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Oil in Coolant Reservoir

My son brought over his “new car” he just inherited from his grandfather. We gave it the once over and when we checked the coolant reservoir it appeared full of black oil. Nice looking oil, not the white frothing stuff I used to seeing fr4om a cracked block or head gasket. Any ideas? No oil loss in the motor. We changed the oil and filter and no coolant in the oil.

You don’t give the year but I suspect that the “black oil” you see is actually Dex-Cool residue. Google Dex-Cool and read about this coolant. I always remove it and flush the cooling system entirely. I replace it with a quality “green” anti-freeze that will blend with any other anti-freeze. It’s your choice.

This is one of those rare cases where I’d flush the cooling system out good (with a flush additive). After refilling it I’d do a system pressure test to find any leaks. Then I’d do a compression leakdown test on the cylinders. If everything checked out okay with the cylinders I might in this case be inclined to run an oil system flush additive through it (follow the directions on teh bottle TO THE LETTER) and refill with fresh oil & filter.

If all of this checkes out okay, then you may want to consider a complete tuneup with new filters.

I don’t usually like additives, but since you don’t know where that black stuff came from or where it’s traveled (in the engine) you want to get everything cleaned out and try to determine its source.

I once had my ford explorer die on the highway without any warning or reason. I had to have it towed to the shop. The next day they fixed it a told me it would be $963. When I looked at the bill they had changed the fuel pump, replaced the filters, cleaned the throttle body and tuned up the computer servor uints. WhenI walked back and asked the mechanic to see the parts he said they were all tossed or returned for core. BUt really it didn’t matter becuase I had just run out of gas anyway. they did all that work trying to figure out if there was something else before they realized there was no gas. The gauge read 1/4 full. This sounds a lot like that ! I wonder if someone just put oil in the coolant reservior by mistake. It doesn’t appear to have mixed.

It is just possible that the very senior previous owner accidentally poured oil in the radiator opening.

However, I agree with the previous suggestions of testing for all possibilities.

Good luck with getting the car in shape.

I thought of that possible error too. But since there’s no way of knowing in this case where the black came from, I figured it’s better to be safe than sorry.

Sorry to hear about your bad experience. Unfortunately, there’s a lot of this “I never checked the basics, I just threw parts at it” type maintenance going around. It’s a major reson why so many people get stressed when they have to take their car to a shop.

When my granddaughters 93 Buick Skylark showed “black oil” in the coolant reservoir I asked my mechanic about it . He said “It isn’t really oil’ I don’t know just what it is and they all do it. It doesn’t seem to hurt anything.” By all do it he was referring to GM cars of that vintage.

The oil might be coming from a leaking transmission fluid cooler in the radiator. have you checked the transmission fluid level/condition?

Tester