Slight traces of Oil? In coolant reservoir only

['98 Roura]

Hey everyone. Got a slight problem with my 98 Aurora, for the past year she's been getting slight traces of a black oily substance floating on the top of the coolant reservoir and on the bottom of the coolant cap, a goopy white film shows up. It's not an awful lot though, like maybe just enough to fill a tablespoon , and I have flushed the system twice and each time, the substance does not reappear for over a month. It seems like it is oil, but I have never found any traces of coolant in the oil crankcase or on the bottom of the cap or dipstick. There is no white smoke coming out of the exhaust, the exhaust does not smell sweet, it is not overheating, and the most annoying part is that I have not found any oil in any other part of the cooling system, only in the reservoir. Also, this Aurora does not have the oil cooler lines going into the radiator, only trans cooler lines. Both times when I flushed the cooling system, I cleaned the oily residue out of the reservoir with soapy water, and after running water through the entire system and out the bottom of the radiator, I never saw any oil come out. Does anybody have any ideas to where this gunk is coming from? Because the car is not acting up or overheating, I've been trying not to worry about it, but I know that these northstars can be fussy, and irritating engines at times. Any ideas would be most appreciated. Thank you

[RCA1186]

Could be the GM cooling system tablet additives. Those tend to foam up in the coolant and make a white frothy film. I would think you would have removed most of it with the flushes though.


I think they sell kits to test for exhaust gasses in the coolant. I'd maybe pick up one of those.

[Marc]

Have a compression test done. That'll find a bad head gasket.

[tigger]

if you've flushed the system twice and it keeps coming back, I'd think you can narrow it down to engine oil or ATF. So failed head gasket or failed trans cooler/radiator...

You could dye and pressurize the cooling system for a day or so, then check your oil and ATF to see where the dye winds up. But from your description, it sounds like a very small leak so that may not be very helpful.

Or you could go cheapest first and replace the radiator. If it comes back, you're only out a hundred bucks and an afternoon of your time. And you'll know.