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Ideal running temperature?


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Hello everyone

I've been troubleshooting a potential overheating problem with my Mk1 Vitesse. Hoping I can borrow your expertise.

From a cold start, the temp gauge (Jaeger) climbs fairly rapidly to hot, almost the whole way. I was worried at first but the engine didn't seem catastrophically hot as the gauge would suggest. Hot, sure! but not boiling hot.

Anyway I picked up an inferred thermometer to check the whole cooling system. The car was probably running for over half an hour as I did this, so I'm certain the hoses and thermostat housing had got up to temp. (I'm not paying too much attention to the exhaust side of the block but recorded it anyway). 

  • Thermostat - 83-85C
  • Upper hose - 60C
  • Radiator core (top) - 80C
  • Radiator core (bottom) - 55-60C
  • Lower hose - 55C
  • Block (exhaust side) - 90C 
  • Block (distributor side) 75C

In your experience, what's an acceptable running temperature? I've heard that the cooling system on the 2l Vitesse was pretty much borderline when leaving the factory, so my results don't alarm me too much but interested what you guys think?

FYI- Stock cooling, mechanical fan, new thermostat and water pump. Recently flushed and replaced coolant.

Thanks guys,


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Those temperatures seem reasonable as on my Vitesse the cooling fan comes on at 90º which is measured by a probe inserted in the fins just under the radiator top tank which corresponds to about 3/4 on the gauge.

So if your gauge is reading incorrectly it could be down to the instrument voltage stabiliser, usually mounted on the rear of the speedo, not giving the correct 10v output. It of course will also affect the reading of the fuel gauge but this can be harder to detect as the real fuel level is difficult to confirm.

Also I believe there are different temperature sender units available so another possibility is that you have the wrong one fitted.....   

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Great. Thanks for the tips. I'll have to check the rear of the Speedo. I don't know why, but I thought only the Smiths gauges ran a 10V supply and voltage stabiliser. Being an early Vitesse with Jaeger gauges I'd assumed they run on 12V and therefore don't have a stabiliser. Don't know where I heard that from, so I must be mistaken.

As for the sender unit it's got GTR108. I'm not really sure how to check it's the correct one for the gauge!? Any ideas.


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Mines a late Mk1 2L and fitted with Jaeger instruments with voltage stabiliser so I dont know when this change over took place. I know the very early cars like the 1600 Vitesse had different instruments that used the full 12v and if you use a sender from one of those the readings of course will be wrong however a GTR108 should be the right one for your car....

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The Smiths and Jaeger gauges are the same, built in the same factory to the same design. They were the same company, following a buy-out. The only difference is the name.

There were, on earlier cars, different gauges for non-stabilised supply, but I'm pretty sure "earlier" in this case means early 1600, probably the single-dial ones. Certainly my early 2L has stabilised gauges.

I've also had the gauge read seriously too high on a Toledo due to a failed sender - they can fail open or short or part-way to either, including the behaviour you see.

As glang says, the actual temperatures you measured are absolutely fine.

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I couldn't say for certain whether any left the factory with them. My Mk1 actually has SMITHS labelled fuel and temperature gauges but Jaeger speedo and tacho, while the Spitfire has Jaeger all round and the GT6 has SMITHS all round. It's likely there's been some swapping of gauges in the past, though, especially on the Vitesse.

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Thanks guys.

Exactly right about voltage stabiliser. Curiously there wasn't one fitted at all!

Furthermore I'm now fairly sure that the fuel gauge is definitely a non-stabilised gauge. Readings are accurate without stabiliser (full is full and it runs all the way down to empty when the tank is down to nearly fumes). The needle “jumps” off the empty position when the ignition is switched on, as opposed to my temperature gauge which gradually sweeps across - from what I've read that the difference in operation between stabilised and non stabilised. If the fuel gauge is original then it explains why no stabiliser is fitted to the back of the speedo from the factory.

I think the matching original temp gauge must have been replaced with the later stabilised temp gauge. Frustrating as the gauges are otherwise identical.

Anyway. I've wired in the stabiliser just to the temp gauge and it reads perfectly in the centre when warmed up, and I've learnt a few lessons about these old gauges along the way!




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Hi All

i have been reading this thread with interest. On the last RBRR our Standard 10, fitted with a 1300 Spitfire engine blew a head gasket due to overheating following water pump failure.

As standard (no pun intended) the original car did not have a temperature gauge. When fitting the 1300 I purchased and fitted a new Smiths temperature gauge and sender, however this never gave a sensible reading and like Oliver using an infra red thermometer we recorded some sensible temperatures despite the read out on the gauge. 

Having now rebuilt the engine and reinstalled it last weekend I am now keen to get to the bottom of the erroneous gauge reading. 

The car uses the original speedo and integral fuel gauge and I don’t believe that there is an inbuilt voltage stabiliser.

My question therefore is, should I have fitted a voltage stabiliser when installing the new temperature gauge and sender?



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as I understand it if the gauge needle jumps when powered up rather than slowly moves to position its an early moving coil type meter so wont need a stabilised supply. If the latter happens the gauge has a heater and bimetallic strip so does need a stabiliser with a constant 10v output.

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its easy  if the needle climbs /moves in a slow damped operation its a thermo bi metal gauge and needs a stabilser to maintain  a steady votage of the gauge will change as driving voltages change 

these use temp sender GTR108 

if the needles swing to a reading as soon as switched on and tend to wag around on bumpy roads its a moving iron gauge with two coils inside  and not voltage sensitive  these must use sender 121997

you cant inter mix the two unless you like  funny results , 

wont be the first time ive come across engine changes for running hot when it just the wrong sender 

they used to be coloured but generic makes all seem black these days 

caerbont instruments still make and supply smiths /jaeger intsruments










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