Thermostats - troubleshooting

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Thermostats - troubleshooting

Post by white exec »

Thermostats

There have been a regular number of posts concerning engine (coolant) thermostats, and running temperatures. Below are a few remarks about them, how they function, and how to diagnose thermostat issues.

The role of the thermostat is to control engine block/cylinder head running temperature, and get it up to useful and efficient operating temperature as quickly as possible, after starting up and driving away from cold. By 'blocking off' the car's main cooling circuit and radiator, the precious heat produced immediately after start-up is circulated (by the water pump) only through the engine block and the interior (cabin) heater. This warms up the engine far faster than if the main radiator were left in circuit, and also provides rapid cabin warmth and demisting.
Traditional 'skeleton' wax-stat
Traditional 'skeleton' wax-stat
A 'housed' 'stat
A 'housed' 'stat

From cold, under normal driving, the water temperature in engine block and cabin heater quickly rises, and at some point it is necessary to prevent over-heating, and this is where the thermostat starts to operate. At a pre-set temperature (typically 80-85degC) of the hot water behind it, the 'stat begins to open slightly, and allows a small quantity of hot water to head towards the main radiator, usually by way of the large radiator 'top hose'. In doing this, cool water (from bottom of the radiator) heads into the engine block and has a cooling effect. This in turn slightly reduces the coolant temperature in the block, and the thermostat closes minutely to compensate.

This gradual 'stat "cycling" continues as the engine produces more and more heat, until the whole system becomes balanced: engine producing heat, radiator (and cabin heater) dispelling it, and the thermostat open just wide enough to maintain that balance.

Thermostats have two rated temperature figures: opening temperature, and fully open temperature. These vary from car to car, engine to engine, but might be something like 82C opening, 98C fully open. What do these mean?

The opening figure is as above – the temperature at which the thermostat begins to open. It also means that that is the engine temperature which can be expected to be achieved and maintained after a short time on the road, even if it is quite cold outside. (The only exception to this is if it is icy cold, and the engine is being driven extremely gently (notably a diesel setting out on a drive downhill) so little fuel is being burnt. In this case, the loss of heat from the engine block/cabin heater may be so great that 'stat opening temperature is never achieved.)

Under normal temperate conditions, though, engine coolant temp will continue to rise, the 'stat will progressively open, and equilibrium will be reached – with the car's temperature gauge reading Normal, or something like 90degC, maybe a little less. At this temperature, the 'stat will not be fully open, but will still be gently restricting water flow to the main radiator.

So what happens if the engine gets much hotter – e.g. hot conditions, heavy load, towing, or hill-climbing? As the engine works even harder, the 'stat will open further to increase the flow-rate to the radiator. The 'fully open' figure for the 'stat is the temperature at which its disc valve is fully open, and offers almost no resistance to coolant flow. This figure (which you would have to look up in a workshop manual) is usually pitched quite high (e.g. 98C), near to where the car's electric cooling fans would be cutting in to aid air flow though the radiator.

Isn't this dangerously close to water's boiling point of 100C, though? Yes, but car cooling systems are pressurised (by the spring-loaded coolant filler/pressure cap), and this raises the boiling point. Boiling can normally be avoided up to 120C or so under these conditions. Electric fan fast speed may not be triggered until something like 102-105C is reached.

Under these very hot conditions, the thermostat will be wide open (or even fully open), and will play no part in regulating engine temperature - that will be all down to the radiator and the fan(s), with maybe some heat being shifted by the cabin heater (or not, depending on time of year!). The thermostat will not have any effect on the running temperature until ambient conditions reduce, or the engine can stop working so hard. As they do, the fans will first cut out, and as things get less hot the 'stat will begin to close again, just enough to maintain normal operating temperature. If temperatures drop enough (e.g. you stop climbing uphill, towing, with a well-laden car in icy winter) it is conceivable that the 'stat might even close fully again, to secure an engine (and cabin heater) of not less than its rated opening temperature (e.g. 82C).

So, that's how they work: (a) to maintain a minimum engine temperature (unless in freak cold conditions of an icy-blast engine bay), and (b) to progressively allow the cooling effect of the main radiator/fans to come into play. At very high temperatures, beyond 'fully open', they play no part.
______________

So why do some cars display a low temperature on their dash gauges?

This is a common complaint, with some owners reporting that gauges "hardly seem to register or get above bottom end", or regularly show temperatures in the 70s. This was happening on cars in the 1960s, and still does today. Some manufacturer's didn't seem to help: I remember Austin-Morris-Leyland-Rover petrol engine cars being equipped with 78degC 'stats! Why? Maybe their cooling systems were marginal, or materials not good enough to stand up to regular higher temperatures. (The most efficient temperature for a petrol engine is around 100C.) Many owners got fed up with this, and fitting of higher temperature 'stats was commonplace, with 82, 86, 88, 92C being favourites. With the eventual arrival of electric fans, nudging the normal running temperature upwards ceased to be any kind of problem, as fan cut-in soon reduced unwanted over-temperature.

Back in the middle of the last century (as the '60s and earlier certainly was), running around without a thermostat was pretty common. When they stopped working (and some could jam closed!), or when faced with persistent overheating, some owners actually removed, rather than replaced, them. Result was cooler running, but very slow warm up, chilly cabins, and icy windscreens in winter. Not good.

Not all cars suffer from a low-spec'd 'stat opening temperature, but some still appear to do so. Increasing the 'stat temp remains an option, and an increase to just the upper 80sC can do wonders.

Worth saying, though, that many electrical dash temperature gauges are far from accurate. Many rely on a resistive temperature sender (which can go out-of-spec) and a simple electrical (current-meter) on the dash, which may never have been calibrated accurately. Some are much better than others, and some are just dreadful. If in doubt about the gauge, try changing the temperature sender (usually found on the thermostat housing) for a new one, or try checking the actual water temperature with a thermometer – a non-contact infra-red one used on the engine side of the thermostat housing would be convenient.

