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?

  • That's crazy I have a 2001 Mazda 626 and I just did the same thing. I barely put any in the coolant but I was checking both the coolant and oil, as they both leak as well. How long has yours been leaking? I have 190,000 mi. on the car. If I poured a small little chug or two should I drain all coolant and remove all oil?
    – user23104
    Oct 14, 2016 at 20:29
  • If you did not run the engine then not much chance of it circulating. Car and naval mechanics back in the early to mid 1900's used various oil type items in the cooling system from water soluble oil, cutting oil and such to keep rust out and help lube the pump. And even kerosene was used as a cooling liquid in extreme freezing temperatures before the newer anti freezes were out. Oct 15, 2016 at 0:26

3 Answers 3


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.

  • 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. Mar 5, 2012 at 11:02
  • 2
    For what it's worth, this is a better answer than it would have been as a comment.
    – Bob Cross
    Mar 5, 2012 at 13:02
  • 1
    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, 2012 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, 2012 at 2:29
  • @kahbou - no need to apologise, I'm glad you clarified this. Mar 6, 2012 at 4:56

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.

  • Just FWIW, oil in the coolant is likely to upset the radiator or the thermostat rather than the actual engine.
    – staticsan
    Mar 6, 2012 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, 2012 at 2:28

You should flush the contaminated antifreeze out. Additionally, use of a surfactant will help clean out some of the residual oil.

The residual oil doesn't appreciably impact thermal transfer, but it may have an adverse effect on hoses. It will not hurt the bearings or seals in the pump, at least for any of the pump seals I have seen.

It's not necessary to use a flush, but for a surfactant, I would suggest about 1/2 oz of Dawn dishwashing liquid. Run that in the cooling system for about 20 minutes after the engine reaches temperature. Then flush with water, again running the engine to full temp and then for 20 minutes. Likely a total of three flushes will be necessary, depending upon how completely your system drains. Pulling a lower hose can sometimes speed the drain, and may get more of the old fluid out.

The oil will not impact the operation of the thermostat. The biggest impact is on hoses, in that it may accelerate degradation. Your mileage may vary, as some hoses are very resistant to oil, and some are less so.

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