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My wife's 94 SL seems to have a problem, but I'm not sure it is just annoying, or if it has damage potential.

For some reason, if you take off the rad cap the fluid is never up to the top. After running it it seems to put a bunch into the overflow container but it stays there. I'm not real sure how it is supposed to work...how does the fluid get back to the rad from the overflow? However it does it, it doesn't seem to work on her car.

I just flushed/filled her coolant this morning and replaced the thermostat and it still did the same thing. I filled it up, filled up the over flow right to the full line, then took it for a drive. After the drive I let it cool down for over an hour then took a look.

I could see the fluid after removing the rad cap, but it was way near the bottom of the neck. The overflow was well above the full line...

What is causing this? I'm afraid that it is going to overfill the overflow and just spill out onto the ground. If she doesn't check it then, she may be running with no coolant in the rad......

Thanks,

Doug
 

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The reserve tank level should rise and fall with heat of the engine, more full when the engine is hot, then lower when the engine cools and coolant is siphoned back into the engine. If your wife's car doesn't do that, chances are there is a leak in the cooling system. I'd try a new radiator cap. If either the seal at the top of the neck or the inner spring-loaded gasket of the cap isn't good, the cooling system won't have the balance that allows the reserve tank to do its job. Incidentally, the owner's manual says to add fluid to the reserve tank and that under normal use, there is no need to remove the radiator cap.
 

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That sounds normal, there should always be fluid in the over flow.
If you look on the side of the over flow tank there are two lines for high and low. If it drops to the point where you need to add more the sensors will pick up the signal and a orange dash light will come on in the bottom left corner next to your speedo.

1 way to make sure you have enough is to mix a jug of antifreeze and water up. And while your car is cold and hasnt been running for a few hours, let it run with the heater on full while you slowly add to the radiator.

Then once it wont take anymore fill the over flow up almost to the top. All the extra will spill out and your car will keep exactly what it needs, Its a little messy but it works.
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><i>The reserve tank level should rise and fall with heat of the engine, more full when the engine is hot, then lower when the engine cools and coolant is siphoned back into the engine.</i>

This actually makes sense, but it doesn't explain something that's always bothered me - why does the manual say to fill the overflow tank to the upper mark when the engine is COLD, rather than when it is hot?

Because of that directive, I got the impression that the engine must suck coolant out of the tank when it's hot, and blow it back in again when it's cold, but I couldn't explain to myself how that process would work.
 
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Discussion Starter #5
Radiator Caps and more...

Okay, I will get a new RAD cap for her car.

One additional point of info that might be relevant though...the cap for her overflow tank is really shitty. It has a very loose fit and often times when I open the hood it has actually popped off.

Could this be the (or an additional) problem? That little plastic cap on the overflow container sure doesn't seem like an important 'airtight' fit like the screw on rad cap though.

Thanks,

Doug
 

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You wouldn't want the overflow tank to be airtight because you don't want pressure there.

The coolant expands when it's hot, so a closed (airtight) coolant overflow system would increase pressure when hot and cause coolant pipe fittings to be more prone to leaking etc. Having an unpressurised overflow tank means that the expanding coolant can be accomodated without a pressure increase, the idea being that the fluid is not lost but will instead run back into the radiator when it cools down and contracts.

Maybe your line from the radiator to the overflow is blocked, and when the car is hot the coolant is escaping directly out of the radiator cap. Detach the overflow pipe and make sure it's flowing freely.
 

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Like Doire says, you don't want the reserve tank to be airtight. It needs to stay at normal atmospheric pressure to keep the system working right.

But, you said you find the reserve tank cap pops out sometimes? If it pops far enough out that the rubber tube inside lifts up out of the coolant, then it can't siphon back in as the car cools.

As for why Mitsubishi says to fill the tank hot, my guess is that its the only sure way to know you have enough. ( errr... edit edit edit) If you fill it cold, it would be more likely to overflow hot? And who wants that spraying out under their perfectly detailed engine compartment?

