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MG MGB Technical - 74 B running hot

My 74 B has been running hot to very hot. Here's what I've done. Changed out thermostat (correct installation), all hoses felt hot upper and lower, hose going into and out of heater are hot, I do have a slight leak around the heater control valve when it is on--I turned it on just to increase flow as I read someone on here said to do that. Oil has been changed and I checked to make sure it's clean and it is. Water pump is not making any noise and there are no leaks. I changed out the temp sensor. I use to be able to drive it and it would cool down some, but now it moves up very rapidly and actually takes longer to heat up while sitting still and idling. What about anti-freeze change out to more water versus the 50/50? Afraid to drive as I don't want it damaged from the heat.
wss stanley

Check your ignition timing. It may be too far advanced. If it is, the increased amount of heat from combustion that is lost into the roof of the combustion chamber will cause very hot running.
Stephen Strange

Or retarded for that matter, more likely to cause running hot than advanced, which is more likely to cause pinking. Even if it doesn't pink i.e. low compression way over advanced can stall the starter. Incorrect mixture can also cause hotter running.

What is the reading on the gauge? If it's not steaming or losing coolant then it isn't overheating. Have you confirmed that with an external thermometer in the coolant? That should be the first thing to do especially with an electric temp gauge. Turning on the heater just acts like an extra radiator, at the expense of comfort. Fair enough in an emergency but you shouldn't need to do it.

An engine will run hotter than it should if either it is producing more heat than it should, or the cooling system i.e. radiator isn't getting rid of it as it should. For the latter you could do with an infra-red thermometer to compare the rad inlet and outlet and scan the surface for cool spots. A big difference between inlet and outlet implies low coolant circulation, which could be a blockage caused by the thermostat (replacement noted), sludge in the block, or corroded water pump vanes. A normal difference between inlet and outlet implies either too much heat being generated, little difference indicates a blocked rad if there are cool spots on its surface. Are the vanes of the radiator blocked e.g. by leaf debris?
PaulH Solihull

My 78B, which still runs a bit hot for my liking, has been running better and cooler since I increased the advance from about 15 degrees to about 18. Any more and I get pinking when flooring it up hills. I currently have it set so it will pink in 4th if I do that but in 3rd it is fine.

Changing the timing also removed a hesitation when cold, seems to provide more 'go' and reduced some running on I was seeing.

Still less than 1000km on the engine after a rebuild. I am not sure how long it takes to break in and start running at normal temperatures.

Simon Jansen

As always the feedback is appreciated. Wondering about sludge or buildup in the block. Is it possible to flush out the block and start over with "clean" coolant?
I've not lost any coolant due to overheating or steam. Frankly when it climbs to the limit I shut it down as I don't want any damage. Radiator is in good external shape with no bent fins or blockage.
In terms of the gauge it will go the edge of top mark if I let it.
wss stanley

Bill, check that the cam in the distributer is free to rotate. These can sieze up if they are never oiled and condensation will cause rust to form. They always freeze up in the fully retarded position so when out on the motorway, you are running very retarded - and hot. Owen
O McNeill

Sludge build up has been a problem in my 1974 MGB resulting in hot running and engine damage during a cross Europe trip.
Removing the brass drain plug on the engine block (behind the distributor) revealed that the engine block contained an amazing volume of what looked like black sand-rust and anti freeze residue.When the cylinder head was removed it was clear that rust had blocked two or three of the water passages between block and head at the position of no 3 and no4 cylinders.
No amount of poking with wires etc provoked a flow of water from the drain. Eventually I fabricated a device from an old motorbike petrol tap(bsp thread) and a length of flexible plastic pipe which was screwed into the drain orifice and a quick blast from a footpump dislodged the plug of debris and resulted in a flow of the dirtiest water I have ever seen in an engine.
I am currently rebuilding the engine after fitting a new piston and rings into no 3 cylnder.The engine has done about 75,ooo miles and is showing signs of bore wear in all cylinders but I am hoping this repair will keep it going for another year or two until I pull it out for a complete overhaul.
My advice is do not ignore overheating and if your car has stood un-used for a few years as mine had check the engine drain to ensure you are getting water flow.
JT Graham

"No amount of poking with wires etc provoked a flow of water from the drain"

This is *very* common, so much so that a working block drain seems to be a rarity! Interesting way of clearing it though, I don't think I have every heard of poking with wire being effective.
PaulH Solihull

I had problems until I had my radiator boiled out. No more problems.
don g

This thread was discussed between 26/07/2010 and 29/09/2010

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