I took my car to the dealership for an oil change and inspection, which includes the fluids being checked and topped off. After the service, I took a look at the fluids in the car.

There is a tank marked "coolant" with F and L marks on it, and the the level of fluid in this tank was (significantly) below the L mark - it looks like there are about two cups total at the very bottom. I understand that this is the expansion/reserve tank that is used when the radiator requires it, and captures extra coolant when it expands.

I asked the service manager about this and he said that it was normal (even for the level to be below L) and that I should "check my owner's manual" to verify. I was somewhat dubious of this, and my owner's manual says the following:

The coolant level should be filled between F and L marks on the side of the coolant reservoir when the engine is cool.

If the coolant level is low, add enough distilled (deionized) water. Bring the level to F, but do not overfill. If frequent additions are required, see an authorized dealer for a cooling system inspection.

When I checked the coolant level, the engine was still warm and not completely cold. Could this have caused the coolant reserve level to be lower than usual? Is there another reason that I could have observed this, or is the service manager pulling my leg and the mechanics neglected to check the fluids?

Should I add fluid to this tank myself? Why distilled water rather than an antifreeze/water mix?

5 Answers 5


There is nothing inherently wrong with what the service manager told you. (As an aside, service managers and mechanics are looking for add-on work. They get paid more money for this work than work you bring to them to do. If he says it's not a problem, believe them, because if they could make some money off of you, they'll do it.)

If it serves your peace of mind to add some coolant to the overflow, there is no issue with this. Follow the vehicle owners manual as to when to check it (hot or cold) and add fluid accordingly. Anywhere between the marks is golden. Check it every so often to ensure you have the right amount of fluid in it. If there is an issue, it will all of a sudden be too high (over flowing high) or too low (bone dry). If it does this, then worry about something being wrong.

You can add distilled water (which you should use instead of tap water ... tap water will have minerals in it which can react with metals in your engine and radiator and cause sediment in your coolant system). I think a better choice is to purchase 50/50 pre-mix antifreeze/distilled water. Purchasing a gallon of it, topping off your system, then leaving the rest for a later date when you might need to add a little more. My understanding is there is no shelf life on antifreeze, so put the top back on it and be good. There are three or four different types of antifreeze you can put into cars these days, so ensure you are using the correct antifreeze for your application. These are all color coded, so it should be an issue (green, orange, yellow, and possibly red). Each one is specific to certain vehicles and you don't want to mix them. It's not that you'll cause the engine to blow up, it's just that the coolant will not behave like it is supposed to for longevity and cooling effect if they are mixed.

  • 3
    They were supposed to do this for free, so I'm thinking that maybe they just skipped over all the fluids that were supposed to be checked. The wiper fluid was not topped off either. Also, the reserve tank looks almost bone-dry to me.
    – Andrew Mao
    Feb 28, 2015 at 22:50
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    @AndrewMao ... My only suggestion to you is do not utilize this service department again. Take it to a different shop which wants to service your vehicle the way it should be serviced. Fill the reservoir up to snuff as well as the rest of the fluids. I never trust another mechanic to do the work I'm more than capable of doing ... as you can imagine, I don't take my vehicles to the shop very often ;-) Also, when you go to a new place, you may want to contact the management of the establishment (the boss of the service manager) and let them know they lost a customer because someone was lazy. Feb 28, 2015 at 23:08
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    That's good advice, but I'm trying to reconcile that with your initial assessment that there was nothing wrong. In any case, I guess we've established that the coolant tank being so low is not "just normal".
    – Andrew Mao
    Feb 28, 2015 at 23:51
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    It all depends on what "low" means to you. Your original assessment seemed like it was just a little below the low line ... Now you are saying it was much lower than that. If it's empty, I'd worry about it. If there's still some in it, refill it to the proper level and keep an eye on it. If it happens to be low again, then worry about it. Mar 1, 2015 at 3:00
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    They probably recommand to add distilled water since to prevent some one from mixing two type of coolant who are not compatible.
    – Rémi
    Dec 23, 2015 at 15:40

I am not certain I agree with any of the assessment on this link, with the exception of the one that states, if it is consistently low / or empty, you have a problem.

I have a 2003 Nissan Maxima with 196.000 miles on it and recently the reservoir tank has been consistently low, even after refilling it.

I would contend, that if this very same scenario is happening to others, there is indeed a problem. The radiator is a sealed unit, so constantly having to refill any overflow tank is , in and of itself telling you something else is going on. Head Gasket, water Pump, Hoses and a host of other possibilities.


It might be normal. Presure builds up in the cooling system when driving so it can look like it's below L. Best way to check the level of the coolant would be on the cold engine or to carefully let the pressure out of the system buy slowly turning the reservoirs cap. WARNING! If the pressure is in the system and engine is hot it might burn your hands or something else so be really careful and turn the cap really slowly and stop as soon you hear fizzling. after it stops fizzling turn the cap a little bit more and do this little by little till all the pressure is out

  • If pressure built up, why would it be low instead of high? Even if this was the case, my coolant was still low when the car was cold, so I don't this applies.
    – Andrew Mao
    Dec 23, 2015 at 2:44
  • the pressure applies to the reservoir as well which pushes the coolant down by a bit. but if the coolant level is the same with the cold engine then it's not the case.
    – Einius
    Dec 23, 2015 at 13:51

I have a Subaru and the expansion in the overflow tank when the car has been running in city traffic is enough to take it from the "cold" mark on the tank to the "hot" mark. That is a fair bit of expansion. The car has to be switched off for 4 or 5 hours for the coolant return to the "low " mark.


I have two cars, both of which the coolant level "assumes" a level well below the min mark.

I have yet to find an explanation on line. If I add coolant, it will escape, I will smell it, and if I ignore it, the level will end up at a lower level and everything is fine in the long run.

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