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MG Midget and Sprite Technical - Baffling cooling problem

I'm starting a new thread as, after correcting the warped head and refitting, the cooling system is acting very strangely. I wanted to check the mixture again, which requires warming up fully of course. After a short journey I left the engine idling, to wait for the thermostatic fan to come on, which is usual practice. It normally comes on at 190 on the temp gauge. But it didn't. The gauge went up to 190 and stayed there, and eventually the radiator boiled. Was it a bad thermostat in the head? I swapped it for one which I verified was opening in water at 90C. This made no difference - this time I chickened out before boiling point. I've had problems with the fan thermo-switch before, and the gauge went way beyond 190 which it should do with no fan at idle. So I think the gauge is OK.

The heater is working fine so water is circulating. I haven't checked the thermo-switch and will do so when things have cooled down again.

I should clarify that the temp gauge sensor is in the head, just below the thermostat, and the fan switch is in the top of the thermostat housing. Conceivably both the gauge and the thermo-switch are playing up, but that seems very unlikely.

Les
L B Rose

Hi Les,

Sorry to hear of your cooling problems. It sounds like a dodgy thermo-switch. I have a cross-flow radiator and electric fan, and had problems with the thermo-switch coming on too late and leaking (it was pushed into the top hose). The solution was to fit a new in-line thermo-switch to the hose, and as a back up I wired in an illuminated toggle switch: this allows me to get the fan running early on really hot days.
Good luck.

Nick
J.N. Williams

Les, is it perhaps possible that the top of the thermostat housing, being the highest point, is holding a bubble of air so that the fan thermostat isn't properly immersed in water, and so not responding correctly.

Many years ago on a 1500, l became used to a sudden drop in the electric temp gauge being an indicator that the water level had dropped and left the sensor clear of the, by then, boiling coolant.
GuyW

>>The gauge went up to 190 and stayed there, and eventually the radiator boiled.<<

Beaten to it by Guy, I'd add have you got the correct and fully functioning pressure cap to whichever system you're using.

Presumably your thermoswitch isn't adjustable to simply turn the dial to get it to cut in.
Nigel Atkins

Well there do seem to be 2 faults. Blown fuse for the fan relay circuit. Came back on with new fuse. Still can't explain why the gauge isn't reading above 190. Not long ago I repaired the sender and calibrated it vs a known thermometer. These things either work or don't - when the ether leaks out. Also need to find out why the fuse blew. Thanks for help guys. Worth looking at the air trap possibility.

Les
L B Rose

Les,
is the fused shared and if so what with also is the fuse the correct size/type (other than getting an instant short).

You could take the bulb out again to test it just to be sure.
Nigel Atkins

I agree with Guy

These little engines are notorious for trapping air when you change out the rad fluid there are alot of hidden places for air to hide inside the engines and cant escape

Jack up the front end so the rad cap is the highest point of the engine by way much RE MOVE THE RAD CAP

Let the engine idel for 1/2 hour then let it cool and put the cap back on and run it around the block a few times if it repeats again then repeat the process


Prop

1 Paper

Also something I do to avoid blowing a head gasket or warping the head

I drill an 1/8 inch hole in the stat face so some of the rad fluid and pressure can escape and take the pressure off the system
1 Paper

Fill the cooling system through the heater. Take the hose from the tap, stuff a funnel in it. Hold it up high and fill the system, with heater tap and radiator open. When it overflows, close the tap, then, when it too overflows, close radiator.
Alex G Matla

Prop
I think 1/8' might be a tiddle on the large side for one of these little tackers
1/16" max. would be plenty to stop little air bubbles getting trapped and building up under the t/stat
willy
William Revit

I already have the thermostat drilled as the bypass on the head is blocked off - standard practice. I have 8 fuses on the car now and the one for the fan relay is dedicated. Yes Nigel I need to get the gauge sender bulb out and test it.
L B Rose

If it's any help my Revotec thermo switch has also packed up. Irrespective of setting the fan doesn't cut in.
Fortunately the friend who wired it up also included a manual over-ride switch so the fan still works it just takes good old fashioned, one eye on temp gauge to ensure it cuts in appropriately.
I didn't appreciate these thermo switches were serviceable items.
Jeremy MkIII

Les,
with dedicated fuse the rating would be important obviously, my next thought was if you have an over ride switch as this would introduce further possibilities.
Nigel Atkins

Jeremy,
did you mean the switch isn't serviceable so needs replacing before you'd expect(?).

I believe that wiring in an override switch can lead to thermoswitch problems and shortening its life.






Just joking. :)
Nigel Atkins

LOL Nigel

you almost got me there!!

Yes I expected the thermoswitch to last longer than it has - I mean it even has a relay wired in!
Jeremy MkIII

Apparently the relay is of a cheaper type which is disappointing considering the high cost of the kit, ah well such is the way of these things.
Nigel Atkins

No I don't have an override switch. But as the gauge didn't go above 190 I would not have used it!
L B Rose

Willy

I like 1/8inch hole as it helps to keep it a little cooler in the summer but it saves alot if the stat packs it in the closed position so there is still some flow to make it to a parking lot or some place safe without cracking the head

It's something I do on all of my rides and always have

Prop
1 Paper

Les,
personally I don't see the point of an override switch but others do. If you've not got one it's out of the equation that's all.

Bear in mind that the gauge pick up where ever placed is only at one point in the system, same for fan as well.

