MGB & GT Forum

MGB just started running HOT!
NO LEAKS!!

Seems I have No water circulation, the bottom hose is soft and no pressure, TEMP. gauge climbing.

How can I tell if its a bad water pump or a bad thermostat?

Also, all of a sudden the cooling fans do not come on?

All these problems started at the same time??

Have you checked the radiator for proper fill of coolant and bled of air

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Fred-
1) Fill the radiator and then idle the engine up to normal operating temperature.
2) Shut the engine off and then loosen the top radiator hose from the radiator.
3) Disconnect the King Lead from the ignition coil, then have an assistant crank the engine and see if coolant comes out. If coolant doesn't come out, then either your thermostat or the coolant pump has given up the ghost and needs to be replaced.
4) Remove the thermostat and repeat the process. If no coolant comes out from the thermostat housing through the top radiator hose, then the coolant pump has passed beyond the veil and joined the choir invisible. Replace it.

My guess is that your electric cooling fans may have ceased to function earlier, but you never noticed the defect because cooler winter conditions prevented the circuit from kicking in.

1) Fill the radiator and then idle the engine up to normal operating temperature.
2) Shut the engine off and then loosen the top radiator hose from the radiator.

DON'T DO IT!!!!!

The cooling is pressurized by the pressure cap, and part of the role of the pressurization is to raise the boiling point of the coolant. If you loosen the cap, a hose, or any part of the cooling system when it is at operating temperature, two things will happen. First the pressure you release will spray you with scalding coolant. Second, by de-pressurizing the system you immediately lower the boiling point of the coolant. While the pressurized coolant may not have been boiling, with reduced pressure the temperature of the coolant may well now exceed the boiling point of the non-pressurized cooling system, which means it could boil explosively, spraying you with scalding coolant.

FWIW, coolant pumps are very rarely the cause of overheating. More likely causes are a stuck/sticking thermostat, low coolant level, or perhaps a mechanical problem like a blown head gasket. If you want to check the thermostat, wait until the engine is cool, then remove the thermostat and put it a pan of boiling water to see if it opens. It also wouldn't be a bad idea to get the use of a non-contact infra-red thermometer and monitor various parts of the cooling system. For instance, if you find that the area right below the thermostat is very hot, and the area just above the thermostat is not, then almost certainly the thermostat is at fault. And the infra-red gun will also confirm that you have an overheating condition and not some other condition.

BTW, you can get a thermostat at any auto parts store. Just ask them for a thermostat for a mid-1970's Chevy 350 V-8. Fits and works perfectly, half the price of an MG stat. You will need an MG-specific gasket....

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Dick-
I said that he should run the engine up to normal operating temperature. That's about 185 degrees Fahrenheit, which is well below boiling (212 degrees Fahrenheit). Pressure does raise the boiling point, but reducing pressure will not create heat. If the coolant temperature is 185 degrees Fahrenheit, then there'll be no boiling, regardless of pressure. However, I concede that I should have mentioned removing the pressure cap from his recovery tank to depressurize the cooling system prior to loosening the radiator hose. Good point!



Edited 1 time(s). Last edit at 2017-02-19 04:16 PM by Steve S..

Actually with the engine at a nominal 185 degrees operating temperature, there is coolant elsewhere within the engine that is significantly hotter than that, particularly adjacent to the cylinders and at the back of the cylinder head. Were it not for this differential, there would never be a concern about removing a radiator cap from a hot engine because, presumably all the coolant would be at 185 and therefore not in danger of boiling (<212...).

Furthermore, "normal operating temperature" may well be >185, since the thermostat only sets the minimum operating temperature once the coolant reaches that threshold. In practice, the coolant may be well above that, if a car is allowed to idle with a mechanical belt-driven fan, or if electric cooling fans are not working properly, or even if they are. And, remember, if liquid boils anywhere within a vessel, it boils everywhere in that vessel. So if coolant adjacent to cylinders is at an elevated temperature, even if the gauge reads "normal" and the thermostat is open, there could well be areas within the cooling system where the coolant could boil if pressure is released.

An alternate method is to go to Home Depot and buy a short length of clear plastic hose that's the same diameter as the upper radiator hose, and temporarily install it when the engine is cold. This way, it is possible to visually observe the flow of coolant when the thermostat opens...

Bottom line is, I would never suggest that someone depressurize a hot cooling system...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Dick-
Your description is apt, to say the least. It points out the oft-overlooked relevant points. Thanks for that!

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Goes the easy route. Thermostats are cheap. Buy one, stick it in. If it fixes your problem, it was the thermostat.

A failing water pump can usually be characterized by coolant coming out of the pump seal orifice or by internal noise - if a pump gets really bad and seizes, your engine will probably stop turning due to belt friction. I'll bet, as others have pointed out, that a bad t'stat or inoperative fans are to blame.

Many do not check the rads in their car for flow as part of their regular maintenance. The rad should see a temperature drop as you trace the coolant flow temperature from radiator inlet to outlet.
When was the last time the thermostat was replaced?

This thermostat seems to work well on Green B.
https://www.carid.com/stewart-components/high-performance-thermostat.html

One way to quickly check for flow is to remove the rad cap, start the engine and watch thru the rad filler neck for the flow to begin when the thermostat opens.
If the rad does not have a good flow when the thermostat opens your rad could be plugged or the water pump may be at fault.