Expansion Tank Overflow Bottle - Explanation

[ibroker]

Could someone give me an explanation of the whys and hows of the overflow bottle, until I got the wedge I had never seen one. Can I replace the tank with a new one without the bottle?

Laymans explanation please....My spanner knowledge is limited...

[Wedg1e]

It's a TVR bodge. In a proper installation the expansion tank would be all you need; as the water in the engine heats and cools the level in the expansion tank rises and falls to accommodate it (same as a sealed central heating system).
I suspect the issue is partly down to the Wedge's tank being a bit small for the V8 so once the water expands far enough, the pressure rises, lifting the cap and allowing water to overflow. On a Granada (where the tank came from) it would just be dumped on the road; TVR piped it instead to the extra expansion tank from where it can be drawn back into the system as the coolant cools down. Or at any rate that's the theory.
If you have any gasket leaks then the system will over-pressurise and overflow (even the extra tank) so if the extra tank is full then you have a clue there's a problem.
If you can keep the engine cool enough (decent radiator and fan, no leaks, no blockages) then you'll probably hardly notice the level in the extra tank change (it should have some coolant in there always to allow for it being drawn back if needed).
In reality of course...

[ibroker]

Are you suggesting then the overflow bottle should be next to empty when the engine is cold?

[Number 7]

ibroker said:

Are you suggesting then the overflow bottle should be next to empty when the engine is cold?

I tend to keep it about 1/4 full

[The Hatter]

I agree it's a TVR bodge and doesn't work very well...

However the W124 Mercedes E-class has the same system with an expansion tank overflow bottle low down in the inner wing cavity. It relies upon an unusual type of expansion tank cap with two seals such that the coolant gets pulled back from the overflow tank into the expansion tank when the system cools down.

I guess TVR did it because the expansion tank is not high enough to ensure the engine is full of coolant when the expansion tank is half empty - however I don't think the the Capri tank has the appropriate double sealed cap, so I suspect they were on to a loser. Maybe TVR specified a special expansion tank cap but I've not seen anything to confirm that. I also suspect that when the engine's head gaskets are spot on then there's no problem; it's when the head gaskets start to leak combusion gasses into the coolant (inevitable with steelies?) that problems occur with the coolant blowing out of the expansion tank.

I now have a W124 expansion tank and associated cap on my 350i... it works superbly!

[wooly350i]

Hi Martin, so is the cap the spring loaded type used for the Merc, if so I assume its ok for the 350i ? I'm hoping it will be sufficient also for the 2.8 ford lump in Delilah as the current one has sprung a leak in the latest twist of events, cheers, John.

[Wedg1e]

Number 7 said:

I tend to keep it about 1/4 full

Ditto.

[The Hatter]

Wooly, I don't think the Merc cap will fit the Ford bottle if that's what you mean - even if the bayonet fitting was the same I doubt the pressure setting would be correct.

[voltage_maxx]

My 350i only has a standard Capri type header tank and no overflow tank.
With a failed head gasket, it was peeing all of the header tank water out, any time the engine got good and hot.

I'm just putting the engine back together now, so will know in a few weeks if the extra expansion tank is actually needed on a car with fresh head gaskets.

[adam quantrill]

The Jeep Cherokee has an overflow bottle, with a siphon, by design. Some systems just work like that.

The theory is that if the expansion tank overflows for whatever reason, the overflow catches the coolant for recycling and it gets sucked back in when the engine is turned off and cools.

P.S. I still have a few V8 pressure caps - double seals. Not sure if they would work with a V6 tank - I don't think they fit properly - we tried a few BBWF's ago.

Edited by adam quantrill on Monday 17th March 22:56

[wooly350i]

No Martin, I meant is the merc cap equivalent to the ones used by tvr and ford as far as you can make out, Tobz needs to change complete tank on the 2.8, so tank, cap the lot, its not worth pissing about with trying to araldite the tank spigots and second hand ford tanks are a horrendous price with no guarantees. So what im thinking is the W124 that you've used is ample for the 3.5rover so surely it should be good for the 2.8 ford assuming there are no issues with one being a cast iron block and the other being aluminium, he's just taxed the old dear and needs to drive her under bridges and tunnels. Cheers, john

[Waynester]

How much coolant should there be in the expansion tank when cold. My car doesn't have this extra overflow bottle.

[adam quantrill]

In the plastic ones if there's a sticky-up bracket thing inside under the filler cap then it should be full to the shoulder on that, I think. If you put too much in it will spit some out until it finds the level.

[KKson]

Gents, I've got the standard set up. When I first bought the car I didn't realise that the overflow tank needed a level so each time after a hot run I lost a little water. Then topped up to approx. 1/4 full on the overflow tank, so that the dip tube is covered, and ever since it's always at the correct level, no matter how hard I thrash it. Standard set-up works for me.

[Waynester]

adam quantrill said:

In the plastic ones if there's a sticky-up bracket thing inside under the filler cap then it should be full to the shoulder on that, I think. If you put too much in it will spit some out until it finds the level.

Mine is about halfway down that square tube thing that's under the cap.
Just ordered a new 15psi cap as I think the one I have looks quite old. Maybe the spring is weaker. Can't hurt to replace it with a new one.

[V8 Fettler]

The blowing and sucking can be a pain in the backside unless everything is well fettled. A long, long time ago, there was a time when radiators had no separate expansion tanks or overflow bottles, the expansion tank was the top of the radiator.

It is possible to have an overflow bottle at a lower level than the top of the radiator, only works if there are no leaks though.

RV8s seem to have resilient coolant systems, mine had a small leak from the radiator which mysteriously fixed itself. I keep the level in the expansion tank just above the outlet, seems to work OK although I avoid long queues if possible.

http://www.custompistols.com/cars/articles/overflo...