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Theres oil in the radiator and the overflow tank of my 2002 GMC Envoy. The oil is fine, and has no water in it. The SUV gets regular oil changes, and the oil presure is fine. Water temperature is mostly normal, never overheating (not yet anyways).

What's got me stumped is that the oil is not low despite the amount in the coolant, and the oil does not have water or antifreeze in it.

Is this a blown head gasket? Or maybe someone at the oil change station had no clue what they were doing and put oil in the overflow tank? I noticed this oil about 6 days after changing the oil, when the sixth cylinder started to misfire.

I changed all the plugs and coils, but in the process found oil in the overflow tank. Engine is running fine and sounds normal at this point.

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    Define "a lot of oil" in your overflow tank. This is a subjective statement which needs explaining. To me, a lot of oil means it takes three flushes of the cooling system to get it clean after fixing the issue. You could just be saying its floating some in the overflow tank only.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 25, 2014 at 21:55
  • Same issue on a 2000 Cadillac Seville SLS. Floating oil in the coolant reserve after being at Firestone in addition no coolant in the reservoir and massive coolant leak( the week before the system was professionally leak tested and was 100% solid. Any ideas? Oil is new and free of and foreign fluid filter is old. Not a clue had 2 ase Mechanics look at it both said that Firestone had to have pour oil in the overflow. I haven't been able to flush the system yet due to weather . What are peoples thoughts? Is it save to let sit til spring when I can work on it, as long as I don't run it?
    – Michael AE Krengel
    Nov 22, 2016 at 20:37
  • oil cooler could be bad but I agree with other people saying the oil change people screwed up and put it in there those places are notorious for screwing up everything leaving drain plugs loose overfilling setting tools on the battery and shorting the computer all stuff I've seen so far this year and it's only April
    – Mackenzie Morter
    Apr 9, 2017 at 16:20

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My sister has a 2000 Cadillac DeVille And it's doing the same thing. A car has two lines that that the radiator cools the motor oil and the transmission fluid her car has oil in the radiator but not in the engine the line for oil has a Crack in it and that's what's causing the oil to get in the radiator fluid. So the line has to be replaced.

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I am going to assume you took the vehicle somewhere and had them change the oil? I would put money on it whoever that someone was, put oil in the overflow by mistake, realized they did it, and put oil in the crankcase while not saying anything about the overflow. The key to me for this is when you said:

Im talking about at least a quart of oil in the overflow bottle im talking straight up oil like i just poured a bottle right in there

If it was this way, you'd expect to see some oil get into the radiator as the expansion/contraction of the radiator fluid inside the cooling system is going on during normal operation. It probably wouldn't draw much in, just enough to make a mess. I think you cooling system and head are going to be just fine. Sounds like you already flushed it at least once. I would run it and see if it comes back. You may see a little residual. If so, flush it one more time.

BTW, the oil under the cap would be normal for this type of situation. Just clean it up really well and it should be fine.

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Could be someone put oil by mistake, I had lady friend took car to have oil changed and after about a block or 2 Engine lock up. They FORGOT to put oil in so things can happen. I'd flush rad a few times and test drive see if it happens again. Also does it have a trans cooler seperate from radiator or does it have lines in the rad for cooling, that could put oil in radiator and I am guessing that it could push fluid back to over flow tank..

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