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MG MGB Technical - Engine overheat

Hi folks,
All of a sudden the temperature on the temperature gauge approaches the red mark before the two electric fans (new) kicking in on my 1980 MGB and the engine gets really hot. I already changed the thermostat, radiator cooling fan thermostat, coolant temperature transmitter. There is no leak. Before this condition appeared the engine temperature was always normal. Could it be the water pump (no leak)?
Reto Schlumpf

Make sure your cooling system is full.
It is possible the radiator switch for the fan is defective.
Make sure the radiator does not have any obstructions.
Impellars on water pumps do wear out, reducing the flow of coolant.

Thanks Kimberly,
just ordered a new water pump and a new radiator switch.
Reto Schlumpf

Measure the actual temperature first, to determine whether it *is* too hot or the gauge reading is too high. You say 'approaches', maybe you are expecting them to come on much sooner than they should. If the coolant *is* getting too hot I'd expect the fans to come on at the normal time i.e. about mid-way between N and H, but not be able to reduce or maybe even hold the reading. A faulty instrument voltage regulator can give high electric temp and fuel readings. On the V8 you can get steam pockets which when they hit the sensor cause the gauge to sky-rocket. Conversely when the coolant level is low and there is no steam the gauge reads about normal.
Paul Hunt 2


Sounds like you don;t have enough coolant flow. In the old days we had to get the radiator rodder out or replace it.
James Huggins

Replaced now all components of the cooling system.
Still the same problem. Just flushed the cooling system and found a lot of contaminant in the radiator. Temperature is little lower now but fans (new as well)still kicking in too late. Furthermore finally managed to open the drain plug close to the distributor and nothing is coming out from there. It's completely dry inside. Tried to 'blow' it but it seems to be completely closed in there. Is this normal? (MG B 1980 American / Japanese specs.) Thanks for help.
Reto Schlumpf

It sounds to me that you have not bled the air out of the system.

I do this by filling the cooling system thru the pluged hole in the top of the thermostat. I have a funnel that jams into the hole. I top it up and bleed it by squeezing the bottom radiator hole. The air will continue to gurgle out of the funnel (keep topping it up as the fluid drains down into the head). When no air comes out, the system is bled. Make sure the heater valve is open while doing this. Also, have the radiator cap off of the header tank until it fills up, then when it is almost full install the cap and keep bleeding until all air is done bubbling up thru the funnel. I plug it with the coolant level flush with the bottom of the plug. This solves this problem.

M Whitt

Yes. To re-emphasize what Mark advises, it's important to keep squeezing the bottom hose while doing this, in order to force air and fluid flow through the system. It's a pain but when done you'll know there's - or little - air in there.

Rick Stevens

It is common for crud to clog the block drain. Some have used a wire to clear it. Some have even put the wire into a drill to snake out the drain. Check with your locals parts supplier to see if there is a product availble to flush such deposits out of the block.

I actually did a good bleeding of the system. What is puzzling me is the fact, that inside the block drain it is dry and i don't know if therefore a part of the block does not get cooled and this might cause the high temp indication (almost red). Is it correct there should be water coming out from there once opened (under normal condition)? If yes how can i de-block it (tried the wire, but it seems to make sharp turns inside).
Thanks a lot for inputs!
Reto Schlumpf

What is the level of coolant in the radiator when cold and when hot? Just where is the temp gauge pointing when the fans finally do cut in? Did you measure the temperature of the coolant at N and when the fans do cut in? What type of gauge is it? Mechanical i.e. dual gauge? Or electric i.e. a single gauge? What happens to the gauge when the fans *do* cut in? What does the temp gauge read when the fans cut out again (if they do). What is your ambeint temperature when this happens (being in the UAE)?

It seems very common to find the drain tap dry, same on mine. The possibility of coolant not reaching all parts of the block is not the cause. All that does is certain parts of the engine will get hotter than the others, which possibly could be too hot if the uncooled area is large. If coolant isn't going past that area then it won't pick up heat from it to transmit to the temp gauge sensor, and that sensor needs to be in hot coolant for it to read hot. In my experience if the coolant drops below the sensor, then until the engine gets so hot it starts boiling what coolant *is* in there, and steam from *that* reaches the sensor, it doesn't read hot (and then it starts pulsing all over the place).

You shouldn't need to 'bleed' the cooling system, as long as it is filled and first run *at least* on the level, ideally a bit nose up, with the heater tap open, after a few heat/cool cycles it should have purged any air into the header tank anyway, which is why you need to check the level each time it has cooled down until you don't need to add any more. I've never had to do anything else in 40 years.

