A The oil may be entering the cooling system via a breach in either the head gasket or the oil cooler, or even through a crack in the

cylinderhead. This may only be a tiny fracture, but as the oil pressure is higher than the pressure of the coolant, the breach is normally only in one direction at the earlier stages.

If there are no running issues, the most likely cause is a failure within the oil cooler. The replacement time given for the oil cooler is 1.5 hours, but it is a tricky job. If the oil is seeping into the system via the oil cooler, you should be able to see a concentration of oil in the cooler area by removing the hoses. If there is a high-level of oil in the oil cooler area, this will confirm the diagnosis. It would be advisable to rectify this problem as soon as possible as once the oil begins to enter the cooling system it will be difficult to clean out thoroughly.