Anyone know why the coolant in my system goes into the reserve tank like it should when it's hot but it won't go back into the radiator when it cools down.
Radiator cap is the correct pressure rating and the hose from the radiator filler neck to the reserve tank is not blocked.
Probably a loose hose clamp or the waterpump seal leaking slightly. It is easier to suck air in through a slight leak than it is to suck coolant back out of the overflow tank.
Maybe I need to have a better look.
Is this likely to cause any problems if I keep the radiator topped up?
there should always be coolant in the overflow tank whether it is hot or cold...there is nothing wrong with your coolant or your system...its not blocked.
relax and dont worry unless your temperature guage is getting near hot...
But the problem is if I keep topping up the radiator then the reserve tank will eventually overflow.
It could be normal that it never completely empties the overflow but if it doesnt go down anwhere as much as it fills when its hot then there is an air leak.
Where are the likely places?
1] Primary suspect is the radiator cap seal under the cap/handle of the cap. The cap neck seal may be OK and blow off at the correct pressure but the top of the cap must also seal against the top of the neck both under pressure and a slight vacuum.
2] The water pump seal can allow air in as it is only the spring that holds seal face together. Once pressure has built up in the cooling system the pressure also forces the sael against the sealing face.
3] As mentioned in another post above a clamp on a connection to the expansion tank could be loose or it could also be too tight and cuts the hose or worse deforms or splits the nipple that the hose is fitted to.
4] If you have to top up the radiator look also at heater taps as sometimes the leak is not obvious but if you are using the GM coolant it generally leaves a telltale stain.
when i first got my Commie it did the same.... wouldnt suck back into the radiator.... i went to AutoBarn and bought a new cap... made it worse... AutoBarn sold me the wrong cap cos as soon as the engine got a bit warm.... coolant started blowing into the overflow....they argued with me that they had sold me the right cap.... I went and looked in SupaCheaps book ... sure enough I was sold the wrong cap.... Now my cap has a rubber seal on the top.... AutoBarns cap didnt... SupaCheaps book told me there is 2 different preasured cooling systems.... higher system uses the cap with the rubber seal and NOT the metal disk seal... told AutoBarn of there mistake... they even had the right cap there too... exchanged the cap.... now after 6 months I dont ever have any probs of the coolant not sucking back..