1.8CVH running very cool

The thermostat's rather more likely & somewhat cheaper.

Thermostat. They fail to open usually on Fords. £4 or £5 for a replacement.

or in this case fail to close

"Fail to open" means that its fail state is "open", as in "fail to off", "fail to safe".

It's the engineer in me....

I understood it as you wrote it; it never occured to me until Mr C's reply that its meaning is completely ambiguous!

Tell him to get out whilst you are typing

Chris

Thanks folks. Thermostat ordered.

Mark

NO! he said it was running cool, not hot. Learn how the cooling system works and what a thermostat does on the CVH. Next you will be saying it stops the heater matrix working!

Well tell him to leave you alone and concentrate. Fail to open means "it doesn't, hasn't or will not open". Stop trying to excuse your total lack of knowledge of cars (and computers) in various groups by posting this rubbish. There is no way you could ever be qualified going by what you post.

It can equally mean "fail to the open condition". This is a common way of describing failure modes in many fields of engineering, and what I understood Conor to mean.

Strictly speaking, in order to avoid ambiguity, it is necessary to expand the statements. However, on this occasion, Conor is not wrong.

Chris

Much the same as it does on most Fords since the early 1960's...

Well, if the engine never reaches its designed temperature, the heater certainly won't be working efficiently ;-)

Chris

Just out of interest what makes them fail in the open position? I'd have thought if anything they would fail to open because of the hefty spring pressure?

Diaphragm splits, crud jams them, the spring corrodes.

Interestingly the heater has never worked well and is at best a tad above tepid. Heater temp goes up and down a bit for no logical reason. Perhaps air, matrix clogged or just because its a 20 year old Sierra. I don't much care providing it moves and gets me comfortably and safely up and down the motorway. As said in original post the temperature gauge usually sits around a third of full scale and is constant with motorway or city driving. However, in the faulty condition the other night with temp gauge reading a tad above cold (and varying up and down a little), the heater was behaving just as it always does. That makes sense to me as although engine temp down, its far from what we humans would call cold.

Mark

Partially blocked heater matrix is very common on Sierras. When working properly, it should scorch your socks, like all fords of that era!

Chris

My Capri certainly does or at least does now after I replaced the thermostat the previous owner decided wasn't needed.

I'm waiting for my stat and gasket to arrive, but drained down the old coolant today and gave it a couple of fills of tap water with a short drive in between to clear out the crud. Seems fairly clean, but when wactching the water level in the expansion tank while on tick over, it goes up and down about 4 inches. This is with the cap off. Watch it for a minute and it sits at about minimum, then suddenly goes up to near the neck then goes down again within a few seconds. Heater is set to hot position.

Is this right? Does it indicate any fault like head gasket? Temp got up to normal this afternoon, but it is a warm afternoon here. It was varying a fair bit though. Normal while holding it in 2nd at 30mph for a minute, but heading for cold while on tickover on the drive. So I am still going to replace the thermostat. Just curious about this water level going up and down so wildly.

Mark

If the thermostat change doesn't work it could be the temperature gauge sender that's giving a false reading. As regarding the heater, a mate of mine had a F reg pinto sierra and the heater on that was crap, he tried everything to sort it including flushing the matrix with fernox and it still wouldn't demist the screen in winter. He's now got an escort and you could cook on the heater of that. Trevor Smith

It definitely had a fault; I had two Sierras, and they both had very powerful heaters. Far better than my Focus in fact.

Chris

flushing doesn't work when they are gummed up, the liquid physically can never get to the end of all the blockages, it just goes round one of the open routes and cleans that. the escort and sierra rad are very similar size and shape so performance is close to identical, both are very good heaters unless blocked up.