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MG MGB Technical - Temp gauge

Hi

I have a question about runnning temps. I know there's lots in the archive, but there's so much stuff about so many cars running in so many different climates I can't seem to find what I'm after...

I have a 77 UK roadster running in UK summer (max ambient temps lately have been something approaching 30C (~86F) but more usually 20C (~68F) or so). My gauge generally runs at around 80% of the way from cold to hot. Can vary from 60% to 90% (very near/just touching red) without any apparent clear link between outside temp or type of driving but always significantly over half-way. As a very general observation I would maybe say (surprisingly) it seems to be perhaps a little cooler at idle/slow traffic and a little hotter when tanking along at 90mph but don't read too much into that.

Timing seems ok. Don't think it's running lean (but need to dig out color tune). The engine is a recent recon (<1000 miles). Have flushed radiator and get good flow through with clear water.

Any ideas how hot (in actual degrees or position of gauge) it should be? Any good tests for gauge - can I connect it to a standard resistance or voltage (don't know their operating principle) and expect it to go to a particular position?

Thanks for any help
Darren
D Lewis

Darren - The minimum temperature that the engine is going to run at is whatever temperature the thermostat is calibrated for. You can usually find that temperature stamped somewhere on the thermostat. I run a 190°F thermostat in our 66 MGB, so it reads a minimum of 190° all the time, with occasional excursions above depending on ambient temperature and what the driving conditions are. We have occasionally hat the gauge go well past 212°F when climbing long, steep grades on extremely hot days. Recognize that anything below the far right edge of the Hot area is pretty normal as far as operating temperature, and your gauge residing in the 80% area of the gauge is really fine.

To check what the actual temperature is for a given reading of the gauge, I would suggest taking an old pan (not on of the wife's good cooking pans) and filling it with a 60% solution of water/antifreeze (boiling point of about 230°F) and put it on a camp stove next to the car. Run a extension on the sensor wire and a ground wire that are long enough to reach the pan, remove the sensor from the head, hook it up with the extension and the ground wire and suspend it in the water/antifreeze mixture. Stick a candy thermometer in the water/antifreeze mixture and start heating the whole mess. Note down the temperature of the water/antifreeze mixture at various positions of the gauge. I think that you will be surprised with what you find. I have been doing some work with gauges on the Magnettes to get them to work with MGB sensors and most of those gauges are calibrated for 212°F at a dot to the right of the Hot area. Others who have worked with the MGB and midget gauges have found that 212°F is somewhere at or in the Hot area. This is nowhere near boiling for a system that has 25% - 50% water/antifreeze mixture and it pressurized at anywhere from 7psi to 15psi. Since conventional wisdom states that a car is not overheating as long as it is not boiling while the car is moving, the Hot area of the gauge is calibrated to scare the hell out of the driver rather than really indicating a overheating condition.

I guess that all of the above says that if the gauge gets into the Hot area, check things out, but don't panic, if steam is not coming out from under the bonnet and the engine is not stumbling or pinging excessively, the car is doing fine. Hope this all helps.
Cheers - Dave
David DuBois

Darren,
I would suggest that you take a look at the archives, as I was involved in a very long thread back in May dealing with this very subject. There was a lot of very good information posted. You may be running the correct temperature - and lower then you think - if the temp sending unit is faulty. I went three before I found a good one. Also, find someone with an infrared temp gun that can shoot your engine and check/verify the temperature. I thought the engine in my 73MGB was running hot, then I used an infrared camera I own, which works like a video camera, and I found out the engine was running at the exact correct temperature, even though the gage said it was running at 80% hot. But as everyone pointed out to me, your engine is not running hot unless it boiling over. Go to the archives.
Good luck.
Robert
Robert Browning

Darren,

As David has mentioned should run at thermostat temp, which should be half way, the bottom of the white hot zone is 212.

Air flow at speed should keep it cool and traffic causes a problem of hot air not escaping from under bonnet so may tend to rise.

I doubt if guage is accurate but you could replace with new with temp markings. However, a problem normally only arise if there is a sudden change to its usual pattern.

If you use 4life this boils at 180c, so you might overheat engine before this boils.

Paul
Paul

That is a very high indication, which may not be the correct indication. In the UK it should really run very close to the N under all but rare occasions, although RBs do seem to run hotter than CBs. Cars with electric fans are also designed to run in stop-start or slow-moving traffic anywhere between N and mid-way from there to H when the fan should cut in, but in free air at cruising speeds it should really be much closer to N. Where on the gauge does the fan cut in if left to idle? It should be when the needle is between the two little dots partially hidden under the chrome bezel. If it does then unless the fan stat is equally incorrect in the other direction, which is pretty unlikley, it would appear to be indicating actual temperature. If it tends to run hotter when running than when idling it does indicate the engine is overtemperature when running.

Even so, perhaps the second thing to do is measure the coolant temperature with the engine running and the needle at N and one of its high points. Unless you have really been thrashing the car and come to a sudden stop you should be able to release the cap without any problems of boiling, but do so with a cloth and slowly, tightening again if you hear any deep gurgling or coolant starts splashing out, hissing is OK. Likewise you should be able to run the engine at a fast idle without a cap without it boiling. The coolant will expand until the fans cut in, then contract quite rapidly, and it depends on how much space you have relative to the volume of coolant as to whether it will overflow or not. Likewise, depending on how full and how hot it is, removing the cap on a hot engine may cause it to overflow.

You say timing seems OK. A 77 should have manifold vacuum which is easy to check, at 1000 with it connected you should be seeing about 34 degrees, dropping to 10 degrees with the vacuum pipe disconnected, the idle speed should also fall, and possibly stall. You are checking and adjusting the timing with it disconnected I trust? Again with it disconnected check the centrifugal advance, you should see about 20 degrees total at 3k and 40 at 5k if you dare.
Paul Hunt

This thread was discussed between 17/08/2004 and 18/08/2004

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