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MG Midget and Sprite Technical - Where is the coolant going?

75 midget 1500. 4-2-1 exhaust, K&N, Elec. fan.

I keep loosing coolant, no obvious drips or leaks. Expansion tank cap seems to be holding pressure OK. When topping up I have tried to ensure all air removed from system especially heater loop. I suspected it could be being blown out of the pressure vent by leaking headgasket, so borrowed a compression tester. Quick check got about 10-15psi variation across all 4 cyls in range 125-140 psi. Plus oil looks v.clean and no sludge in filler cap. Any comments on the above gratefully received, especially as to expected compression variation.

Thanks in advance for your help.
Best Regards Steve Lowiss
s lowiss

after the engine is at normal running temp. check your tail pipe for water dripping, it can be craked between the valves, and never gitting into the oil system

Prop
Prop

also see if you got a white cloud out your tail pipe also.
Prop

how are you filling your system?

sounds stupid but there are two ways to put coolant in the later cars

If the car has an expansion tank, the "filler cap" on that is not how you fill the system, though it has a trad radiator cap to close it

there should be a brass cap (Might be a black plastic/fibre moulding)on the top of the thermostat housing

it is the same size as a spark plug spanner ISTR

you remove that to fill the system and put a drop in the expansion tank too when the plug sized hole fills up.

Give it all time to let air bubble free.

do not do it if the engine is hot

I know, you already know that...

if it isnt this suspect head gasket failure, if you are an AA member call them and ask the patrol to do a HGF test with his "chemical sniffer" and test the system pressure with his Sykes kit

HGF causes exhaust gas to leak into the water jacket, the chemical test fluid changes colour to sickening green when gas is in the water

Most garages can perform this test for you too
Bill sdgpm

Unrelated, perhaps, but how much coolant are you putting in your expansion tank? If you fill it to the brim, the excess will be forced out when the engine heats up. You need about half the tank devoted to air space (I use the top half. Ha...) so the air will compress when the coolant gets hot. That's the factory spec, by the way.

Good luck,

-:G:-
Gryf Ketcherside

Thank you for comments, I'm ok about topping up via filler cap on thermostat housing, and leaving expansion tank half full. I'm now fairly sure coolant is going into the cyls, therefore have ordered a new headgasket & manifold gasket. .. my w/e job. Hopefully its only the gasket that has failed and not a cracked head or worse. Any advice on what to check for on strip down, or what parts to inspect..and what defects to look for.. eg. on tappets? valve seats?

Once again any comments appreciated. Thank you.
s lowiss

This thread was discussed between 02/06/2009 and 03/06/2009

MG Midget and Sprite Technical index

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