I know engine temperature should go down to "C" mark before opening radiator cap.

However I'm finding that the temp just stays on normal for a long time. I think I've read people say you need to give an engine 30 minutes or so to cool down before you drain oil. However after more than 30 minutes my engine temp is still on normal.

Is this normal? How long should I wait before I can expect an engine to cool down? When should I consider that I might have a problem and what type of problems might cause long cooling?

  • 2
    How long it takes for the temperature gauge to go to "cold" depends very much on the weather. If the air temp is 25C or 30C it might well take an hour or more for the temperature gauge to drop. If the air temp is 0C, you won't have to to wait so long! – alephzero Oct 10 at 17:54

The exact time taken depends upon various factors:

  • The ambient temperature
  • The amount of heat insulation in the engine bay
  • Volume of coolant in the engine
  • How hot the engine got while driving
  • material used in construction of the engine block/head (aluminium blocks cool quicker than steel for example)

and many more besides.

Typically though it should be left to cool for 1-2 hours before it's cooled sufficiently to open the rad cap safely. If you're actually trying to check the coolant level then you probably need to wait more like 3-5 hours (as if the coolant is still warm it will show a falsely high reading due to expansion).

I realise that sounds like an awful long time but you have to remember that after you turn the engine off a fair chunk of the residual heat from the engine is actually still being passed to the coolant (which is no surprise as the raison d'être of coolant is to remove heat from the engine!) and since you are no longer running the water pump and rad fans (in most cases - some cars will have after-run systems for these. Mine still runs the fans for quite along time after ignition off if the car has been driven hard for example) and there is no longer any significant airflow over the radiator (unless the wind happens to be blowing that way I guess) the cooling system's ability to remove heat from the coolant is a now purely passive and a fraction of what it is when the car is running.

The reason you can do engine oil changes after only 30 mins is because you aren't waiting for it to be "cold" - in fact oil is much easier to change while still warm as viscosity drops and it will drain much easier and faster.

  • 1
    +1 for raison d'être. Also, where the oil comes out, when you drain it, is NOT under pressure. – mike65535 Oct 10 at 14:45
  • @mike65535 yep.. the real "danger" with opening the rad cap when the coolant is hot is that you are generally right over said cap (in order to open it) and that physics takes over, the reduction in pressure sees the coolant boil and you get a nice face full of hurty steam. If you open the oil cap a bit too soon you might get the odd bit of hot oil on your hand and some minor superficial burns. – motosubatsu Oct 10 at 14:50
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    Also, oil that hasn't had time to cool also hasn't had time to settle, so you're likely draining more junk out of the oil pan when you do it warm than when cold. – computercarguy Oct 10 at 17:41
  • Even if you open it too soon and there's no coolant immediately visible, it may surprise you after a few moments with more of said, "hurty steam." Also, I LOLed at "hurty steam" – seizethecarp Oct 10 at 17:52
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    Also if the coolant level is very low and you refill it with a hot engine, you might be pouring cold water straight into a hot cylinder block, with the risk of cracking the head gasket if you forcibly cool the block but not the head (or vice versa) or even cracking or warping the block or the head themselves in the worst case scenario – alephzero Oct 10 at 17:57

If you have any doubts, don't do any of this, just wait til the system is cool to the touch. Steam is extremely dangerous!

With that said, you can open the system a lot sooner than a matter of hours, and possibly much sooner than 30 minutes, if you need to. The easiest way to gauge if it's under sufficient pressure that you shouldn't open it is trying to squeeze the upper radiator hose. Get a feel for what squeezing it is like when it's cool, and compare when it's hot (using a cloth or insulated gloves since it's going to be unpleasant to touch with your hands). If you can compress it with your fingers, it's not under extreme pressure, but there could still be enough pressure to shoot out hot coolant/steam when you take off the cap.

Cooling can be accelerated by leaving the key in the on position with the engine off, on vehicles where that causes the fan to run. You can also start, quickly rev (to circulate the coolant via the water pump; 1-2 seconds is a plenty), and stop the engine a couple times while it's cooling to speed up the cooling process. In a real pinch you can spray/pour water on the radiator. Rapid cooling like that can lead to cracks, but the radiator itself should handle it fine because that's what happens in a heavy rain anyway. Doing the same on the engine is not a good idea.

Once you think it's cooled enough, cover the cap in large, thick towels, one of them laid out across the entire area around the cap and one or more folded in layers at the point of the cap itself, press down forcefully on it, and turn. Very gently reduce the force and listen for escaping steam/coolant. If you hear it, quickly apply more force and turn clockwise to close the cap again, or, if anything starts to feel hot or you feel like you can't get it back on due to the pressure, let go and back away from the vehicle fast. This makes more sense and is a lot easier if you've practiced it on a cool system beforehand.

  • 1
    I can't say I've ever lined up towels over my radiator to open it up. Hot to the touch, I leave it alone, but turning it slowly and listening, as you mentioned is the key. While in a pinch, back in the day, I have used a car floor mat to open the cap on an overheating car, without scalding myself. – Gary Bak Oct 11 at 11:10
  • The towels (or floor mats or whatever) make a huge difference in safety/protection if something goes wrong. At worst I've gotten clothing slightly wet/dirty using this kind of method, never any injuries, even when the cap blew off and coolant spewed out for several seconds. – R.. Oct 11 at 12:57

The reason you wait is that when the engine runs, the water absorbs its heat, and transfers it to the radiator where it is released to the environment, and cools. The water becomes pressurized and the temperature rises to boiling. So if the cap is released before the system has cooled, first the cap will blow off explosively, and second boiling pressurized steam and water will escape (possibly higher than 100C, due to the pressure), and scald anyone/anything nearby. So you are waiting to allow the water to cool enough to be safe.

The fact that the heat arises from the engine gives you a good way to check the radiator cap safety. When still, the water will broadly have a similar temperature to the engine block. It shouldn't be possible for the engine block itself (the core metal part of the power unit, where the pistons are) to be lukewarm or cool to the touch while the water in its cooling system is still boiling and pressurized. So if in doubt, you should be able to safely double-check that way.

Three other safety tricks if you do.

  • Be sure you're checking metal and that it's in intimate contact with the cooling system. That means only trusting the feel of the engine's power block, or the radiator grill, not plastic or rubber or other things close to them. And of course, be careful as these can/will be very hot too.
  • Use a folded (multi-thickness) dry towel to open the cap. It's a safe good practice that means if you did somehow make a mistake, your hand and body has at least some protection from the heat and steam blast. Also more of it would soak into the towel (which you can drop), not your clothes (which you can't).
  • Only crack it open a little at a time. That way, you should have some hope of hearing escaping pressure. (A bit like if you open a shaken/dropped bottle of soda a little at a time instead of all at once, to stop it exploding fizzy water out!)

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