I've got one question; I was looking over this car that I was going to buy and there was engine oil in the radiator. I asked the guys why it was in there and he said he didn't have enough coolant to fill it so he used engine oil. He topped up the radiator and topped overfill bottle with oil.

Will having a radiator full of engine oil do any damage to the motor in the long term or short term?

  • 8
    oil in the water should be harmless (unless is could damage the thermostat?) but If he's lying there's a serious fault with the motor.
    – Jasen
    Commented Jan 24, 2016 at 10:14
  • 4
    I'd be VERY suspicious of this explanation. I guess it's POSSIBLE that he had a small radiator leak, didn't notice it until the engine overheated a million miles from nowhere, and he had no antifreeze or water in the trunk but had some oil. Or some such scenario. But the far more likely scenario is that, one way or another (see @Zaid's answer), oil is leaking into the antifreeze. I wouldn't buy this car, there's about a 99% chance that the seller is lying.
    – Jay
    Commented Jan 25, 2016 at 14:16
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    Turn tail and run as fast as you can. If he's truely saying this, it means he's so incompetent, he's a danger to himself and others. Using oil in the cooling system means that many seals may get damaged, the sensors for coolant temperature may get damaged and the heater core/matrix may get clogged. I would not even SLIGHTLY consider this car unless it was practically free and I was going to sell parts off of it for profit. I've done that many times with other badly-cared-for cars, but.... Oil in the radiator? Nah, that doesn't sound real to me.
    – yollooool
    Commented Jan 26, 2018 at 20:18
  • Engine oil in the radiator? On purpose, and the guy is telling you he topped it up himself? There is not a chance I would buy this vehicle. Walk away.
    – Miller86
    Commented Jan 31, 2018 at 15:16

8 Answers 8


The seller should not be selling the vehicle with oil in the cooling system. That supposed "quick fix" should have been rectified at the earliest opportunity. Have him flush the cooling system and put another few tens of kilometers on the odometer before you even consider such a vehicle.

If the seller is topping off the radiator with oil instead of water, then he is either lying, incompetent, or was caught on the side of the road with no other options. Leaving the oil in the radiator makes each option more likely in decreasing order.


I would be more concerned about whether engine oil is actually finding its way into the radiator from the engine.

If it is, this would be indicative of a compromised head gasket, warped cylinder head, or damaged oil cooler (if the car uses radiator coolant for cooling the oil). The first two items are not trivial to replace or fix. The third one isn't far behind.

If you consider the seller's claim of topping up the overflow tank with engine oil to be legitimate, the engine itself should be fine, but the cooling system will need thorough flushing at minimum.

  • Or he has a cracked cylinder block, and fixing that basically means replacing the engine.
    – Jay
    Commented Jan 25, 2016 at 14:11

Yes, it's possible some of the seals designed to withstand water and glycol could get damaged

I am thinking you are creating a fictitious scenario here, so I'll roll with it.

If you filled your radiator with oil and started your car and let it run for awhile I would be most concerned with damage to seals that were designed to withstand water and glycol.

Petroleum based products could damage these seals by penetrating and degrading them. I would imagine that some manufacturers are using material that would not be effected by petroleum products and some are not.

This is assumptive on my part and I don't have any citations to back my concern.

Aside from the possibility it might damage some seals, I don't see engine oil doing damage to your cooling system by simply being in the radiator.

  • 1
    The original question didn't specify damage to the cooling system only, but to the engine in general. You might want to expand your answer to include the possibility of engine overheating: cooling systems are designed to work with water/antifreeze. Oil has higher viscosity and lower specific heat than water, so thermal transfer will be affected. Also, film of oil lining the channels in the block could reduce heat transfer or even block small channels, frothing can produce a foamy emulsion which could block small channels and possibly cause water pump cavitation, etc.
    – barbecue
    Commented Jan 24, 2016 at 23:55
  • Also, IMO, this answer does the best job of actually answering the question as asked.
    – barbecue
    Commented Jan 24, 2016 at 23:57
  • If the cooling system was actually filled with oil then the small passages of the radiator would clog, and the water pump would churn the oil into froth. I believe that the OP meant that there is a non-negligeable amount of oil in the cooling system, not that the cooling system is completely filled with petroleum product.
    – dotancohen
    Commented Jan 27, 2016 at 13:23

Strange - engine coolant is antifreeze diluted with water. So the seller is claiming he had oil but not water? I would not buy that car.

For the seller, a coolant system flush would in order, both forwards and reverse, and then refill with antifreeze and clean water. If engine oil still appears in the coolant water then the oil is moving from the oil galleries to the water galleries. Check the oil filler cap for white creamy emulsion like this

enter image description here

This would be water going the other way, into the oil lubrication system.

If this is the case, engine oil in the water is evidence of existing motor damage.


I consider it quite more likely that it was the engine that had been putting oil into the coolant. Of course, it may also be that the seller poured some obvious additional oil on top in order to mask the defective motor seal, a very expensive defect to fix.

  • 1
    "The anti-freeze is only critical when you are parking your car" - could you qualify this statement or back it with evidence?
    – Zaid
    Commented Jan 24, 2016 at 12:43
  • Agree with @Zaid ... if you qualify what you are saying, I'll actually upvote your answer as it is along the lines of what I was thinking. Don't think anti-freeze is the only purpose for putting it in your cooling system. This, I'm sure, is why you have suffered two downvotes (at this point). Commented Jan 24, 2016 at 13:46

Have into account two factors: oil viscosity may be much or very much higher than the antifreeze fluid (glicol-polipropylene derivates) normally used: the radiator grille has capilare-thickness tubes and an hydraulic circuit (pump, filter, reservoir...) prepared for much less viscosity. Furthermore, the oil will becoming more and more viscose while the temperature increases... until a cold night (not so cold: just under 50º F) will become absolutely blocked (yes: around cylinders will regain the low viscosity state, but the radiator grille... well: I wound't like to find myself in a such situation).

  • 2
    One small point, I believe engine oil thins out as it reaches operating temperature. This post says it thickens. Otherwise right on the money. Do you know what viscosity oil was in that radiator? Not to mention, I bet he didn't flush out the coolant 100% so now there is an awful mix of the two.. Screams bad idea.
    – cdunn
    Commented Jan 25, 2016 at 13:44

As oil has a higher viscosity than water, I think it could damage the water pump because it would put more stress on it. Also, the oil would flow slower than water and this could lead to overheating of the engine.


The question I could do myself would be if that oil on radiator could'nt be the consequence of a damaged head gasket which pours oil from lubrication system to the cooling one (then, the guys could have improvised along the way any bad or worse excuse). Of course, I've purchased 4 2nd hand cars, and I would have rejected any candidate which had oid in the radiator: that means a both deffective lubrication and cooling systems. I will aquire a such car the day I feel I am too much happy.

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