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The radiator caps in all cars seem to be very tightly installed. In some cars (at least in my 2011 Toyota Yaris), the radiator cap is not round so you can remember the position it was at, but in other cars the radiator cap is round and there is thus room for mistightening it. Now suppose I remove and reinstall the radiator cap but forget to tighten it as well as it was tightened previously.

What problems could such loose radiator cap cause?

Can the coolant evaporate through the loose cap, requiring filling up the coolant?

Is it possible that the engine overheats if the radiator cap is slightly loose?

I found a related question, Can a loose radiator cap cause P0420? where it was asked whether a loose radiator cap could cause an error code. However, there was no question about other problems that a loose radiator cap could cause.

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Cooling systems are under pressure to increase the boiling point of the coolant. This allows the system to operate efficiently without boiling off the coolant and overheating the engine.

A loose radiator cap will cause the system not to pressurize, resulting in overheating. In this case, the coolant is more likely to be lost to boiling off than by simple evaporation.

Depending on the vehicle, it may or may not have the ability to give codes. Can't answer this because your question is not vehicle specific.

Finally, the resulting overheating may strand the driver since the cooling system will have to cool down before it can be refilled and driven. If the engine is operated while overheated, severe mechanical damage may occur.

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  • This answer is not completely correct because it is true in rare cases only. Because most engines work at about 90C which is below boiling temperature. The 50/50 glycol/water mix has a boiling temp of 106C. The pure water boiling temperature is 91.9C at altitude of 2438m so if you are living on a very high mountain, you may have boiling problem (but most of us do not live on mountains). In either case, the only thing to do is to run the engine and look if liquid overflows from the cap or not to figure if it is tight or not. – Evren Yurtesen Feb 6 '17 at 19:41
  • Maybe 90C is the average. Other parts of the engine are probably hotter. All I know is that when I had a bad seal on my radiator cap on my 2000 Toyota Sienna minivan, on warm days or long idling at stop lights the temp gauge would run abnormally hot. I would slowly lose coolant and would have to add coolant every few weeks. Took me a while to figure out it was as simple as a bad seal; the rubber gasket had somehow gotten folded over in one spot. – Vince Patron Feb 6 '17 at 20:04
  • My objection was against boiling of the liquid because it won't reach boiling temp normally. You lost liquid because it expands when it gets hot and your cap is not tight (therefore it will leak). But you will see that you are loosing liquid. Especially if you are trying to figure out if your cap is tight enough, you can simply check after a drive for leak. You told yourself you drived weeks with leaking cap, so it won't destroy an engine just like that. You can just keep an eye on the liquid level for a while after opening cap. In either case, liquids in engine bay must be checked regularly. – Evren Yurtesen Feb 6 '17 at 20:24
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Open the hood, check that your radiator is fully filled by opening the cap. Then tighten the cap and run the engine until it reaches operating temperature. If you see liquid around your radiator cap, then it is loose or broken and must be fixed. The most used coolant liquid is pink/red color (see your car manual for the color/type) so it is easy to see even after it dries, therefore easy to identify leaks.

If you do not see any leaks, you are good to go. If your liquid level is going down (remember to check with cold engine), this means you have leak. I can't think of any reason for coolant to evaporate. Because system is under pressure and the reservoir is the highest place where the liquid is, it would just leak out if it is not tight enough.

The information about boiling and destroying your engine is possible but unlikely to happen as long as you are careful. You have to be at altitudes over 1500M for your coolant to boil at normal operating temperatures. Because pure water boils at 95C at 1500M and glycol in your coolant already increases the boiling temperature (50-50 mixture has boiling temp of 106C at sealevel) and normal operating temperature of engine is between 85C-95C (on most car the needle sits on 90C mark when car warms up to normal temp). In addition, if you have a leak in your system, you can loosen the cap and drive slowly/short distance while avoiding pressure buildup [ref] Because if you have a leak, with pressure you will loose liquid much faster than evaporation or through the cap. You will be fine as long as you have liquid in your reservoir and you are not living on very high altitudes. :)

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Short answer: engine overheating by loosing coolant. And other less talked effect: rotting of metallic parts around the radiator, especially the support underneath it (even more pronounced below the rubber supports).

Cooling system is an under-pressure, isolated system that shouldn't have leaks and/or air intake. When the engine reaches its operational temperature, coolant is just a pinch below boiling point (thanks to be under pressure). If there is a leak, pressure is lost, boiling point is reached, increasing the leak and loosing coolant to the extent to get the engine overheating because a lack of it. If the system is "open" somewhere that could suck air, it is also an engine overheating risk point, since air pockets interferes with the system function as well.

Loose cap = possible leak from the cap (liquid or evaporated coolant) OR cap internal valve damaged or not sitting well in its base. I could be possible to have a cap tight without no leaks, but having its valve damaged, which would lead to overheating as well, since the cap won't be managing system pressure as it should, probably sending coolant to the system's expansion tank.

Be careful when replacing caps: the new one should have the same operating temperature AND system pressure (PSI) than the one original for the car. Lower PSI will make it send coolant to the expansion tank too soon, higher PSI will put the system under too much pressure with the risk of blowing a hose, damaging the pump, probably even the head gasket...with high temperature coolant!

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