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Right, so this expansion tank... I'm trying to understand yet more cooling-system weirdness courtesy of Mazda, since I'm riding high from beating the mind-bogglingly elaborate AST setup on the FD into submission. Here we go with stupid questions installment #372

So, on my Series 3 there's a plastic expansion bottle mounted to the side of the radiator shroud. It has a single hose going to/from it that is sourced from the filler neck for the radiator. The hose leads into a kind of big straw integral to the cap, that then sits in the coolant in the expansion bottle and goes down to nearly the bottom of it. There is a "high" and a "low" level marked on the tank.

Now, by my understanding the coolant is fed into the bottle from the radiator when it expands so much from heat it pressurises into the filler neck, forcing the cap mechanism to allow it past. But then how does it return to the radiator from the bottle, given there's only the one hose connecting them? Is it simply sucked back by the change in pressure as the coolant...well, cools down? The expansion tank doesn't appear to hold pressure, or be designed to. The reason I ask is I'd quite like to replace the ugly plastic bottle with an alloy one, but I don't want to screw anything up. Would it be OK to install one where the inlet/outlet union is on the bottom, because the sort of straw thing attached to the original cap setup seems a bit of a stumbling block? How crucial are the level markings on the tank? I always just kinda check the rad is full by looking in the filler cap. I know it seems a bit pointless just for aesthetic reasons, but as an FD owner I have a morbid dread of cooked plastic bottles splitting and costing you your cooling system. If, as it seems, the tank is just a little overflow reservoir that does next to nothing then happy days! If I'm going to cause endless coolant level problems then maybe I'd better leave it...


Cheers, thanks for being gentle with me, lol

I'm pretty sure It just sucks back into the rad Nik. The bottle is not part of the pressurised system.

So any container will do the job; you need to consider the graduation marks Max/Min. The dip pipe has to be well under the coolant when cold and you don't want it venting out hot coolant either.

's what I figured, cheers Dave.

So would it be OK to do away with the dip pipe and go with an inlet that just goes in at the base? Presumably it would find its own level that way anyway?

yeah, everything rad side of the cap is pressurised, the expansion bottle isn't. I don't see any problem with changing to an alloy expansion tank or bottom feeding it..... the dip tube just prevents air being sucked back into the system.....
It only need one pipe to it ..... When the coolant heats up it expands, which increases the pressure in the cooling system. When that pressure reaches the value of the pressure cap it lifts allowing coolant into the expansion tank. When the system cools down the coolant contracts which causes a partial vacuum in the system, the vacuum draws coolant back out of the expansion tank and into the system.
I suppose the clear plastic tanks are cheap to produce and also make it easy for owners to check the level although if you're system is in good condition, the change in levels/ fluid loss should be minimal I'd think.

Cool! It's gone then, lol.

well, soon as funds allow anyway...

I was thinking of something along the lines of an alloy oil catch can, as they have a "sight glass" transparent tube on the side so I could still judge the level. And if I mount it at the same height as the original then the level ought to be the same or thereabouts. Thanks for setting my paranoia at ease, guys

I don't reckon the level of the bottle is particularly critical( as long as it's not miles out). When I modified my air intake, I dropped the level of my expansion bottle to get the air pipe over the top of it....




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From a rotary engine POV, overheating is most often catastrophic. You have different metals with contrasting expansion rates so the outcome ain't gonna be sweet.

So with an expansion tank, there's a decent chance of keeping the correct level in the radiator with a fairly simple method of checking. I guess mazda didn't reckon on bottles with two and a half decades of rusty residue coating the insides of the tank

And for no reason apart from vanity, here's mine after the equivalent of an expansion tank spa day ... the patented steradent and dishwasher treatment

Shite Nik........(Not being coarse, just pointing out the danger with a bottom connection).

codge wrote:Shite Nik........(Not being coarse, just pointing out the danger with a bottom connection).

lol, is this just an arbitrary bottom joke or are you saying there's a good reason why not to go in from the base... ahhhh... plumb it in the bottom....urghh... I give up Ya knas wot I means

I was thinking of this kind of thing, and just blank off the top spigot, then plumb the hose from the radiator neck to the bottom spigot

http://www.carbuildersolutions.com/uk/a ... ectangular

Dave's cracked it. Sediment in the bottom of the tank is the reason not to draw from the bottom.

Keep your bottom clean of course and nothing to worry about..!

The pressure action has already been described of course - but if you lift the cap you'll see another sprung valve that opens under very gentle vacuum pressure so the overflow water can seamlessly draw back into the system.