Good advice from everybody in this thread - if one of these doesn't do it, try the next one!
 

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Andrew Caple said:
><i>The reserve tank level should rise and fall with heat of the engine, more full when the engine is hot, then lower when the engine cools and coolant is siphoned back into the engine.</i>

This actually makes sense, but it doesn't explain something that's always bothered me - why does the manual say to fill the overflow tank to the upper mark when the engine is COLD, rather than when it is hot?

Because of that directive, I got the impression that the engine must suck coolant out of the tank when it's hot, and blow it back in again when it's cold, but I couldn't explain to myself how that process would work.
I may be way off base here, but I thought that when the car warms up, it uses coolant to cool the block. That is why if your thermostat goes bad your car overheats even though there is still water/coolant in the radiator. I figure this coolant must flow somewhere and then come back (although I may be wrong). Then when the car shuts off, the coolant goes back through the hoses to the radiator.

Regardless, it seems like your car uses more coolant when it is hot than when it is cold, so if you filled it when it was hot, when the car shut off, it would drain back into the overflow (which would already be full) and therefore it would all just spill out on the ground through the little hose at the top of the overflow. Then, if you checked it again when it was hot, it would be "low" again, since some had spilled, and this would go on ad infinitum. However, apparently all that is needed is the amount that is used when the car is hot, which is the amount that is in the radiator when the car is cool, plus a portion of the overflow.
 

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Well, there are no pumps or electric motors or any other kind of intricacies involved here - it's just a hose from a plastic contained to the engine block.

So what physical processes are at work here that cause coolant to get sucked in from, or belched back out to, the overflow tank from the engine? Why does a hot engine cause coolant to get sucked in, why does a cold engine cause it to get blown out?

What is the significance of the fact that the hose to the overflow tank is connected immediately below the radiator cap?
 
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I thought that...

it was as simple as something like this:

When the car is cold and you fill it up to the cap that is as much as you can put into a cold vehicle.

Since once it is hot you will scald yourself if you remove the cap, you cannot do that.

I just thought that once you are running the engine and it is warmed up there are places within the engine that fill up with coolant as it flows around. Because there is now all this coolant flowing around, but the total volume needed for the running/warm vehicle is more than what is in there it creates a vacuum as it rushes around. That vacuum sucks from the only place it can suck...through the little hose just under the cap. The hose needs to be just under the cap because once the fluid reaches that point the vacuum pressure is gone since there are no air pockets left within the closed system. Once you shut the car off and the engine cools, I figured that the since the cold system is now 'overpressurized' from the intake of coolant from the overflow, it just forces it back into the overflow.

Of course, I have absolutely no background in physics and no education relating to engines of any kind.

Hmmm, now considering the problem with my vehicle and the reason I started this thread, I might need to adjust my hypothesis though...

It must not be the case that upon cooling the system is overpressurized and that is why it puts coolant back into the overflow, because with my wifes vehicle, I don't think it ever sucked it out of the overflow to begin with (probably a leaky cap like everyone mentioned). However, upon cooling, it still put a bunch into the overflow, leaving my level way below the cap. So I think that when the engine is shut off, the 'current' of the coolant flowing round and round is gone...therefore there is nothing making sure that the superhot coolant flows back to the radiator to be cooled but yet it is hot as hell...so it rises up to the highest point.

Since my cap might leak, but might only be a very slight leak, the path of least resistance might be down the hose and into the overflow. Maybe a little comes out from under the cap, but most must take the escape hose to the container...

Am I way off base?

Doug
 

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Interesting theory.

I don't know if it's right, but I can think of a reason why it's wrong - I don't think that there *are* any "places within the engine that fill up with coolant as it flows around".

I base this on me experience of flushing the coolant (see my FAQ for how to replace the coolant). The end of the process was to go for a drive, come home, let the engine cool down, pop the rad cap and top it up with coolant, and to repeat this day after day until it was impossible to add any more coolant. The whole point of this was to "burp" any air out of the coolant system, so that, when done, the entire coolant system (rad, hoses, block) contained coolant only, and no air.

