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MG MGB Technical - disappearing coolant!

Hi all,
My 1970 CB Roadster, AFAIK Bog standard engine, Old style 'back fill' radiator (the one with a 'neck') so I cant guage the water level. 82Deg Thermostat, 15psi rad cap is loosing water somewhere. This has been happening since I got the car. I just topped it up (cold) and could feel water in the bottom and top hoses when squeezed. Out of pure guess I put the vent pipe into a bottle. Drove for 5 miles so nicely up to temp. The guage varys regularly between 70 and 90 (ish) as the thermostat does its thing. When I got home the bottle was full (500ml) and the pipe was venting air (Bubbles into bottle). I can no longer 'feel' water in the top or bottom hoses. Rad is hotter at the top than the bottom. The water in the bottle is clear so no sign of oil contam. The exhaust is not steamy. I have no loss of power. The oil is clean (changed -100 Miles was black as ink) there is no 'Mayo' in the oil filler cap. So it looks like it is overpressured somehow. Any thoughts?
Thanks
Bernie
B Anderson

Air bubbles in the catch tank is worrisome. It could be a cracked head or gasket failure with exhaust gases getting into the coolant. You could have a mechanic check with a combusion analyzer for CO and hydrocarbons.

I doesn't sound like you are overheating but to get a 15psi cap to relieve you'e overpressuring. Again pointing to exhaust gas.

The top of the radiator should be hotter than the bottom. I assume the bottom is above ambient temperature however. That would indicate some flow.

I'd do a compression check also to see if anything is amiss. Also double check the temperature gauge. One way is to put it in boiling water and see what it says. Should be 100 or close to it.

Assume no water is coming out of the heater core.

I'm sure you'll get better advice than mine.

Bob
Robert McCoy

Just a note. Heater is not leaking :)
B Anderson

How old is your radiator cap? If it is faulty, it will allow air to be sucked into the cooling system when the engine cools off and can cause the water pump to cavitate , that is draw in air, at its impeller. RAY
rjm RAY

Rad cap is less than 8 months old I replaced it as the original was only 4Lb!
Bernie
B Anderson

Top up the rad and then run the engine with the cap off. See if there is a constant stream of bubbles or if the level suddenly rises and overflows in which case it is almost certainly a blown cylinder head gasket. Just let's hope it's not a cracked head.
Iain MacKintosh

Bernie,
How full are you filling your radiator ? The correct level should be some way down fron the top as the coolant need to expand somewhere as it warms up and the pump pressurises the system. The later systems used an expansion bottle to catch this "overspill" and then draw it back into the cooling system when the engine cooled down. In your earlier system there is no expansion tank so you need to leave room for expansion in the top of the radiator.
I run my early system just above the fins in the rad (it will find its own level) and its been like this for 20 years now , without trouble.
My point is , don't fill the early radiators to the neck or you will be constantly refilling it as the system expells coolant to find its own correct level.
Regards
Jim
J Robinson

Good point Jim. Mine has that horrid neck so I can't see the top of the fins. i just fill it so there is water in the neck part (not much). As teh temp remains stable I am not too worried and there are no other symptoms (yet) I will get round to a comp check and pressure test but it will have to wait as I don't currently have time
Bernie
B Anderson

Your local independant garage will have a device that pushes in to the rad filler neck with a small amount of special fluid in it. Colour change of the fluid will inicate -or not- head gasket failure.

Once the first 500ml is expelled, does it repeat the next time-keep a close eye on the temp gauge! If not would seem to point to overfilling. Tho venting bubbles does sound odd.
Michael Beswick

Been out today for about 80miles. Gauge steady as usual (within usual range) catch bottle was empty on return. I'll get it to the garage for that check though.
B Anderson

I fill my 65 radiator (old style) until I just see liquid at the bottom of the elbow when I shake the car. It generally stays stable at that point. Maybe I have to add a slight bit a few times a year. If I add too much it will discharge some liquid the first trip or so. Then nothing.

I run a 7# cap, 192 deg F (I think that's 89 deg C) thermostat and 50% glycol year round. Without a catch tank there will be air expelled during heatup and air sucked in during cool down. I can hear my vacuum breaker opening when the car is cooling off.

So, if you've over-topped you could expel air until the coolant expands to fill the top tank and then gets expelled.

Also, a radiator shop or mechanic can put a pressure test on the system for you. That should tell if you have a leak and if so maybe where it is.

Bob
Robert McCoy

This thread was discussed between 07/09/2012 and 09/09/2012

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