Running too cool?

[uzbek]

I've had my car only just over a month and yesterday did only my second decent long trip. I'm worried it runs too cool. It is a 'second sanction' MkI i.e. the MkII style rad and air filter with no mods except a stagweber radiator cowl.

First run (back from the seller) was 2 hours on the motorway, quite a warm day and included a moderate traffic jam. The temp gauge rose slowly over 30mins or so and then never budged above 90, even in the tailback. As the speed dropped and I bumbled along the slower roads nearer home it dropped a bit but still in the same general area.

I did some short trips and noticed the needle never getting much above 70deg approx. Yesterday (v frosty, heater on) we had a nice long run and it never rose above 70. On a long steady 3 mile downhill section it dropped almost to the first quarter of the dial before rising a bit on the level and uphill etc. The car runs extremely sweetly, it also warms up quite quickly ie only neededing the choke for a couple of miles or so.

I have a feeling that the thermostat is coming in too soon-and presumably this is a bad idea. Or could this be the air intake vacuum flap stuck 'open'?

How to test this?-or do I just relax!

Bruce

[DJT]

"Stag runs too cool" -shock horror!

Bruce,

What thermostat have you got fitted? Some fit a 74degrees stat in the belief it helps cooling, but an 88 degree stat is fine, especially at this time of year.

The gauges themselves are not particularly accurate. Have you checked with an infra red thermometer?

[uzbek]

I did not fit the thermostat so cannot say what has been fitted. I have not drained the coolant down to have a look. Would a low temp one explain my findings?

I have not checked the temp with another thermometer (don't have one). I appreciate the dials are not objectively very accurate but it seems odd to me (even in very cold weather) that it would not settle to run at around the same temp on yesterday's sustained run.

The irony of a too-cool Stag is not lost on me!
Bruce

[V Mad]

I would remove that stat and replace it with an 88 deg one. Also worth checking the old to see what temp it opens.

[Westfaling]

Bruce
I agree with Dave it might be worth buying, borrowing or stealing an infra red thermometer and checking your actual under bonnet temps. Never trust fixed instrumentation.
Mind you if your heater is working well enough why worry.
Nick

[uzbek]

i'll see if i can track one down.

If it is running at lower temps is this bad for the engine?

[staginhiding]

Just earth the temp gauge wire.
If you pull it off the sender on the engine and earth to the engine to see if the gauge goes its full travel to Hot..
Simple test to rule out a possible faulty instrument.

Does your fuel gauge read ok,as there is a stabiliser fitted that controls both gauges..

Sam

[V Mad]

Yes, running too cool is bad for the engine in the longer term. Also will waste fuel. I would say if it really is running at 70 that's too cool.

[uzbek]

Just earth the temp gauge wire.
If you pull it off the sender on the engine and earth to the engine to see if the gauge goes its full travel to Hot..
Simple test to rule out a possible faulty instrument.

Does your fuel gauge read ok,as there is a stabiliser fitted that controls both gauges..

Sam

Fuel gauge is fine, emptying with appropriate speed! Thanks for the gauge test tip.

Since it ran at 90deg the first long trip I think it unlikely the gauge is the problem. From heat soak into the cabin then (and lack of it at the weekend) I think it is runninng cool and the gauge is giving a fair indication.

I'm guessing it has a low temp thermostat.

[StokeyStag]

FWIW the temperature gauge on my '72 Mk1 usually sat between 25% and 33% all summer. When I took it out for a decent run last Thursday (cool but sunny) it didn't get above 25% at any point. I've assumed that now the ambient temperature is that much colder that the engine is running cooler.

Mark

[uzbek]

I can probably borrow an infra-red thermometer. Where do i point it and what should I expect as readings?

bruce

[ampotter]

I had the same problem with the engine running cool, especially during the winter period. I found that it was fitted with a 74C thermostat which I replaced with a 82C (an 88C would improve further) and made a big difference. Do ensure though that you fit the correct type of thermostat ie with or without the foot (by-pass shut off) this will depend on the type of manifold fitted. Andrew

[uzbek]

Thanks Andrew

Would that be something I could work out from the engine no. or is it only going to be apparent when i remove it?

[ampotter]

Look into the manifold where the therostat (removed) sits and on the later manifolds there will be a hole (by-pass). If you have this type of manifold then you will need a thermostat with a by-pass (foot) shut-off. The purpose is to shut this
by-pass hole off when the thermostat opens and therefore preventing some of the hot coolant from returning to the engine. I also believe there are some external visual differences between the early and later manifolds - something to do with the external pipe work. LP Parts sell the correct thermostats. I'm sure others will advise. Andrew

[Mike Stevens]

Hi Bruce,

You said that it ran at 90 in the summer and then 70 when the weather was cooler. But how do you know these figures? The standard gauge doesn't have figures on it, just blue and red bits (I think!). If you have got numbers on the gauge, then it's probably not an original and is therefore unlikely to read correctly with the original sender.

If the heater is working Ok and keeping you warm on these cool days, then I suspect that the temperature is probably OK. However, a check with an IR thermometer is well worth it. Look at the termostat housing cap, top and bottom radiator hoses and left and right of the radiator. The 'stat housing/top hose/rad LHS should probably be the hottest and around the 'stat opening temperature when the engine is up to temperature.

Cheers,
Mike.