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MG MGB Technical - engine running too cool

Hi Folks,
I am running a standard 1800 1975 MGB, which I have owned for approx 3 years. It is regularly serviced with carb balancing etc.

During all this time, when the engine has warmed up, the temperature gauge has consistently registered only one third of the way over the gauge.

If I get caught in heavy traffic, the temperature can creep up to the 'normal' point..., but once on the move it will return to its one third position again.

The cooling system has filled with the 'forlife' cooling additive for approx 13 years. My concern is that the fuel consumption seems high at approx 19/20 mpg and the heater is not particularly hot...

Is the reduction in temp. reading caused by the forlife product, a dodgy thermostat or something else?

Thanks in advance

Peter Cosgrove

Hi,
I used to have this problem too, it appears it was caused by a badly gunged up cooling system (it was full of brown sludge when I purchased the car) I spent a morning taking the radiator out and running many gallons of water and a cleaning solution though the entire system and the car has run at 'N' ever since
I don't know if this is a common cause of running cool though?
David Hayes

Either that or you could patent this and sell it to the overheating crowd which seems to be more frequent.
John Bamford

Hey

This is not a common complaing on this BBS, most people's cars run hot! :o)

Don't forget that it might be your temp. gauge that's mis-reading... if you can double-check it with another guage and sender you can be sure.

If it is running cool, it may well be from a failed thermostat - they're only simple devices and are designed to fail 'open' so as not to melt your engine. I would think it unlikely that it's your coolant additive that's causing a problem, as even if it's super-efficient at taking heat away from the engine, the thermostat should make sure that everything stays at the right temp.

For 4 quid and 10 minutes' work you can change the thermostat (don't forget to buy a gasket too) and relax knowing that your engine will warm up properly.

Where in the West Mids are you?
--
Olly
Oliver Stephenson

19 - 20 mpg (UK Gallons) is way too high a consumption for normal motoring. Unless you spend your life in crawling queues, or have an amazingly heavy right foot, mid twenties to low thirties would be what you should aim for. I should think a compression test and thorough tune-up is called for (preferably on a rolling road), paying particular attention to ignition timing and mixture setting. Oh, and check for binding brakes too.

Mike
Mike Howlett

Check the thermostat rating - I would recommend the 88 degree one rather than the 82 degree one - you may find yours is either stuck open or is a 74 degree one.
Chris Betson

Peter-
It's possible that the heat sensor is defective, but that seems a long shot as they usually just fail completely and the gauge then gives no reading. It's equally unlikely that the gauge could be that inaccurate. It's also possible that your thermostat has failed and is stuck in the open position, but from what you describe, I have to wonder if your thermostat has been replaced by a blanking sleeve. Have you ever looked?
Steve S.

Peter. What you need to do is measure the actual temperature that you are running at. I believe the 75 cars still used the standard type radiator with the pressure cap on the radiator instead of the sealed radiator with the seperate expansion tank. If so, run the engine up to operating temperature, carefully remove the radiator cap and insert a meat thermometer (inexpensive and fairly accurate) into the coolant to see what it reads. That is your baseline.

I have a 79 LE (US spec) which consistantly reads about 1/3 of the way up the gauge when being driven. The engine was a very good, reliable unit, but not original to the car. Thus, I rebuilt a spare engine and installed it. The newly rebuilt engine shows the same gauge readings as the old engine. I installed the old engine into my daughter's car while rebuilding her car's engine. Her engine always ran mid range on the temperature gauge during our "winters" and about 3/4 during summers. The engine out of the 79 runs exactly the same, using the same cylinder head temperature thermocouple. Thus, I find it perfectly possible that the gauges themselves may read differently and that the only true test is what the actual temperature reads on a thermometer. Check what the actual running temperature is. Les
Les Bengtson

Unless that car has six hidden rads, my money is on an innacurate temperature indicating system. I do not believe there is an MGB alive that constantly runs at 1/3 of normal temp- even in the Antarctic in July. Of course, this is April first. . . .
Ken R
Ken Rich

Ken - oh yes there is - my B ran at 1/3 N for years - combination of clean cooling system and 74 degree celsius (centigrade) thermostat.

Remember that the temp guage scale is not linear - the N range goes from approx 70 degrees just above C to approx 90 degrees just below H. So 1/3 is around 77 degrees, on the N is about 80 degrees and 2/3 N is about 83 degrees.

My V8 uses the same guage and commonly runs around 95 degrees, which is well into the H, without boiling or any other problems.

The hotter the engine runs (within reason) the more efficient it is, which is why I normally recommend the 88 degree thermostat.
Chris Betson

If it creeps up to normal when stuck in traffic but reads low when running the stat is stuck open. A bad gauge will read under in all conditions - except when the engine is overheating! - but I would expect a low-rated stat to be similar.
Paul Hunt

In sub 70F weather mine will read that low, but goes up to N in hotter weather.
Greg Bowman

just as a point of interest;

Some modern all-bells-and-whistles engines have variable temp thermostats... when the ECU decides that you're cruising along the motorway and want efficiency, it lets the coolant up as high as 105 deg. C. When you put your foot in, it opens up the thermostat to get the engine back to normal operating temp and retards the spark until it has done so. It generates a bit of lag on full power being achieved, but it saves fuel.

Of course, these engines are designed specially to be able to cope with the higher running temps for long periods of time. I wouldn't recommend running a B at 105 degrees, it would go all wobbly.

ttfn,
--
Olly
Oliver Stephenson

Chris, is Essex Antarctic or Arctic?
Ken R.
Ken Rich

If I remember correctly, the catalogs list two different electric temp gauge sending units, depending on year of manufacture. It has happened many times that the wrong one was fitted causing an incorrect reading. This is just another possibility and certainly a lot cheaper and easier to check out than rebuilding or replacing the gauge. Let us know what you find! Cheers!
Bob Muenchausen

Not applicable to the original poster as the UK didn't get the electric temp gauge until 77.
Paul Hunt

Hmm, on the "which 'stat" issue, I've gone back to the old-fashioned way of doing things. My car gets an 82 degree 'stat in the warmer months (which saves having the leccy fan on all the time and absolves me from studying the temperature guage continually) and an 88 degree 'stat at cooler times (thus keeping things warm and snug).
Going back some time, swapping thermostats every six months used to be part of everyday motoring life. I tend to the opinion that this is omitted from the MGB service schedule due to punter pressure and not because of some revolutionary rework of the B series engine's cooling jacket(!)
The car certainly seems better behaved with this regime in place and the guage reads within 10 degrees (angular) of half-way at all times once warm-up is complete.
It also means that the coolant is always tip-top as I can't be ar*ed to collect it and pour it back in each time for the sake of 5-odd quids worth of antifreeze :-)
T J C Cuthill

Thanks for all the responses - I did not think my posting would generate this volume of help & knowledge.

It looks like I will be checking out the thermostat for a) being present, b) being faulty & c) being too low a rating... I will purchase an standard thermostat and let you know the results..

Thanks to all

Peter Cosgrove

This thread was discussed between 01/04/2003 and 03/04/2003

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