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My 1990 Mazda 626 leaks oil like a sieve, so I regularly top up the oil.

In a moment of stupidity, I opened my radiator cap instead of my oil cap while topping up my engine oil, and poured about 50-100 mls of engine oil into my radiator.

What implications is this likely to have on the car?

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Normally, I would ask first in a comment but that seems to be a privilege not available to new users, so here goes:

Are you sure it was the radiator cap and not the overflow bottle? The radiator is usually full and getting 100ml in would be a struggle, not to mention that the green coolant would be staring you in the face. If you radiator wasn't full then you either have an empty overflow bottle, or your system wasn't burped properly the last time the coolant was changed, or you have a leaking head gasket introducing combustion gasses into your coolant system.

If it was the overflow bottle, there's a good chance that you car hasn't drawn the oil in from the overflow bottle yet. I say this because the pickup for the overflow bottle is near the bottom of the tank, engine oil floats on water and is insoluble in water.

My advice then is to remove the overflow bottle, clean it with a degreaser and refill with coolant. If you did actually get it in the radiator, let me know and I'll edit this answer.

As far as implications of oil in the radiator go: Oil will, in theory, attack the hoses and water pump seals. This would probably be a long term degradation rather than a sudden failure, and given the oil problem I doubt your engine will survive long enough to see the long term effects. My father's vehicle has been running with a slight oil-to-coolant leak for about two years, without any (visible) ill effects. The vast majority of the oil (after you've removed the head of oil floating in the radiator filler) will collect under the radiator cap as a creamy goo, which you can periodically remove.

Replacing the coolant wont do much to remove the oil that has coated the inner surfaces of your cooling system. If you have oil in the radiator and haven't run the car yet, siphon out the oil before it touches any more surfaces in the cooling system.

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Thanks for the detailed answer - it was definitely the radiator, not the overflow. I haven't put proper coolant in the car for a while, just water. It's a very old car (with 350,000kms on the odometer) and I'm just keeping it going until I find the time to look for a new car. It sounds like, from your comment, that if I'm looking to replace the car in the short term, then oil in the radiator won't cause me too many problems. –  LeopardSkinPillBoxHat Mar 5 '12 at 11:02
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For what it's worth, this is a better answer than it would have been as a comment. –  Bob Cross Mar 5 '12 at 13:02
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No worries. Sorry to question your question like that, I thought I'd mention the overflow bottle vs radiator cap thing as I've had a few friends fail to make the distinction. What's the correct etiquette here? Do I remove the overflow bottle bit and pretend it never happened? That seems to be the stackoverflow way... –  kahbou Mar 5 '12 at 17:26
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I think that you should leave your point about the overflow bottle: this isn't stackoverflow where code is code and sometimes there's only one right answer. The next person to find this question may have just poured oil into the overflow tank. That said, it's your answer - what do you think would be best? –  Bob Cross Mar 6 '12 at 2:29
    
@kahbou - no need to apologise, I'm glad you clarified this. –  LeopardSkinPillBoxHat Mar 6 '12 at 4:56
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You need to drain the coolant and replace it.

You should follow the instructions in your owners manual. If you don't have one handy, the ehow page is a fairly general set of instructions but it'll get you into the right general location for the coolant drain.

Keep in mind, engine coolant is toxic and should be disposed of properly. This is a perfectly reasonable time to consider having a professional flush the coolant system for you: they will dispose of the coolant properly, allowing you to avoid the hassle.

Oil in the coolant won't kill the car. It's something that can happen with a bad head gasket. However, it is definitely not a desirable state.

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Just FWIW, oil in the coolant is likely to upset the radiator or the thermostat rather than the actual engine. –  staticsan Mar 6 '12 at 1:20
    
@staticsan, I know: in this case, there's no implication that coolant is getting into the combustion chamber (or anywhere else). –  Bob Cross Mar 6 '12 at 2:28
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