If there is insufficent coolant in the radiator it may not reach the thermo switch to turn it on.

For the temp gauge to get to the red either a) the engine is generating more heat than it should, b) the radiator isn't able to get rid of normal heat fast enough, c) the pump isn't circulating it fast enough to allow the whole of the radiator to be used, or d) the temp gauge is simply indicating a higher temp than exists.

a) can be detected by the radiator outlet being almost as hot as its inlet and hot all over. Common causes are timing over-advanced or retarded.

b) as the radiator outlet being almost as hot as its inlet with cooler areas elswhere in the matrix.

c) as only the area near the inlet being hot, the rest as you get closer to the outlet being cooler.

d) should always be checked with another thermometer in the header tank as a first step.

With electric cooling fans they could be too slow (electrical problems), turning in the wrong direction (ditto), the blades being on back to front (less efficient).
Paul Hunt 2

Thanks for all the inputs!!

At present the temperature in UAE is only 25 degrees.
The engine seems to get hotter than the radiator. The coolant level is correct and there is no air inside. The main problem is the fans kick in late (almost approaching red on the gauge). Furthermore the fans can not reduce the temperature when the car is stationary. Once driving the temperature drops to the level i am used to (just below middle on the gauge).

According the workshop manual the fans should turn anti clockwise, which they do (pushing the air through the radiator). The odd thing is the fan blades actually turn against their logical (air wing/propeller) design, but i had them installed according the manual...

Unfortunately i do not have an independent thermometer.

I just would like to try to open the blocked area where the plug is because in the summer it gets very hot here.
How much of the engine block does not get cooled?
How can i de-block it?

As mentioned before i have replaced the complete cooling system...

Strange… only this 1980 model (the only MGB in Dubai) gets my into trouble...
I never had any troubles with my previous older B's...
Reto Schlumpf

Don't worry about the block drain not allowing coolant to escape. This is a very common occurance and the coolant passage leading to the drain is extremely small. Little or no difference in cooling will be obtained by waisting large amounts of time clearing the drain. You should be able to purchase a relatively inexpensive thermometer at an auto store or even one desinged for cooking. Ray

those fans should rotate anticlockwise when viewed from the front of the car. check the blade fitting again. i have found many fitted wrong way round. the thick rounded edge should be the leading edge i.e. to the left when viewed from above . as a check the hollow side of the curve on the blade is nearest to the radiator. jim34
j soutar

Reto. As a follow on to Kimberly's advice, here in the desert Southwestern part of the US, we have highly mineralized water. When people use tap water to fill up their radiators, it is common to find that the rear portion of the block is filled with crud when the rear core plugs are removed. This is commonly removed by inserting a stiff wire into the area and breaking up as much of the particulate matter as possible. Peter Burgess is recommended the use of CLR, I believe, before an engine is rebuilt. Do not know if that would work with a block that is not being torn down, then cleaned after the flushing process.

As to your drain valve, I am not familiar with them. My 68 GT does not have one, nor do my two 79 Bs. I do remember such a feature being on my 61 Sprite. Thus, your engine is at least 26 years old and may be older. Worth investigating. Also worth discussing if anyone has used one of the more agressive calcium cleaning products on an engine which was to remain in the car and not be torn down for rebuilding.

Infa-red thermometers are relatively inexpensive here in the US. Mine is a professional model costing something over one hundred dollars. Hobbyist versions, at about $70 are available and test accurately against my version. Worth considering. Should be able to order it over the internet.

Les Bengtson

I have a 75 B without the plug on the thermastat housing, no header tank and a centre fill radiator. I have today replaced all hoses and fitted a new radiator. I have read this thread with great interest and would like to ask the best way to bleed my system. Thanks, Chris.

Reto - it does sound like there is a problem with the fans if driving the car reduces the temp gauge to just below N but the fans don't reduce it all. I agree with Jim that they rotate anti-clockwise with the thick edge leading and the concave face facing the radiator, i.e. as in conventional wing design. Without being able to test the coolant temperature (a meat thermometer will do) in the header tank - remove the metal or plastic plug in the top of the header tank *carefully* - you really don't know whether it is gauge, fan switch, engine or radiator.

Chris82 - just run it through 3 or 4 heat/cool cycles, heat valve open, checking the level each time when cool and topping-up if necessary. If you overfill it will purge the excess from the overflow tube as it warms up. Probably not a bad idea to put a small catch bottle under the overflow if you are driving it rather than running it stationary, after the first couple of cycles, and overfilling it just the once to get some overflow (the catch bottle will show you there has been overflow, check for that when still hot). Then when it cools you know where the 'cold' level of coolant should be. I'd avoid taking it on a long journey for the first cycle in case it purges a lot of air and the level drops. The header tank is at the top of the radiator, it is the later remote expansion tank you don't have.
Paul Hunt 2

Thanks for the info. All done. Judgeing by the thread, its a lot easier to do than the later type. Thanks again, Chris. (P.S Good Website).

