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MG MGB Technical - Coolant temperature

My Gilbern GT has a 5 bearing MGB engine (and other mechanicals). I had the engine rebuilt by Aldons when I rebuilt the car a while ago and have since found that the water temp is all over the place. Some of the time it runs steadily at about 170degF but it can suddenly rise to 230, especially when held up in traffic, and even goes off the clock, before plummeting to its lowest level. It doesn't seem to lose water. The electric fan comes on and brings the temperature down. I've tried changing the thermostat twice to no avail. The alternator belt is tight and the rad was overhauled when the rebuild was completed, though the heater matrix wasn't. I have concluded maybe the block's waterways were not cleaned out properly at the time of the rebuild so the water doesn't circulate properly. Another other suggestions, and if I'm right is there anything I can do to clear the block out without a complete dismantling job? Any help would be much appreciated.

M E Ingall

If "The electric fan comes on and brings the temperature down", then why does it suddenly rise? If it has to rise to above BP before the fan cuts in then the fan thermostat is faulty or set wrong. Maybe the temp gauge is erratic not the actual temperature!
Hot running can also be caused by; ignition too advanced; binding brakes; too lean a mixture.
Allan Reeling

Yes, those are all very useful thoughts, especially the gauge idea. I will try another one. The mixture and ignition timing were set up on a rolling road but that was a while ago. Very many thanks Allan. cheers Martin
M E Ingall

X2 on the problem being in the temperature gauge or sender. Yours is a classic sign of this. RAY
rjm RAY


If the gauge is correct, I would think that air in the system could be a likely cause. The rapid temp swings are what makes me think it could be air. You might be able to fill it from the bottom up and force air out the top, depending upon how it is plumbed.

Cleaning crud out of the block would mean removing the core plugs and water pump. Not impossible, but no fun either. It is a pretty nasty job even with the engine out. You can get some idea of how clean it is be seeing if the drain plug works.

Nice looking car. Do you have a view from the rear?

C R Huff

"The rapid temp swings are what makes me think it could be air."

The V8 has a 'steam pipe' between a water passage in the inlet manifold and the thermostat cover, i.e. bypassing the thermostat. When that gets blocked (as mine did) the temp gauge can react very violently, swinging between very low and off the scale as here. I've also seen similar to that in an conversion where a more modern V8 was used that didn't have the steam pipe. In that case at least opening the heater control valve stabilised things, as that also offers a bypass. I'm not offering it as a cure, but if the heater valve stabilises it then perhaps the bypass port in the head has been blocked off, or something similar.

Conversely, I've had my V8 loosing coolant, and the temp gauge stayed rock steady. It was only when I noticed a very slight misfire that I stopped and investigated, to find the bottom hose punctured by the fan belt and coolant was reaching the plugs that side.

But checking the gauge with another instrument is the first step.
Paul Hunt

V8 Register Workshop Note No. 16, January 1980, mentions the steam pipe on the B V8.

Tony Lake recently posted some comments about the system, with particular reference to conversions, and mention of the steam pipe, on the V8 Register Bulletin Board.

"The factory GTV8 cooling system is pretty well designed in that it vents well during fill and needs only occasional checks to make sure that coolant in both radiator header tank and overflow tank is at the correct level.

mgb roadster and MGBGT conversions often use V8 engines from Rover cars that have a thermostat housing with a near vertical outlet. This makes the outlet hose the highest point in the cooling system, it is then very difficult to fill the system completely. A vertical outlet is required because of congestion beside the thermostat housing where the distributor vacuum unit is located on Rover spec engines. On the factory GTV8 the vacuum unit is positioned 180 deg away on the exhaust side of the engine. This leaves plenty of room for a horizontal outlet from the thermostat housing and a corresponding gentle hose rise to the radiator header tank which is then truly the highest point.

Another important and often overlooked point is that the factory GTV8 has a vent line from the top of the heated intake manifold to the radiator side of the thermostat housing. During coolant fill air is continuously purged to the top hose. Without the vent line complete engine fill is very difficult, all the air has to pass through the thermostat jiggle pin, if it has one, ref recent posts."

The thermostat should have the small bleed hole ( with or without a jiggle pin ) and ideally be mounted with that hole uppermost. Some suppliers will supply one without the hole even if they claim the correct part number.

The fan belt ( actually water pump and alternator belt, there is no engine driven fan ) rubbing on the lower radiator hose is usually when the wrong water pump is fitted. The correct pump has the inlet stub at an angle which routes the radiator hose clear of the belt. The attached image showsthe difference; the dirty pump is an original BV8 pump. Possibly for conversions the engine mounting position might have an effect on the hose routing ?
J N Gibson

Dunno what happened to the picture, lost in the ether ?

Maybe it's here ...

J N Gibson

"On the factory GTV8 the vacuum unit is positioned 180 deg away on the exhaust side of the engine."

Spot the deliberate mistake :o)

Thermostats can have the original jiggle valve which opens up a reasonable hole for filling, but closes when the system is full to stop circulation when the thermostat is shut. Then instead of the valve some got a tiny V-notch in the edge of the main valve disc, which despite being so small seemed to be adequate for filling, but did allow a little circulation when cold. The last one I bought had nothing, and made filling impossible on my 4-cylinder, I had to drill a small hole in the surround.

You shouldn't need to check the radiator as the design of the system with the remote expansion tank has the outlet pipe from the rad header tank at the highest point internally. So if there is any air in the radiator, this will be pushed into the expansion tank as the engine heats up and the coolant expands. Conversely the inlet to the expansion tank is at the bottom, so as the coolant cools and contracts it will always be coolant that flows back into the system - assuming the level in the header tank is correct. 4-cylinder cars with the remote header don't have the facility to check the radiator anyway.

My V8 does have the correct water pump, but it is a matter of choice as to whether it's too close to the fan belt or too close to the oil cooler hose as there is a huge range of position depending on how one twists it on the pump and rad. With the last two bottom hoses I've had to trim the radiator end of the hose back about 1/4" or it is pressed against the cooler hose - the turned out flange on the crimp which is not good. After having the hose cut by the fan belt I fashioned a guard which has stopped that problem. But despite trimming the hose to move it away from the cooler hose, after 5 years and 12k (I was warned I'd be lucky to get 2 years out of it as they were the only ones he could get at the time) it was weeping from a wear notch from that. It was also showing a lot of surface cracking, and split when removing it from the pump, showing how rubbish those Kevlar hoses were. The original was still smooth, shiny and pliable after heaven knows how long. I do have a better quality hose now, but have chosen to fit a guard for the cooler hose as well.
Paul Hunt

If the temp gauge is electric, change the voltage stabilizer. This stopped my B from "overheating".
Glenn Mallory

This thread was discussed between 01/04/2016 and 14/04/2016

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