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MG Midget and Sprite Technical - Did I or didn't I overheat my 1966 1098 mk 2

Ok, driving along I happen to notice my temp gauge rise at stop lights to around 225! Going down the road it falls to about 160, maybe a little higher. I have since taken the heater to the rad shop to be boiled and flow tested (passed), newer thermostat is installed in the car and I swapped a brand new thermostat before taking the cooling system apart and did the same thing, high temp indication at idle, not driving. Water pump has about 2k miles on it and looks fine (I pulled that out to check it). So the things I know

-Gauge going up at idle and down driving average 50 degree fluxuation
-Randiator has good flow and thermostat and waterpump are newer/new
-No leaks and no loss of coolant are visible
-newer radiator cap

I have recently gotten a hold of an IR thermometer and will hook everthing back up and see where the temp. sits.

Last night I took a pot of boiling water out to the sending unit and at 150 degrees it was reading 170 on the gauge. I don't know if there is a difference in temp when the system is under pressure or not. It seems the gauge is off by about 20 degrees.

I have had no smoke, no funny smells and no loss in performance, just the gauge going to the scary zone and falling drastically down again.

Thats all I know, does it sound like a gauge issue????? Any good way to check that?


JS Jesse

Even if the temp gauge is wrong there should not be a gap that big between stationairy and on the move.

What fan do you have on the engine what where the ambient temps and is your heater open or closed?

Onno K

4 blade metal fan, no clutch (stock) and heater was on, only started (at least when I noticed and I check my gauges often) after I topped off my coolant, only took about a quart of coolant after four months, I had some very small leaks in my radiator for the longest time, just got them fixed but they were there since I bought the car and drove it since this summer. seemed to happen after I topped up the radiator.....

JS Jesse

160 to a bit above 200 with an engine driven fan I'd kinda expect and a gauge just being a gauge but that does sound high at 225 uncomfortably high

are you sure you've not got a "hotspot" or air trap from adding water or when changing items

I'd also check you have the correct pressure and depth of pressure cap and that the seal on the cap is good (yes I know it's new)

over here I always run with anti-freeze coolant as it also help to lubricate and protect parts of the coolant system

fan belt tight, fan in good condition and tight and stable

hold on 4 blade fan, is your fan with any rubber grommets or spacer behind it?
Nigel Atkins

no rubber or spacers behind the fan, should it have them????? I have ordered a new radiator cap specifically for the car, only $6 bucks to be safe and I do run 50/50 mix. Should I have spacers behind my fan??

JS Jesse

no no spacers but I thought a 1098 came with a two blade fan is it different for Us car?

have alook at this for rad caps though as there are two lengths for 7lb caps -

50/50 blimey how cold does it get in Virginia
Nigel Atkins

Do you have a thermostat with a little hole in it? If not, that could be it; Thermostat opening and closing, but no gradual transition due to the missing hole.
Alex G Matla

This sounds like an air flow problem.
If the temp drops when travelling then the radiator is cooling OK when the car is travelling and presumably the water circulation between engine and rad is OK. So I would say that the "wet" side of your cooling system is in good condition.

To me it sounds like a problem with air circulation so maybe something to do with the fan. Loose belt? Or belt that is worn or polished so it isn't running properly on the V - sides of the pulleys?. I assume also that your fan is correctly assembled to its hub and to the pump? It would be possible - although it would look odd - to fit the fan blades the wrong way round. The convex sides need to face forwards. The fan spins clockwise (viewed from the front) so the blades need to scoop the air and fling it rearwards. You also need to have the proper radiator shrouds fitted - I assume these haven't been modified in some way?


I have a 1098 engine fitted with metal 4-blade fan. I suspect they were supplied differently for different markets so yours may not be the same. The 4 blades are made up of two blades bolted at right angles through a spacer/hub and into the water pump spindle flange. The space is about 3/8" thick to bring the fan a bit closer to the radiator. I suspect if it was further away the fan wouldn't draw air through the rad quite so effectively as it would pull air in from the sides (path of least resistance)

I wasn't aware of a spacer being fitted with a metal fan and Moss don't show one either.

Note that the entry for the fan blade says "1 or 2 as required".

It is important to ensure that the stiffener is fitted, as the blades are prone to failure without.
Dave O'Neill2

>>I wasn't aware of a spacer being fitted with a metal fan and Moss don't show one either.<< the space I was thinking of was if the wrong fan had been fitted (and as Guy perhaps wrong way round in my earlier post)

>>Note that the entry for the fan blade says "1 or 2 as required".<<
thank you David and Guy that's the bit I missed and explains the 4 blades and the shape of the stiffener

>>It is important to ensure that the stiffener is fitted, as the blades are prone to failure without<<
yes I was wondering about the integrity of the fan and its fixing

same as Guy I was thinking of fan belt in earlier post but forgot about rad shroud

anyway, more ideas . . .
Nigel Atkins

(Jesse photos would be handy)

in the meantime questions/thoughts

What temperature thermostat is in now and when it was running to correct temperature?

