rayjones Posted March 30, 2004 Report Share Posted March 30, 2004 Hi All, Have had a persistent coolant loss on my 7 for quite a while now. There are no signs of leakage anywhere other, than from the short pipe on neck of the expansion tank, just underneath the filler cap. The spring, and rubber seals on the cap look OK. Reasonably sure the head gasket is OK, no emulsified oil, no overheating, and there is never any pressure in the tank if I take the cap off, even when hot. On checking the Haynes manual for the pressure rating of the cap, they claim that it should be 15 pounds pressure. Mine has 13 pounds stamped on it, and says something about being a waxstat type. Can anyone confirm that I have in fact got an incorrectly rated, and/or possibly the wrong type of cap. Ray Quote Link to post Share on other sites
malcolm Posted March 30, 2004 Report Share Posted March 30, 2004 .....and there is never any pressure in the tank if I take the cap off, even when hot. On checking the Haynes manual for the pressure rating of the cap, they claim that it should be 15 pounds pressure. Mine has 13 pounds stamped on it.... Ray. I looked at the cap on my FHC and it is clearly marked '15'. Not sure how much of a problem this would be though. Wouldn't you expect to have pressure in the tank when hot?? Malcolm. Quote Link to post Share on other sites
drbob Posted March 30, 2004 Report Share Posted March 30, 2004 I cant imagine the difference between the 13lb and 15lb rating making a difference to coolant loss. The waxstat monica refers to the thermostat and if its shown with reference to the cap i think they have made a mistake in the manual. The rear most waterway in the block can weep into no.4 pot without pressurising and with little or no noticable effect on running this could be the source of the loss if its not apparent elsewhere. What sort of quantity over what sort of period is being lost? Rob Quote Link to post Share on other sites
andrewt Posted March 31, 2004 Report Share Posted March 31, 2004 Agree with Malcolm that there should be some pressure when the engine is hot. (to help prevent the cooling boiling). I suggest you beg or borrow a coolant pressure tester and pump up your system manually. This will allow you to locate the leak. If no4 is a potential problem, then i would expect that removing the plug would allow enough visibility to see if this is where the leak is. I have seen leaks in non running engines (not Tr7 though) that are drastic enough to cause the engine to hydraulic lock after sitting for two weeks, such that it wouldn't turn over on the starter. Quote Link to post Share on other sites
marvmul Posted March 31, 2004 Report Share Posted March 31, 2004 When there is only minimal failure of the head gasket, leakage of this gasket will only occure when the engine is under load thus normally only when driving the car. Firmy blipping the engine is also a load : so you can take of the cap of the expansion tank and blip the engine several times to see if air bubbles appear in the expansion tank and/or the level of the cooling fluid rises. Quote Link to post Share on other sites
andrewt Posted March 31, 2004 Report Share Posted March 31, 2004 Not to disagree with marvmul, but if the head gasket leak (if that's what it is) was only apparant under load, then there would be times when the system was under pressure (possibly excess pressure) which would get vented by the radiator cap into the catch tank or overboard. Although this scenario is possible it doesn't seem to fit exactly with the symptoms. I would ask the question as to whether the leak leaks when the engine is running, the engine is left off, only when it is cold or all three. It could be a leak that weeps when cold or when no pressure is present in the cylinders, but isn't apparant when the engine is running or hot. It seems likey that it is an internal leak or evidence of dried on antifreeze should be visible. Performing a pressure test when the car is cold should show up the problem area. Andy Quote Link to post Share on other sites
rayjones Posted March 31, 2004 Author Report Share Posted March 31, 2004 I too would expect pressure in the radiator when the engine is hot, The water loss amounts to say 1/4 of a pint if I go out and drive it hard for 20 or 30 miles. The loss seems to to be very little worse if I go further, I have driven it 200 miles without having to top up. It is definately losing it from the vent pipe, as there are streaks of antifreeze on the outside of the tank. I did wonder if I was over filling the header tank, but if it is much less than half full, the top hose starts to get air pockets. As Marvmul suggests, I have tried revving the motor which does not disturb the coolant at all, it only happens when I drive it. The car is often stood in the garage for two weeks or more, and does not lose any then. You can leave it idling for ages, again no problem. I assume the waxstat marking on the cap refers to a waxstat that is actually in the cap, and I'm wondering if this is faulty, and releasing the pressure as soon as there is any, and therefore maybe the coolant boils over to some degree due to lack of pressurisation. I will try a replacement cap as soon as I can, thanks to malcolm for confirming the correct pressure rating before I buy another Ray Quote Link to post Share on other sites
rayjones Posted March 31, 2004 Author Report Share Posted March 31, 2004 Further to previous posts, I checked out the header tank again tonight, looks like the cap is faulty, there is what I take to be a waxstat bypass valve in middle of it. The valve head is located in the centre of the rubber seal, and does not seal very well, it is fairly loose and will rock from side to side. I can blow through it to some degree, so obviously prime suspect. Only thing is now, if I replace the cap, what are the odds on the water pump leaking when I finally get some pressure in the system! Thanks for your thoughts everyone, they do help to clarify your thinking on problems like this. Is this waxstat device a normal fitment in these caps? I assume it is an additional safety device which will open if the temparature of the coolant gets too high, to supplement the pressure spring! Ray Quote Link to post Share on other sites
rayjones Posted April 1, 2004 Author Report Share Posted April 1, 2004 Tried a new cap tonight, It is at least pressurising the radiator now, although not had chance to take the car for a decent run, so looking good. Have realised that the disc valve is not some sort of waxstat as I thought, (and as the Logo on the old cap implied) but a back release valve, so that if the cap is taken off while the coolant is hot, it will bleed air back into the rad on cooling, to prevent a vacuum forming. obvious really. Anyway something to watch out for, if you are losing coolant! Ray Quote Link to post Share on other sites
andrewt Posted April 2, 2004 Report Share Posted April 2, 2004 I have a feeling Waxstat is a trade name rather than a suggestion of a part being fitted, but your explanation fits with the symptoms. Hope you've cured it finally. Cheers Andy Quote Link to post Share on other sites
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