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Discussion Starter #1
When checking the coolant level in the overflow tank should the engine be hot or cold?
 

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if its hot then its under pressure and will blow boiling coolant all over you and melt off your flesh.

I've seen it happen.

Check it cool
 

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I'm not trying to contradict anybody here.

But actually, now that I am thinking about it, if it's just the overflow tank level you are concerned with, I seem to remember a mechanic telling me you actually want to check it when it's hot, to get a true indication of the coolant level in the overflow tank at running temperature.

You don't have the built up pressure in the overflow tank like you do in the radiator - and it really doesn't matter anyway, because you don't have to do anything except pop the hood and look at the overflow tank to see the level that the coolant is sitting at, that's why the overflow tank is see-through.

The coolant level at running temperature should obviously be higher then when the engine is cold, which is why you want to check the overflow tank level when it's hot.
 

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When checking the coolant level in the overflow tank should the engine be hot or cold?
From the Owner's Manual:

Engine Coolant Level
The coolant bottle provides a quick visual method for
determining that the antifreeze/coolant level is adequate.
With the engine idling, and warm to normal operating
temperature, the level of the antifreeze/coolant in the
bottle should be between the ranges indicated on the
bottle.

The radiator normally remains completely full, so there is
not need to remove the radiator cap unless checking for
antifreeze/coolant freeze point or replacing antifreeze/
coolant. Advise your service attendant of this. As long as
the engine operating temperature is satisfactory, the
coolant bottle need only be checked once a month.
 

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Discussion Starter #5
Thanks for the the replies, BB you grasp the question, your answer makes sense. Thanks
 

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The Proper Way to Check the Coolant Level:

When the Engine is Cool
Remove the Pressure Cap and check the coolant is within 1" of the opening that the pressure cap seals against.


The overflow tank is for the overflow from the fluid expanding. When the fluid cools and contracts, it will suck back coolant from the overflow tank. Unless there is a leak in the Cooling System, then instead of sucking back in coolant from the overflow tank, the system will suck in air through the leak.

I've seen plenty of vehicles, including my own, have the overflow tank right on the level and there was 2 gallons of coolant missing out of the system.

I also have a good friend that spent a week in burn center of the hospital because he opened the pressure cap on a hot vehicle, what Luckyse7ens says is very true!

The only way the overflow tank will indicate the actual level of coolant in the system is if the system working perfectly and has nothing wrong with it. And if the system has nothing wrong with it and working perfectly then why would you need to check the fluid level?

In short, wait for the engine to cool, remove the cap and make sure the coolant on the pressure side of the system is filled on the pressure side. If it is, then you top off the overflow tank if needed.

Oh BTW, I service the Overflow Tank to the Full Mark when its Cool. The overflow tank usually has a vent and will vent over coolant if it gets too high. So on the Full Mark when Cool means I have the most coolant the tank can hold. It really makes little difference cause again, its the OVERFLOW Tank, NOT the actual Coolant Level of the Cooling System. You have to remove the Pressure Cap (when the engine is cool) and see if the Coolant comes all the way to the top of the system.
 

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Yeah there are typically a "Cold" line and a "Hot" line on the coolant overflow tank if you just want to visually see that.
 

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Discussion Starter #8
Not on my XK just one line. I just had the thermostat housing replaced a week or two ago and want to top things to the line at the right time to insure I still do not have a leak. I am well aware of pressurized system, cutting my may limited mechanical skill as a gas pump jockey in the mid 60's. Yes there was a time only the real MACHO's pumped their own gas. Now NJ is the only place it may still happen that I know of.
 

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From the Owner's Manual:

Engine Coolant Level
The coolant bottle provides a quick visual method for
determining that the antifreeze/coolant level is adequate.
With the engine idling, and warm to normal operating
temperature, the level of the antifreeze/coolant in the
bottle should be between the ranges indicated on the
bottle.