Do you also have an engine driven fan?
Nigel Atkins

No Nigel I don't have the engine driven fan. The whole point of the electric one was to avoid the power loss.

I have now found that the gauge would not read over 190 so have recharged the bulb with ether. It now reads OK but the fan still doesn't come on! Fuse is OK and it comes on if I short out the switch, so next job is to get the switch out and test it. I have not found these switches to be very reliable in the past.

So far I have found 2 faults (gauge and fan fuse) and now I think there is a third, the thermoswitch. Wondering if that failed and blew the fuse.

Les
L B Rose

Les,
I'm with you I don't have the engine driven fan either, altho' I don't think I gain much power when the uprated alternator is running for the fan, also don't have a manual override switch.

In one way it's good that it's the gauge that was faulty and you're able to repair it.

I wouldn't have thought a faulty switch would affect a fuse of the correct size but some sort of consistent shorting could.

Is the thermoswitch Kenlowe, Revotec or other, preumably, adjustable?
Nigel Atkins

Nigel

I have tested the thermoswitch and after repeatedly dunking in hot water it came on. So I thought it was OK, put it back in but on a 2 mile journey it stuck on and would not go off. Obviously faulty (I checked it was the switch and not the relay). It's an Intermotor one like this:

https://www.ebay.co.uk/itm/92-82-C-Range-Intermotor-Radiator-Fan-Temperature-Switch-Engine-Cooling-Genuine/352082374963?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649#vi-ilComp

I do wish I could embed links in this forum! Anyway it's surely time for a new switch.
L B Rose

Les,
at those prices I'd buy two or change location and type if you find these switches unreliable in your installation.

Sounds like the switch might have cooked itself by not working in the first place. I wonder what temperature the switch can tolerate, its range is 82-92c a 90c water stat (IIRC) might not fully open until 102c, or if a mistype 80c stat up to 92c, but at what temperature did the coolant boil.

I wonder about your coolant system and switch location as you've put you've had problems before. I see from your vehicle profile your engine's not standard and you've put you've deleted the by-pass so I wonder what other changes you've made as the original Frogeye was 43?hp and had to have changes made to the original cooling system, and ran on a 7lb cap. No doubt you've consulted the likes of Daniel's book (it covers the Frogeye?) and others.

29m spanner for that switch, wow.


Nigel Atkins

Less

You mentioned you added ether to the gauge bulb

How did you accomplish that... I always thought that was black magic specialty work


Prop
1 Paper

Nigel
The switch goes in the top of the thermostat housing, which came from a Marina along with the original switch. It was marked 95 which is maybe a bit hot, but worked for several years. Way back I blew 2 of them until I installed a relay - they don't seem able to take the current of the fan motor. I have now fitted a new switch and it works OK. But I found that there was an air lock above the thermostat so the switch sensor wasn't bathed with water. I think this installation may be a bit high for a Sprite, as I need to keep the rad well topped up to fill this air space. After doing that, the fan came on at the correct temperature according to the gauge, although in winter it hardly comes on at all.

The thermostat is 82 and I have replaced it as it didn't open fully when tested in boiling water. I always drill a hole in them for bypass, as stated above (and recommended by Vizard whose book I followed). I can't really say when the coolant boiled as the gauge was not working properly. That's what misled me in the first place.

Prop
I have tried recharging the gauge several times. The first method was to solder a brass tube to the end of the bulb, and fit a small reservoir with plastic tubing. The liquid comes from a can of engine start spray - it's mostly ether. I put a couple of ml of it in the reservoir, then cooled the bulb with freezing spray from the electronics shop. It was sucked into the cooling bulb, then I crimped and soldered the brass tube. But this was hit or miss as you can't measure how much you put in. The latest method involved soldering a small brass nut to the end of the bulb (which already had the hole), pre-cooling with freezing spray, and injecting ether with a disposable syringe. I attached a bit of brass tube to the end of that using heat shrink tube, small enough to get into the bulb. Then I just had to insert a screw into the nut with some metallic resin to seal it. This worked much better.

Of course, I had to calibrate the gauge versus boiling water (the cooking thermometer is not allowed back in the house now!). It may not be perfectly accurate over its full range, but it's OK at the point on the scale that matters. Easy to pop the needle off and refit.

I can't take credit for either of these methods, as I found them on the web, and if you want to try it I'd strongly recommend the second one.
L B Rose

Les,
thanks for your reply and well done on sorting it. Cigar to Guy then for air bubble in stat housing.

As I put I'd see about relocating (another) fan switch to somewhere lower, a variable switch would give more choice of location and working temperature. Once cooling is lost you can soon get into more troublesome repairs.

Or top up coolant very regularly.
Nigel Atkins

Yes indeed, thanks Guy. The only other position for the switch is either in the head where the gauge bulb is, or in the rad which would be better (I'd prefer the gauge to measure the head temperature). Each has a smaller hole of the same size, so I might look for a switch that fits - preferably adjustable.
L B Rose

I used an adapter that fits into the top hose and is designed for a standard screw in thermostatic switch. Available, I think it was from Burton Power. The switch then actuates the fan via a relay.
This one, but there are nicer looking ones at a higher price
http://tinyurl.com/yamazl5y
GuyW

Later model I know but the same idea only more expensive.

Nigel Atkins

This thread was discussed between 11/01/2018 and 24/01/2018

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