When at stop and the engine is fully warmed up when you put the clutch in do the revs drop?

What are your revs at idle when full warmed up?

Are you sure the water is leaking from the rad and not blowing off out of the cap and down the overflow pipe?

A US quart (2 US pint) to me seems a lot to put in to a system that only holds 6 US quarts (12 US pints)

Your radiator is clear of external debris on both sides?

From Haynes:-
Has your bottom hose collapsed in?

(Ignition timing set up correctly, loss of engine power or misfire, carbs set too weak) – can these be discounted by >>no loss in performance<<?

Exhaust partially blocked?

Oil level in sump too low?
Nigel Atkins

It could still be a gauge reading fault. Especially as you seem to have no other faults. The gauge is obviously a bit out as you have shown by your test 150 / 170 F so over reading by 20F. But this is at a lowish temp and there is nothing in your test to show if the reading differential is constant or even linear! If the guage is suspect at that one figure it could be way out by the time your engine is at a normal hot running temp.

It is of course natural for an engine to gain heat (or rather, not to loose heat as effectively) when standing. The fan only operates at relatively marginal efficiency and has little effect once you are moving. Most of the rad cooling is normally by forced air pressure when moving.

I am not convinced that you actually have any problem at all - other than perhaps with your gauge.

PS, photo of the fan off my 1098, showing stiffener plate, 4 blades and spacer.


Just thinking about your temp readings again. Sorry but I have long since forgotten my Fahrenheit scales so the temps you were quoting didn't immediately interpret for me.

You say that when running the gauge reads around 160F. Given that you tested your gauge and it is reading high this represents maybe 145F actual temp (= 63C) This is below the opening value for your thermostat and is cool for a running engine - it won't be at its most efficient.

When standing you got a reading of 255F. If because of the gauge fault that represented an actual temp of 235F (112C)it is on the high side and would probably be boiling over, loosing water at that temp. Does your car have an expansion tank? I say probably boiling over because I cannot remember the formula for the raised boiling point of water when under pressure. With the correct rad cap it will be operating at either 7 lbs or 15 lbs above ambient and this, together with the addition of antifreeze, will raise the actual boiling point.I think 235F (112C) would be marginal as to whether it was actually boiling or not but maybe someone can provide the correct boiling point under pressure. Either way it is on the high side for correct efficient running.

I still think therefore that you probably don't have a problem. But I would check that you have the correct rad cap (as Nigel says, there are 2 lengths and Moss and other suppliers don't necessarily distinguish between them!) and I would check and refill the cooling system to be absolutely sure that you don't have any air trapped in there,m e.g. in the heater core

Jesse, (Guy),

I use the blurred photo below to compare your gauge readings with my 2/3 year old gauge readings

Mine shows the same as Jesse’s at 160 or 7 o’clock/well before the N and usually only just goes above N/about 5:45 (190) when I switch off

My gauge occasionally wanders to around 7:30 (140) rather than 7:00 (160) but goes back there

Just thoughts - when you took the reading was the sender under the surface of the water where it could be a bit higher or did you direct the IR onto the sender out of the water, and have you checked the accuracy of the IR, also an errors with both in opposite directions will widen the range of error

From my 1974 print of Haynes, page 54:
‘The Radiator cap is pressurised to 7 lb/sq. in. and increases the boiling point to 225F’

From quick search of t’interweb - 50/50 antifreeze(?) gives an increase of 11F(?) so if cap seal is in good order then a boiling point of 236F(?) off the gauge scales

also as an aside from my 1982 print of Haynes, page 79 is: Antifreeze fluid 50% - Protection to –37C (-34F)

and I forgot to include the heater (0.6 US quart) so your system is shown as 12.6 US Quarts in total, still makes 2 Quarts a lot to add at one time – did you have the heater tap fully open when you filled with this?

Having made some many mistakes recently I’m researching a bit more and relying on my memory less

ETA: I don't know how much, if at all, standard antifreezes have changed their properties over the years

Nigel Atkins

Jesse, sorry

I meant a (1) US Quart, I was probably thinking of of 2 (US) pints - typing (and reading!) I'm not good at no matter how slow I and how many times I rereading what I've put
Nigel Atkins

4 blade fan is equipped, I have had my radiator steam boiled and flow tested, flow test was fine. I have oredered a new radiator cap to be on the safe side. I have not seen steam or a boil over, I was even able to take the radiator cap off (hot but not unusually hot) when it was "overheating", no gueiser just coolant sittign there like normal and after I pulled the cap off it went down to 190 on my gauge (I am sure this had to do with a pressure change). No overflow tank, I topped it off because I had four very tiny pinhole radiator leaks since I owned the car, even the radiator shop said they would have been ok but I had them fixed.