The radiator normally remains completely full, so there is
not need to remove the radiator cap unless checking for
antifreeze/coolant freeze point or replacing antifreeze/
coolant. Advise your service attendant of this. As long as
the engine operating temperature is satisfactory, the
coolant bottle need only be checked once a month.
I've seen it happen enough, despite what the owner's manual says, you really need to remove the pressure cap (only when the engine is cool) and make sure the pressure side of the cooling system is full.


If the system never develops a leak and is working perfectly, then what is in the owner's manual is true, the level on the overflow tank will indicate if you are topped off on coolant.


But if the system develops a leak, and you're just following the advice of the owner's manual, you likely will miss the few momentary and subtle clues of running low on coolant until you're down to barely any coolant left in the system, but think you're just fine on coolant, until the engine overheats and leaves you stuck on the side of the road for several hours waiting for the engine too cool then get coolant to refill it.


OR- you could check all the fluids once a month, when the engine is cold remove the pressure cap and make sure the radiator is filled with coolant. If NOT, then you know you have a leak, you can start looking for the leak to repair it, and top off the coolant in the system so you don't end up stuck on the side of the road.
 

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Yeah there are typically a "Cold" line and a "Hot" line on the coolant overflow tank if you just want to visually see that.
Not on my XK just one line. I just had the thermostat housing replaced a week or two ago and want to top things to the line at the right time to insure I still do not have a leak. I am well aware of pressurized system, cutting my may limited mechanical skill as a gas pump jockey in the mid 60's. Yes there was a time only the real MACHO's pumped their own gas. Now NJ is the only place it may still happen that I know of.
My XK has "Full" and "Add" lines.
And often cold its at the "Add" line, but at the "Full" line when warm.


Don't get wrapped about it, think about it, its an "Overflow" Tank.


The proper level of coolant is the pressure side of the coolant totally filled with coolant and no air in it. The overflow tank holds extra that is forced out from the pressure side of the system when the fluid expands as it gets warmer, so it can then suck it back in when the coolant cools and contracts, without sucking air into the system.


So all you really need in the overflow tank is that amount that is forced out/sucked in as the coolant expands/contracts. Plus a little extra in case there is a slow leak.


The actual level has little effect on the cooling system, as long as its as its enough.


Know what happens if you overfill it? Perhaps when the fluid expands, the level might go up to the point it spills over the vent in the bottle and an ounce or two spills on the ground. Know what happens if its underfilled, nothing, if its way low, perhaps you'll suck a few onces of air into the cooling system, which it can handle just fine.


The other systems, engine oil, transmission, axles, over filling or under filling has big consequences. NOT the coolant overflow bottle, it won't make much of difference one way or the other.


What will make a difference with the cooling system, is if there is a slow leak you don't notice, you'll slowly loose coolant and very likely won't show up on the level on the overflow bottle. That is why you need to open the pressure cap (only when the engine is cool) to check the cooling system is filled to the top (or within an inch) under the pressure cap. If its not, then you have a leak, and if its a slow leak you can't find, you need to keep doing this, to keep the system topped off.
 

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Another clue if the system has the proper amount of coolant, and a good one since you check it while hot (since you can't remove the pressure cap while the system is hot).


Touch the pressure cap and/or filler neck of the radiator (that the pressure cap mounts on). If the pressure cap is as hot as the other hoses, then the system is full. If the pressure cap is significantly cooler than the hoses or other parts of the cooling system, then you're likely low on coolant.


If your low on coolant on the pressure side, then coolant will NOT be touching the pressure cap, so heat won't transfer to the pressure cap. Doesn't mean the pressure cap will be cool, it will just be significantly, i.e. very noticeably, cooler than the hoses and other parts of the coolant system (the overflow bottle excluded of course).


If your good on coolant level on the pressure side, then coolant will touch the pressure cap while the engine is warm and it will transfer heat to the pressure cap and it will feel just as hot as the other parts of the cooling system (the overflow bottle excluded of course).
 
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