i know this can be a pretty generic question and would apply to all cars but I'm no mechanic so here it goes.

i have a bmw 318 2007. I have recently topped the coolant up correctly so the float rises to the correct position without over filling.

I always check my levels regularly under the engine but my question is.

when checking your coolant levels , obviously when the engine is cold , what should the levels be? say vs hot?

I understand where you top up up is an overflow but should coolant always be visible and to the top when the engine is cold?

I am just trying to understand this.

thanks for your help! nathan

1 Answer 1


There are two types of coolant level management system. The goal is to ensure no air remains trapped within the coolant system. Air voids in heater core degrade heater performance and make noise. Air voids in the top of the head lead to poor cooling, possible warping. Bad stuff.

In one type there is an overflow bottle, fed by a single small diameter hose attached to the fill port on the top of the radiator. The radiator cap (on the top of the radiator) has two valves.. One to blow off excess pressure (13-18 psi) and the other that works when the car cools down. In this type of system, there should be NO air anywhere within the radiator and cooling system. The overflow bottle is marked with recommended level so the system can do its job purging air during multiple drive cycles (hot, 8 hour cool down, repeat) When the vehicle is cold, and you open up the radiator cap and look it, the coolant should be at the very tippy top. Note, in my experience it generally takes about three drive cycles to purge all the air out of the system.

The other type of system has a surge bottle. Generally the top of the bottle has a pressure fill cap. There is no cap on the top of the radiator. There will be generally three hoses running to the surge tank, and the diameter of two of those hoses will be 1" or more. The bottle will be located high in the engine compartment. Because of the surge tanks high location, all the air in the system will accumulate there where it is harmless, and not remain in other parts of the coolant system. These systems self purge air from the system during normal operation, because of its height and fluid flow in Thermostat open / thermostat closed conditions. Again, recommended fluid levels are molded on the surge tank. These are important. If you are low here, its bad.

  • hi thanks for your reply. that makes sense, what in theory I guess I'm trying to understand is let's say now I have added coolant, so the float in the expansion tank has risen to the correct markings. let's just say I do a 20 mile drive round. obviously then if i was to check again when the engine was cool I.e the next morning, should the coolant level still be at the same level in the expansion tank? I have heard different theories on this that levels can range but I mean from manual check should their always be water visible in their. and what would it be cold v hot? I'm just intrigued Commented Oct 4, 2019 at 16:14
  • Can you help me understand which system you have? How many hoses are running to the overflow/surge bottle? One small one? Or multiple large hoses? A photo is best, but not sure you can post one until you have X points.
    – zipzit
    Commented Oct 4, 2019 at 16:20
  • it doesnt seem to allow me to add photos? but from the overflow their is 2 pipes/hoses one at the bottom and one at the top. Commented Oct 4, 2019 at 16:32
  • So probably a surge tank, with coolant flow into the bottle all the time. Keep it simple. I'm assuming the bottle is marked with Coolant min/max markings (When Cold). Hot coolant expands a little bit so the levels may be off, relative to the cold fill lines. Check the coolant when cold. Just stay between the lines. Have a nice day.
    – zipzit
    Commented Oct 4, 2019 at 16:45

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