MGB Register Forum

I've had a disastrous summer, my B has been off the road for more time than on in what has been the best open top summer for years. Having replaced fuel pipes, coil, water pump and alternator this summer, my B now loses coolant ( 1 - 2 litres) after a high speed ( c65mph) run. There are no noticeable leaks, but the coolant appears to be blown out of the header tank overflow ( its a 79 with separate header tank) . There is no 'mayonaise' under the rocker cover or evidence of oil in the coolant. I'm tempted to replace the head gasket as a precautionary measure - any other ideas?

Before changing the head gasket have you checked that the thermostatic switch grommet / seal at the top of the radiator isn't leaking as you say that you think it is in the area of the header tank overflow. This may only leak when the system is under pressure.What about the pressure cap itself? Are you getting any leaks from the drain hole underneath the water pump pulley which happens if the car hasn't been run regularly & i have had issues there when the car has been warming up / cooling down?
Cheers, Charles

This will sound as a daft question but what colour is the expelled coolant? When the cooling system on my 77 BGT was pressurized due to a cracked head, the expelled coolant was the mayonnaise colour.
If it's not that colour it could be a leak from the cooling system as Charles said. Get the engine up to working temperature and look around the electric fan switch in the radiator and check the hose to the header tank. A compression test will show up a head gasket failure.

Stating the obvious, have you checked all the coolant hose clips, particularly the one at the bottom of the radiator & the one joining the heater to the pipe over the rocker box as you will have disturbed those when changing the water pump & removing the rocker box? ( I had a small leak which took me ages to find following an underbonnet refurb & found a slight weep from the latter).
Incidentally after 2-3 months having changed all the hoses i found quite a few clips needed tightening again. (I have sealed 4 life coolant which is pink that makes it easier to trace leaks).
Cheers, Charles

Thanks for the responses.

I've had the car on the drive and run it up to temperature where the electric fan kicks in, and looked all over for leaks, there are none evident.
The expelled coolant is a slightly rusty colour (I'm topping up with plain water until I find the source of the problem) and there is evidence of droplets all over the new alternator, coil, distributor and the RHS inner wheel arch, with a small pool of water in the base of the channel below the brake servo. All this leads me to believe that the coolant is being blown out of the overflow in the expansion tank. I can't see any bubbling in the expansion tank when the engine is running. I'll order a compression tester as this will also give me an indication of the state of the valve seats. I'll also get a new filler cap, as my current cap seems to have been replaced by the previous owner and I've read in other blogs that there are potential issues with replacement caps although this has not been evident in the past 4 years.

I take it there are no leaks coming from the heater valve,which is a known weak spot. Look out for drops on top of the distributor which can then can get shaken round the engine copartment? It is not unusual for the expansion tank to contain fluid that is not as clean / clear as the rest of the system.
I have had a water puup yhat spraiys fluid in a line on both inner wings that matches the belt.
Cheers, Charles

Make sure you get the correct cap for the expansion tank. It's rated at 15 psi.
There should be an overflow pipe attached to the expansion tank. Is the coolant coming out of that pipe?

I suspect this is a head gasket failure between one of the cylinders (I would guess at Nos 3 or 4 cylinders as these run the hottest) and one of the water feed holes between the block and the head or a cracked head (although a 79 car should have the strengthened head). The reason is I think this, is I had exactly this problem on my MGC GT many years ago. No mayonnaise in the cap or oil in the water, and it only lost coolant after a run above 4000 rpm. The water is being boiled off in the cylinder that it is leaking into so it won't contaminate the oil very quickly, hence no mayo. Make sure the all things mentioned above are correct, but as you have the expansion tank system you won't have an overflow pipe from the radiator. If it hasn't been run for any significant time with very low coolant levels, then you will most likely get away without having the head skimmed, but do make sure you clean up the head face and the block face to get rid of any old bits of gasket, or resin residue. You might have to remove the head studs to clean up the block - use two head nuts locked together to do this.

The expansion tank has an overflow pipe as that is where my BGT lost its coolant due to a cracked head.

I replaced the radiator expansion cap with a brand new 15lb cap, went for a 30 minute run at c60mph, when i got back the coolant had once again blown out through the expansion tank overflow. I let the engine cool down and after several hours, when stone cold, I released the cap and coolant erupted out of the overflow tank. Something is pressurising the cooling system, so, unfortunately head gasket / cracked head seems to be the obvious diagnosis.