Why aren't any smart people replying to this thread, dammit.
 
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Discussion Starter #12
How about this....

Is it a fact that the overflow tank will actually be 'more full when it is hot?" That is the root of the entire problem.

In my feebleminded scenario the overflow would be 'more full when cold', then as the engine got hot and the vacuum sucked the coolant from the overflow, it would be lower.

Oh..and no offense taken.


:) Doug
 

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Misc. cooling trivia

(I'm replying again because I'm a self-proclaimed 'smart person' )

As the engine warms up, the _coolant_ expands and _increases_ in volume. In a well-designed and maintained cooling system, the warming and expanding _coolant_ forces past the area of lowest pressure, the spring-loaded washer in the radiator cap, and so the reserve tank begins to fill.

Turn the hot engine off and walk away for a few hours and as it cools, the _coolant_ contracts and decreases in volume. That creates an area of low pressure in the engine, and as soon as the pressure inside the engine is sufficiently lower than outside atmospheric pressure, outside air begins to push coolant from the reserve back past the radiator cap and into the engine block.

Air in the cooling system can muck this all up. Air is compressible, so if it's inside the now-cooling engine, it reduces the pressure differential that is going to let the reserve tank put coolant back in the block. (That's why hollywood's post is good stuff, doing that flushes air out) Air inside has other ill effects as well. For instance, it tends to let a layer of steam form above the combustion chamber and reduce heat transfer where the engine needs it most.

When a cooling system is in good shape, it's free of air inside the block, the cap seals the neck, the inner pressure seal holds its rated pressure, there are no other leaks and the return tube in the reserve tank is always immersed in water.

The thermostat's primary purpose is to block flow through the radiator and thus warm the engine faster. In a modern car, it has the secondary purpose of maintaining engine temperature for good emissions control. The engine water pump is always turning, but the flow through it is regulated by the thermostat.

(editions thanks to pak, who pointed out my misconceptions)
 

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Ta da!

At last, a plausible explanation.

I never would have figured that expansion of the metal in the engine would be so much as to require the overflow tank.

<i>Oh..and no offense taken. </i>

None was given.
 

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Re: I thought that...

d-dog said:
snipped..I think that when the engine is shut off, the 'current' of the coolant flowing round and round is gone...therefore there is nothing making sure that the superhot coolant flows back to the radiator to be cooled but yet it is hot as hell...so it rises up to the highest point.

You're describing what we call "heat soak" - but it's not that the coolant rises to the highest point in the radiator; it rises to it's highest pressure. Without the flow created by the pump, it stays in contact with the hot metal around the combustion area and boils (among other things).
 
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Very Interesting...

I'm glad I started this thread...now I just hope that changing the rad cap fixes my problem.

Doug
 

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The real explanation if fairly simple. It has nothing to do with expanding metal. As the water heats, it expands slightly - the expanding volume of water needs somewhere to go - it pushes past the valve in the cap and travels to the overflow tank. When the coolant cools, its volume decreases, creating a vacuum in the radiator. This vacuum sucks coolant out of the overflow tank until it returns to normal pressure.

In the old days, before overflow tanks, we would allow about an inch of space in the radiator to accommodate the volume change. In our environmentally friendly times, we got tired of dumping overheated coolant onto the ground. This called for a tank.

The cap would be my first guess at your problem. The other possible problem is a leaky overflow tube. This would still allow coolant to move into the tank, but would not allow coolant to be sucked back in.

Andrew - I suspect it says to service when cold to keep someone from getting scalded (however unlikely). But it should say only to fill up to the cold mark only. Check your manual again. (sorry, I can't get to mine right now.)
 

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wupps

You're exactly right. It's the thermal expansion of the coolant itself, not the engine material. I left out coolant expansion for the sake of simplicity thinking it was the minor part of the equation, but now I see glycol and water expand quite a bit more than I thought.

(going back to edit now....)
 
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