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MG MGB Technical - O'Heating Question

Hi All,

This has been my summer for unusual problems.

I have a '77 B that developed a leak in the radiator overflow bottle, which led to an(almost) overheating condition, I caught it in time, and kept adding water until I got the car home. It did manage to get to about 7/8th. on the Temp gauge.

I repaired the leaky o'flow bottle and went about driving.

The temp gauge keeps going up to almost overheating, because it keeps "pushing" the coolant out of the engine, and into the overflow bottle, and then into the atmosphere.

My first thought was a head gasket leak, so I've changed the head gasket, installed a new 180 degree thermostat, flushed the entire cooling system, and replaced both heater hoses.(they were due for a change in any case).

The car is still "pushing the coolant out of the overflow until it begins to overheat.

I've pressure tested the cooling system both cold and hot, and it will hold 20lbs. for over an hour under both conditions.

This car has never given any cooling troubles over the seven years I've had it, until the event with the leaking overflow bottle.

Grasping at straws, I'm going to replace the water pump next, but I can't believe that would be the source of the problem.

I'd welcome, and appreciate any thoughts, and/or suggestions.
JR Jim) Ross

Pressure testers usually fit in place of the pressure cap. Have you replaced the cap?
Boiling can be a sign that the system is not pressurised.
Allan Reeling

If you have access to an infra red non-contact thermometer (available from Harbor Freight $35) you can do a quick scan over the radiator, if you aren't getting a temperature gradient across the radiator you could have a problem with the pump.
Also handy for checking engine for hot spots.
Mike
Mike Ellsmore

Jim

Like you my first thought was HGF, the symptoms do sound like the system is being over pressurised and blowing the coolant out, although of course there can be other causes. I've recently had the same issue on an F and the tool I use is an HGF test kit. The tool consists of a clear plastic tube with a rubber cone on one end and a hand pump on the other. You put about 1 inch of the supplied blue liquid in the tube and replace the pump. The cone fits into the filler neck and forms a gas tight seal, you then operate the hand pump several time to draw air from the radiator, or in my case the expansion tank, this bubbles the gas through the liquid. If the liquid changes colour to yellow it’s a sign of exhaust gases and hence either HGF or possibly a crack.

I know you’ve pressure tested your system to 20lbs but combustion gases are a much higher pressure. Have you tried pressure testing the system with the engine running?

Bob
R.A Davis

Does sound like HGF to me as well, been there, done that.

I made a Tee that fitted in the rad to expansion tank (77 and later) hose, connected to a gauge in the cabin which allowed me to monitor pressure while driving, which confirmed the cooling system was getting up to cap pressure. In fact for as long as I can remember the cap has been hissing - only heard on switch-off of course, but otherwise it ran just fine. It was only when it started chucking out coolant that I investigated further, finishing up with the HGF tester. That took several minutes to change colour even though I had driven to the local garage and it was fully up to temp.

Another cause can be the water pump sucking in air, or even a bottom hose clamp that needs tightening.
paulh4

Let me guess Jim, everything was fine until you fiddled with the ignition timing, right?
Sam Coleman

Uh oh ...
paulh4

This thread was discussed between 30/08/2016 and 01/09/2016

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