How do I measure my car's engine coolant level

I have a 2007 hyundai Sonata. Under the hood there is a sticker on the engine coolant reservoir it says to measure coolant levels while the engine is cold. When I do that, the coolant levels are low.

In the owners manual it says to measure coolant levels when the engine is warm. When the engine is warm the coolant levels are correct, within the min and max lines.

Which one is correct? Should I go by the official manual or what is written on the engine coolant reservoir on the car?

What is the higher risk, too much or too little coolant? If I increase it to the minimum when the engine is cold, will that damage the engine if after warming up the level increases beyond the max?

My impression was that if you put too much coolant in then when the car gets hot the excess coolant is flushed out via an overflow valve–and thus wasted. So you should worry much more about too little coolant.

But some experts will shortly show up.

Too much coolant it will simply blow out the excess making a mess. If you find a cars coolant tends to ride in the same place I just leave it. My plastic reservoir only has a small amount of coolant when the car is hot. If I fill it within a few days it goes right back to the same place. I haven’t added coolant in years.

Most cars in my experience have two marks on the coolant reservoir: a lower one marked ‘cold fill line’ and a higher one marked ‘warm fill line’, or words to that extent. Do they not do that anymore?

Only open the radiator when the engine is cold.
You can look in the overflow bottle at any time.
When warm the level should be between the two lines on the bottle. If you have it at the midpoint or a bit above hot it will probably be above the min line cold.

Are you confusing that with one of the other fluid reservoirs? It’s the power steering fluid (IIRC) that has separate hot and cold markings (at least in my car, a Toyota Camry).

I’m sure some automotive expert will come along soon to correct me, but the exact level of the coolant in the overflow tank doesn’t matter too much. Just make sure that it isn’t so high that it overflows and runs out (though that problem will correct itself eventually), or so low that the overflow tank completely runs out when the engine cools down so there’s no more fluid for the radiator to suck in.

Much more important is the level in the actual radiator/cooling loop. Wait until the engine is cold. If you can squeeze the hoses that go to the radiator with little resistance it should be cold enough. Then remove the radiator cap and make sure the level is close to the top.

20 year mechanic here.

  1. Too much coolant will not hurt, there is an overflow hole/tube that spits extra onto the ground, as previously mentioned.

  2. There is no standard across all cars for how to check it. Some specify hot, some cold.
    I have seen coolant bottles with hot and cold fill lines. I’ve also seen them marked min/max with no mention of temp.

If the manual is a factory manual (not a reprint/aftermarket) I would pop over to the dealer and ask the service guys. Point out there are conflicting instructions on the factory bottle and factory manual.

Lastly if it were my car, I’d fill the overflow bottle to the top while the car is stone cold and let it purge what it doesn’t need.

You’ve answered your own question. You said it’s between the min and max lines when hot. You’re technically a little low but it’s OK. The tank you’re looking at is a combination of overflow and reservoir. This use to be done with a tank built into the top of a radiator. It had to run with an air gap because there was no place for it to overflow other than the ground and you don’t want that because it’s a deadly poison to animals who find it tasty.

Your radiator runs full all the time and uses the reservoir as a buffer. You always want to have enough fluid in the tank so it’s available to be drawn back in when the engine cools down. You don’t want to introduce air into your engine as that can cause problems. You also don’t want too much in the tank when it gets hot and ends up flowing onto the ground. When it’s stone cold there should be fluid in it and when it’s been run hard on a hot day there should be an air gap in it.

You’re doing the right thing by keeping an eye on it but it doesn’t have to be at an exact level.

unless the purge line goes into a temporary container you’re promoting the dumping of a serious poison on the ground.

There’s no point in filling it to the top. Starting with a cold engine, the level in the overflow reservoir will rise – no sense in making a mess. I’d fill it about halfway.