Gonna keep this one simple and to one narrow topic...

Are the markings on the coolant reservoir meant to be adhered to when the engine is cold? Or hot?

I've been told both by mechanics.

There is a low mark, and a high mark. One mechanic told me it should not be above the high mark when at running temperature. Another one said cold. I've been filling to the high mark when cold. (The high mark is at the seam, correct?) Of course, it should not be below the low mark. But... hot? Or cold? Or low mark cold and high mark hot, or....

Sorry to be so dense, but I am seeing ... things... that don't make sense and it hinges on first getting the proper level.

I've had the usual series of unfortunate events with cooling, you can probably guess which and in which order. I MAY be down to just my own paranoia and possibly over-filling repeatedly.

OK, I lied - one more question: where is the overflow? Is it just some pressure release in the cap? Or somewhere else? If the system is over-filled, where will I see evidence of coolant overflow?

Kewl photos taken with UV flashlight and UV dye in the coolant later. But first, I'd like a clear consensus on the how to interpret the fill markings, and would be great if some definitive reference and rationale.

Thanks!

 

On mine, it always stays at the seem or full level when everything is all sorted. I did just replace the water pipe, and had to bleed the whole system. When you have a leak is when these overflow tanks seem to have a mind of their own. For example, I went down to a pull and pay to pick up and intercooler boot I needed, coolant was fine heading down there. They had a very rocky and rough driveway. Came home and noticed a small spot of water, thought nothing of it since I had used my ac. So I changed out bad part, topped everything off, took a 5 minute drive and everything looked good. Overflow tank was at full mark. Next day come out and noticed water on ground, check it and it’s coolant, so I popped the hood and the overflow was damn near full. I just shut the hood and walked away. 2 days later she was completely empty. Ever since I have replaced pipe and bled system it has stayed at the full mark hot or cold.

 

Hmmmm, that seems inconsistent with known chemistry!

Water is a funny thing. It expands in volume when it gets hotter. And contracts in volume when it gets colder. Until it doesn't any more. (When it freezes). And then it expands when it gets colder.

So, if it doesn't change level as it gets hotter or colder, I'd think that actually indicates something wrong. It's expanding/contracting somewhere else in the system. Maybe a gnome installed a second expansion tank? Maybe not bled? I dunno.

Thanks, though, it's helpful to know that the level can be "wacky"!

I just want to get the fill level right for starters, and then see if I still have evidence of leakage.

 

There are no leaks and I know my system holds 15 psi of pressure. I have been a little more attentive since I did the repair. Check level yesterday before going on 5 hour trip, while it was cold, and it was at the full level. Came home, waited for cooling fan to kick off, yes fan not aux water pump, popped the hood and she was at the full mark still. Water temps during drive never went above 222, in fact the stayed around 216-218.

 

The factory procedure says to fill the expansion tank to the seam. It doesn't make reference to hot or cold, but the step is right after the vacuum fill of the system, so my assumption is when cold.

 

Yes, I would assume the same, but mechanic told me otherwise. It may be different manufacturers quote different criteria for the meaning of high and low levels?

Have you noticed if the level rises when hot?

I can idle with the hood raised, wait for it to heat up, and you can see the level slowly rise above the seam. I took a sequence of photos the other day. Doesn't rise dramatically, (not to overflow), but I have it filled right now to a bit above low mark, and it rose to maybe 1/4" above the seam. Actually, about the same amount above the seam as it is above the low mark when it is cold.

Maybe I am the odd one out. If it is SUPPOSED to stay the same, it would seem there's something wrong. (But where does the coolant go when it expands?)

I am taking it in on Thursday to the mechanic who changed to cross tube and thermostat housing. I will ask about specific pressure value when he pressure-tests it. Good to have a reference.

I am able to monitor temperature using DashCmd app, and have not seen excessive temperature.

 

The coolant volume most definitely expands when the engine heats up. That's why on modern cooling systems the tank is called the expansion tank

(On older cars you had an overflow tank, which was at atmospheric pressure and external to the radiator cap seal)

I fill mine cold to the seam on the tank. Seems to work fine. Needs a top-up now and again, I assume because of vapor loss through the vented cap.

 

Well, that answers my question #2. The cap is vented, thus that is the "overflow" I guess?

Or not? Is it just because otherwise, the expansion tank would be subjected to positive and negative pressures as the coolant level goes up and down, and likely would fail catastrophically....?

Is there any separate "overflow tube"?

Showing my ignorance about cooling systems!

 

The overflow from the expansion tank goes back into the radiator. Diagram courtesy of RealOEM.com

 

The cap is not vented. The system is under pressure when hot. All the hoses and so on expand as well.

Fill when cold. Never touch the system when hot. Don't open the overflow tank when hot. As long as the system is vented properly, you shouldn't see a big change in level.

 

Thank you for the clarification post. I know it seems excessive but not that hard to take a peak under the hood when cold and after running for a bit, which I’ve been doing and there hasn’t been a change. But what do I know, Not like I’m running an n14 with 162,000 miles or anything, and haven’t replaced a radiator, aux water pump and crossover tube, in the past month, not to mention I had a mini tech show me how to pressure test the system. Seriously if you think you are loosing coolant put 15 lbs of pressure on the overflow tank and look for the leak, if it isn’t holding at 15.

 

The cap is absolutely vented, and actually it has two valves. The first opens at a 2 bar (29 psi) pressure in the cooling system to prevent cooling system overpressure. The second is vacuum relief and lets air back into the system if needed as the coolant cools down. If you didn't have the first valve the coolant hoses or tank could pop, if you didn't have the second the hoses could collapse as the coolant cooled back down.

The system builds up pressure as the coolant heats up because its vapor pressure increases and because it expands and reduces the head space in the overflow tank. In practice many cars can vent a bit of vapor as the cooling system "breathes", so to speak.

Not really, no. If you look at the system the radiator is below the level of the expansion tank, it should be completely full with coolant. I think that line is just for pressure equalization between the expansion tank and the top of the radiator, or to let any trapped air escape from the radiator.