The drain for the block is not a valve. It is a threaded plug. Look in your owner's manual or if you don't have one, look in the owner's manual section of the the Bentley repair manual. It is in the section labeled cooling system, figure 42 #4. You will find the drain plug on the passenger rear side of the motor next to where the oil line fitting connects to the block.

Kimeberly. Thanks. Will take a look for it. Les
Les Bengtson

Mk I cars did have a drain tap, it was replaced by the plug with the introduction of the Mk II beginning in Nov 67. See Clausager in engine production changes on page 140. The taps are still available in the Moss catalog.

During my engine rebuild I cleaned the plugged area around my drain plug with a long screwdriver, hot tanking didn't clear it. My car has never seen tap water since I purchased it in Jan. 98, always used distilled water (according to the label) yet when I changed coolant a year after the rebuild the area under the drain plug was dry. I have been unable to clear it with stiff wire or anything else. I assume it doesn't hurt anything, the wear in the rear cylinder wasn't any worse than the the other cylinders when the engine was rebuilt and it doesn't make any noticeable difference in coolant temperature.


Clifton Gordon

Just a couple of thoughts....and as always I bow to the more knowledgable MGenuises here......since you have been pretty well through the coolant system, Is your engine getting lubricant?.....Friction causes heat......Have you considered one of the after market high output fans?
Best of luck.
J A Kelly

Thank you for all the inputs.

The fans were installed the wrong way around...!!!
The efficiency is better now and during idle running the temperature is staying just shy of HOT. When driving the temperature drops well below middle. Is this normal now or is it still too hot as the workshop manual just mentions that the fans will be kicking in below HOT. Would it be better to leave the thermostat away (install dummy case)? Are there radiator sensors available with a lower threshold?

Does anyone have info about the area affected within the engine block (dry drain plug next to distributer, Kimberly described the location)?

Thank you as always...
Reto Schlumpf

Again, until you measure the temperature of the coolant neither you nor us will know which part is defective and needs replacement.

In your location it probably doesn't need a thermostat at sea level, although I believe it snows and freezes in the mountains further north at least. A restrictor must be fitted in its place, or just remove the spring and bulb from a standard stat, or you can get too rapid circulation of coolant which can create turbulence and hot-spots. But I'd say that if the temp gauge drops well below the middle when driving then either you don't have a stat anyway or it is defective. And if the gauge is indicating the correct temperature then you *do* need a stat, at the moment anyway, as it is being overcooled.

Again if the temp gauge is correct it would seem that the fan stat is coming in at too high a temperature, which may be defective or the wrong unit, it should cut in at 90C 194F, which is about mid-way between N and H, and cut back out again about half-way from there back to N again. But if the temp gauge is staying at or near H at an idle, and the fans are running permanently, they they still aren't cooling as they should. If driving it gets the temp gauge *below* N, in an ambient of 25C, then it can't be a problem of the engine producing too much heat for the cooling system. Measure the voltage on the motor side of the 2-pin connector for each fan, when running of course, and compare that with the voltage at the brown fuse.
Paul Hunt 2

Reto... sounds to me like you are having the same problem I HAD. 1979 MGB. Here is what I did "after" I replaced all the hoses, put in a new radiator, installed a new thermostat, sending unit etc, etc...oh yes , and made sure the fans spin the right way too, squeesed the hoses, etc, etc,.

I found out I was NOT putting in the coolant correctly. If you have the same radiator set-up in your 80 as my 79... fill by OPENING up the fill plug on the top of the thermostat elbow... turn the HEATER to on, move to highest temp. Fill the expansion tank about 1/2 full. Then use a screwdriver to hold open the thermostat and fill, let it gurgle a bit, take your time and keep filling until you get the level even with the plug opening. Remove screwdriver, put in plug. Start engine and allow to run a few minutes (3-4 minutes) remove plug and add more coolant if needed.... you may need to prop open the thermostat again. Repeat until full.

My service manual recommends checking and topping off at the "plug" every 250 miles! Dumb me, I was just checking the level of fluid in the expansion tank. Once I started to make sure the coolant level was at the right level at the PLUG on top of the thermostat, my MGB started to run at the desired temperature.

Simple, but it works ok for me,

Safety fast....

Don 1979 MGB


This thread was discussed between 07/12/2006 and 20/12/2006

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