JS Jesse

right, I thought it was the heater matrix you had tested - or could be both rad and matrix - anyway with what you've just put and taking account of all posts then it may be a faulty gauge

as it's not throughout the range I don't think you'll be able to reset/recalibrate it (others may know better about if it's the gauge at fault and if you can repair it?)

I thought perhaps your idle was low so that at a stop your pump and fan are rotating at a too slower speed

that the problem with the engine driven fanslow when you want it fast and fast when you don't need it at all but the system has worked for years

I prefer an electric thermo control fan that allows the car to warm up quicker especially in cold weather and is off when you don't need it

let us know how you get on if there are no more answers from here
Nigel Atkins

This what you need Guy?

Lawrence Slater

I purchased a new belt as well as a precaution besides the radiator cap, I thought the old one felt a little "smooth."

Thanks for the boiling chart, handy to have

JS Jesse

Guy. I have a '65 Sprite with a 1098 engine which was totally unmolested when I got it and it didn't have a spacer like yours on the fan. I wonder when they changed that?

b higginson

Maybe my fan isn't as originally supplied. It isn't the original engine for the car which is a MK1 Sprite so that could account for the spacer being added in at some stage. I assumed it was all part of the standard fan that went with that engine.

Still thinking that Jesse doesn't have a problem!

Some 1275s have a spacer with the plastic fan.
Dave O'Neill2

"In an automotive cooling system, an overpressure condition is maintained to raise the boiling point. For example, a cooling system having a pressure cap rated at 22psi (1.5 bar) would raise the boiling point of pure water 268°F (131°), a 56°F (31°C) increase."


So a 7lb cap will raise the boiling point from
100°C / 212°F to 110°C / 231°F

Doug Plumb

That was my point - if Jesse's gauge is over reading by 20 F then his 255 F could be around 235 F true temp, and at just about boiling point for a system with a 7 lb cap. If he has a 15lb pressure cap then it is well below boiling point and he probably doesn't have an overheating problem (his original question)

Aside:- I don't think any Spridgets were supplied with a 22lb cap, I suspect the older radiators wouldn't stand that

If anything, his running temp at 160 F (or 140F) looks to be a bit low for efficient operation.

>>could be around 235 F true temp, and at just about boiling point for a system with a 7 lb cap.<<

and at just about boiling point for a system with a 7lb cap and with 50/50 antifreeze

according to to the research in my post below

I also put -

as it's not throughout the range I don't think you'll be able to reset/recalibrate it - the gauge -
(others may know better about if it's the gauge at fault and if you can repair it?)

sounds like it is the gauge from Jeese's posts and no reports of the 'sweet' smell of overheating
Nigel Atkins

Yes Nigel, the contaminant in the water will also raise the boiling point. I don't know if this works cumulatively or not but I guess it is more complicated than that. It certainly isn't linear, but it is all going in the right direction of a non-overheating car!

I missed the point about the error on the gauge not being throughout the scale? The needle is just a push fit and if it is reading wrongly it could just be moved around. For more accurate setting there are two small pairs of datum marks on (some of?) the dials. I think the higher pair are at 100 C and the lower at 50 C. So the gauge can be empirically tested and corrected if needed.

I missed the bit about contaminants in the coolant, what has Jesse been putting in the rad to cool it, the dirty devil, the Mrs would never approve :

See my comments about my ‘new’ gauge as regards empirically (I had to look that word up) adjustment

In my photo the marks are around 212F (100) at about 210 to 215(?) but it’s too blurred to see the 122F (50) marks

having a combined gauge becomes a bit of a pain in instances like this I did think about putting in separate gauges when mine went wrong but didn’t bother I wish I had now this 'new' gauge water temp needle likes to sometimes wander towards C

good luck anyway
Nigel Atkins

By contaminants I meant that anything added will change from pure water and will raise the boiling point. In this case the contaminant is antifreeze at 50:50 ratio.
Sorry to confuse you!

I think there are pairs of calibration marks on the other C-N-H gauge as well, but even harder to see!

The 'calibration' points on the scale as described by Guy

Doug Plumb

Thought I would upsdate the situation. Bolted it all back together last night, I am now sure it is a bad gauge. I had a friend of mine touch the radiator and hefelt the difference between the thermostat shut and open, when he felt the thermostat open, my gauge said 120F, I am running a 195F thermostat due to having a heater equipped.

Thanks for the help shes back on the road,

JS Jesse

This thread was discussed between 10/01/2012 and 12/01/2012

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