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merseyman58

Expansion Tank?

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Hi - I own a Toyota Avensis 1.8GS petrol engine 2001 and have noticed that the plastic tank which 'feeds' the radiator is completely empty, however the radiator is full and does not need topping up. I don't know how long this tank has been empty for or what purpose it serves, but the car has been running OK and is not overheating.

I've read a few posts on coolant and understand that this may be called an 'Expansion Tank' but not sure what purpose it serves. I'd be grateful if anybody can explain what it's purpose in life is and what the possible reason(s) why it's bone dry,

Cheers,

Ken

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The expansion tank moderates the quantity of coolant in the system. As the engine reaches operating temperature the coolant heats up and increases in pressure. At times the coolant pressure forces past the metal pressure cap on the radiator. (it compresses the spring) The coolant can then vents into the expansion tank. As the engine cools the coolant is drawn back into the system until the spring on the pressure cap closes. Directly after a long/hot run the level in the expansion tank may be above the max line.

You must have a leak. Is the expansion tank actually dry or virtually empty? Is the tip of the tube which dips into the expansion tank wet? If it's dry you will now be drawing air into the system.

The leak is (i) a hose: (ii) water pump bearing failure (can be seen from underneath - There may be water marks on the housing next to the pulley wheel) (iii) Head gasket failure.

Fill the expansion tank to the MAX marker and monitor.

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Gary - Thanks for getting back to me that all makes perfect sense. I'll follow your advice and see what happens. The only problem I noticed was that there is a small hole in the expansion chamber but it is at the very TOP of the tank so unless it was full to capacity the water could not escape from there, but is the tank in a pressurised system or simply an overflow?

Also I'm guessing (i) is pocket money and (iii) is a remortgage?

thanks again,

Ken

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No- The expansion tank isn't under pressure. The key for the unit is that the dip tube must always be covered by the depth of coolant so that it will suck coolant back into the system as the engine cools. Unless you've noticed a plume of white smoke following you while driving, I bet that it's the water pump. A good quality copy part is around £35. Labour is 2-3 hours tops. (it's a bit fiddly - small hands required)

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Earlier cars did not have expansion tanks.The radiator cap fitted to the top of the rad had two valves in them,one to conrol the max pressure of the coolant in the system and another to control the vacuum produced when a hot engine cooled down.

without the second valve the vaccum would collapse the hoses and some cases the rad itself.

In some of the earliier cars i had i fitted a windscreeen washer bottle with its mounting and fed the overflow from thr rad that was just beneath the rad cap i put a small hole if there was not one in the cap to stop pressure building up in the bottle.

doing this meant i only needed to look at the bottle to see that .the rad was full to its max.

Some cars still have a rad cap fitted as well as a expansion tank. the rad cap is just like the earlier ones.the tank is not pressurised.

but my Auris has no rad cap fitted. the pipe come straight from the rad to the base of the expansion tank.so the preesure control must be in the tanks cap as must be the ant vacuum valve.So the tank must be under pressure when at full running temp.

we have all read about the problems with some of the toyota diesels with owners saying about marks caused by antifreeze on the engine covers so the has to be pressure in the tank above normal.

the system is designed to be working at a fgure above atmos pessure to higher the to increase the boiling point of the coolant..

earlier cars had the the caps marked the pressure was released. Indeed my wifes Hyundai has a rad cap marked 0.9 what ever that is,in my day they were marked in PSI.anything from 4 PSI to !5 PSI

Not sure about a Avensis but if it is like my Auris then the tank will have pressure when running

The cap is marked do not remove if hot. So if no pressure why the warning.

to add further i took the cap and tested it by sucking on it and you can feel the anti vaccum valve working. there is no way i can test the pressure valve without a lot of hassle.

If others disagree to what ive stated and please explain how the pipe from the rad to the tank has no valve in it to atmospher how the tank cannot have pressure in it .

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Hi Peter - Many thanks for the explanation. I've since taken the car down to the local garage and explained the problem, much shaking of head and sucking of teeth and mutterings about fuel pump and cylinder head gasket. I left him inspecting the underneath of the engine bay looking for water leaks. If/When he finds out what the problem is I'll post details here, but I'm crossing my fingers that it's just the radiator cap or a leaking hose............

thanks again,

Ken

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I've just picked up the car from the garage and when I asked the mechanic what the problem was his English became very poor, all I managed to pick up was "escaping steam". Anyway I was only charged 30 Euro's and felt so releived I gladly handed over my cash and drove off.

Thanks to those who replied with advice and explanations,

regards,

Ken

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