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MG MGB Technical - Temperature fluctuations

Recently replaced the tstat in my standard 66 BGT. I use an 'early' stat (74 deg C tstat) as per manual. The reason I changed was that on the early morning commute into work the temperature could drop a tad below 165F (74) according to the gauge in the car. After fitting the new tstat (again a 74 deg C) on my journey to work (crusing about 60 mph for 10 mins) the temperature suddenly rose from 165 F to just under 190F before falling to 165 again. 30 seconds later, did the same thing again, and again and so on. Is this the tstat opening and closing, reacting to the cooling from the radiator? Seems odd behaviour? Should I live with it or do you reckon it is faulty. The only difference with the old tstat it replaced was the lack of a small hole in the metal surround (with some kind of float in it).

Alternatively I could use an 82 deg C as many in the archives recommend?

thanks for any advice

Richard
Richard Atkinson

My roadster has *always* fluctuated up and down at certain temperatures, particularly during warm up but it can also been seen when changing from climbing a hill to descending in cool weather, and I have always assumed it was the thermostat opening and closing. The V8 was the same during arm-up, so I assumed they all did that. Then after some work on the cooling system of the V8 to rectify a problem it no longer pulses during warm-up, just rises fairly quickly then stops just short of N while I'm driving, only going above that in traffic as it has electric cooling fans. There is another known effect with V8s and that is when the 'steam pipe' between inlet manifold and thermostat housing is missing or gets blocked, then you get massive fluctuations, I've seen it as high as 40psi on the oil gauge!

Thermostats are a relatively crude mechanical device, and they don't maintain an exact temperature, but will result in a slightly higher temperature as the stat opens compared to when it is closing, but there should only be 2 or 3 degrees difference. In theory it *should* be a continuously variable device, balancing the heat output of the engine with the cooling effect of the radiator and ambient air over the block, rather than opening as the engine warms up then staying fully open until it cools right down again. If it did that the the temp gauge would be all over the place as the load on the engine varied. having said that Clausager talks about a 'latch-open' thermostat being used from 1967, which seems to indicate they *do* latch open. When I've tested ones I have they don't latch, but exibit a gradual opening and closing as the temperature goes up and down.

The hole with or without the 'jiggle valve' in it aids air expelling when filling the system. Thermostats without this hole often have a little V-notch in the edge of the valve which does the same thing. If it doesn't have either then you could be getting air or steam trapped under the stat which will affect the gauge sensor.

It sounds like your new one could be faulty, sticking and giving a much wider variation in temperature between heating up and cooling down, you could test it in a pan of hot water and see what happens. Or just treat it as an infant mortality and replace it.

Paul Hunt

Paul, thank you for your advice. It was the wide fluctuations (20 F) in a few seconds that troubled me. I'll replace it with another one.
cheers
Richard
Richard Atkinson

I could be wrong, but I think the small hole is there to let a small amount of coolant circulate as the engine is warming up. Without it I wonder if the coolant trapped in the engine and head get to the point where they open the thermostat rapidly, then allowing a flood of cold coolant through from the rad, closing the thermostat again, and repeating the cycle. I would think that eventually things would even out though, and the fluctuations would level off. I'm curious, is this the type of thermostat with the sliding sleeve or a modern one? MGAs had the sliding sleeve type originally, but I think most people here in the colonies have replaced them with modern 'stats. I've heard of some issues with the sliding sleeve type in MGAs probably more of a problem here where we can get some very hot weather in the summer.

Ralph
Ralph

Ralph - this was a modern type with a very small v notch as Paul mentioned. Last night in the process of refitting the housing, having changed the stat for a new one, I managed to strip the thread from one of the housing studs. A trawl through the archives to find the best way of removing these things beckons - you just know is going to be a b***er of a job!
Richard Atkinson

Double-nuts is the usual way, hopefully there will be some good threads further down. After that heat and grips. Failing that weld a bit more stud on top and use spacers between the cover and the nut!
Paul Hunt

This thread was discussed between 20/07/2009 and 21/07/2009

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