Expansion Tank Level

We are all a bit anal in this forum aren't we? ;)

To the OP:

1. The coolant level in your bikes expansion tank has absolutely no affect on the running temperature of the engine.
2. The proper level for the tank is simply exactly as it says. No less then the Min and no more then the Max. It is as simple as that.

Example
A. Fill it up to much above the Max level and as the engine heats up the coolant would spill over into the street
B. Too little coolant below the Min line would result in air being sucked into the engine coolant as it cools down.

Ok, here is an explanation;
An expansion tank, also known as "overflow bottle", is used in the cooling system of most internal combustion engines, to allow the coolant to expand with rising temperature and pressure. The tank is also called a "coolant recovery tank", since it prevents permanent loss of coolant, by allowing it to be sucked back into the cooling system as the engine cools.

^^^ Without the tank air would be in the radiator, as the coolant would expand and overflow into the street when hot, then suck air into the radiator as the engine cooled down.
 
Owners manuals want you to maintain the bike in acceptable shape, but not worry about it. To it, anything in range is fine. The manufacturer doesn't want people obsessively worrying. And levels change with temperatures. And there's normal, slight evaporation.

Repair manuals want you bringing the bike to the highest state of tune, which is why they recommend filling to the max. There's a chance the bike has just been serviced, has air bubbles in the system that will burp out, sucking some of that excess back in.

When I have a nonpressurized overflow, I fill it to the full line. When the radiator cap lets coolant out, it's suddenly depressurized and might flash-boil. This sends steam down the hose, which condenses when it hits the liquid in the overflow. The more liquid there is in the overflow, the greater the chance is that all that steam will condense. So it'll lose less fluid, in theory. Being low on coolant starts a positive feedback loop that makes you more low on coolant.
 
That post above makes very good sense. And I subscribe wholeheartedly to fill it to the max line. However there are varying opinions on this issue regardless of what is done to every vehicle I've ever had or known of.
 
Minimum line has always been to my mind if you don't maintain stuff for a long time and it gets down here fill it back up. Max line is there so you can top off regularly, lol.
 
If filling it to the max line was all that was needed, why do they bother having 2 fill lines when 1 is all that's needed?.,,
They need a range. Same as an oil dipstick having "full" and "add", so you aren't opening the hood every Saturday putting two tablespoons in. And then calling your machine "unreliable" because of its constant need for top-offs.
 
So your saying, all you need to do is keep the fluid at the min line and you'll be good?

Any cold level between the "Low" and "Full" lines is good ... that's what most manuals are going to say. If the cold level is at the "Low" line, then add coolant, but don't fill over the "Full" line.

I like to fill to the "Full" line when the engine is fully hot under normal driving conditions, because that is the scenario that will result in the most constant coolant level, and will show more accurately if there is any coolant loss for whatever reason.
 
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