_________

Back to thermostats: how do you test whether yours is working?

Thermostats can be:
- ok and working
- missing! (unusual nowadays)
- stuck closed (severe overheating; rare, and designed not to)
- stuck fully open (rare)
- stuck partially open (much more common)
- not opening fully (symptom is persistent overheating)
- leaky (slow warm up, chilly/erratic cabin heater)

A typical 'stat will have a wax-filled piston, which pushes open a metal disc to allow the coolant through. A good few of these discs incorporate a rubber washer surround (so they seal when closed), and this rubber can split, or fall away. When it does, the 'stat goes leaky, and always lets an amount of coolant through, even under cold conditions. This results in chilly running (and inadequate or erratic cabin heating). In winter, only a bit of spirited driving will get the temp (gauge) up, only to have it fall again when you have to ease off the gas. That's typical of a leaky 'stat.

There is a way to check for a leaky / missing / stuck-open 'stat:

- With a cold engine, with your hand, feel the temperature of the radiator top hose, and engine block side of the water outlet (where 'stat is housed). They should both feel cold.
- Start the engine, and allow to idle for several minutes.
- Feel those two spots again: you should feel the engine block warming up (be careful!), but the radiator top hose should remain cold. This is because the thermostat should be shut.
- Leaving the engine to idle for some time (maybe 10mins+, or a good bit longer if a diesel), the rad hose should remain cold, but will eventually begin to get warm. This shows that the thermostat has opened. See what the dash gauge reads, out of interest.
- Engine should be getting decently warm, cabin heater producing useful heat, gauge starting to climb.
Left long enough (takes forever if a diesel; they use almost no fuel at idle) the radiator top hose will get really hot, and fan(s) will cut in.

If all that happens, the 'stat is there (!) and not leaky. It may be opening prematurely (or be a chilly one) but only a real temperature check will reveal that. (If your 'stat is the simple skeleton type, you may be able to remove and test its opening temperature (and whether it goes fully open) with a thermometer in a pan of heated water. They're so cheap, probably best just to renew it, and maybe check the old one out of interest.)

BUT, if the radiator top hose warms up from cold simultaneously and at the same rate as the engine block, then the thermostat is either missing (!), leaky, or failed open. Renew it.

A straightforward check to do.


Hope helpful.
Chris
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Re: Thermostats - troubleshooting

Post by Rhothgar »

Nice write up there Chris.

I have a problem with the green Xantia X1 at the moment. I can't seem to get enough heat into the cabin.

The water is very brown. Could probably do with a strong flush. Just come back from Aldi and doing some shopping and they have Kilrock descaler in there at the moment which I know is very effective as a descaler but probably shouldn't be used in a car engine. I am tempted though...

Thermostat in my car at the moment is an 83. Seems to test OK but not 100% sure it's closing fully. That aside, I went for a drive and deliberately went up a few hills and then took a route over some speed bumps to reduce speed and accelerate. I then left the car running for 15 minutes and got the temperature up to 90 on the dash. Cabin heater still mild if anything.

I noticed that the block by the thermostat has two sensors:- one with two wires and one with one wire and wondered which one serves what purpose. One obviously is the dash temperature but not clear on what the other one does?

Topped up the system earlier using an upside 4 pint milk bottle with the bottom cut off but that seems to have made little difference.

Oh what joyous fun!
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Re: Thermostats - troubleshooting

Post by white exec »

Up to three sensors fitted to many PSA engines:
Brown - temp info to control the cooling fans
Green - temp info for Injection system
Blue - for dash gauge
On later cars, this has been reduced to two (first two merged into one).

Leaky thermostat
With a cold engine, start up and feel both block (which should heat up fairly quickly) and the large hose leaving the 'stat housing (usually it's the radiator 'top hose'). Until the 'stat opens, the large hose leading away from it should feel cold. If it warms up at the same rate as the head/block, it means the 'stat is leaky/not closing properly.

Heater only warm, not hot
Locate the two hoses (in the engine compartment) that lead off to the heater matrix. With engine coming up to its working temperature (and your 90deg reading on the gauge suggests it is getting hot ok), both of these hoses should also be at the same temp as the block/head. One hose is feed (usually off the 'stat/water outlet housing, and on the block side of the 'stat), and the other is return. If both are only lukewarm, then heater matrix is likely blocked, preventing flow through it.
You might ?? be able get to the heater matrix from one of the footwells, and feel the other end of those two hoses there as well.

Note that the coolant flowing through the heater matrix should be at the same temp as the engine block, as it is fed directly across it, and should be the same temp as the gauge (hopefully) displays. It should warm up as quickly as the engine block does - even on a very cold day, and long before the 'stat opens to put the main cooling radiator into circuit. When only warm air is selected using the heater controls, this reduction in warmth is achieved by mixing incoming cold air with the very hot being produced by the matrix. This is the function of the (motorised) heater flap valves. Even in high summer, when cabin heating is not required, the heater matrix is still having engine-hot water pumped through it.

If the matrix is blocked, or ancient, replace it. Nissens brand (eg from Mister Auto) has a good reputation.

To flush/de-calc the system, I'd recommend Liqui-Moly Radiator Cleaner #2506. 300ml can, and just follow the instructions on the can or on their website. Excellent stuff that will dissolve calc, oil and grease, and leave the system clean. Do not use commercial/domestic decalcifiers, as they can seriously damage aluminium copper system components, as well as the main radiator.

Let us know how you get on.
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Re: Thermostats - troubleshooting

Post by Rhothgar »

Hopefully changing a matrix on a Xantia isn't as bad as a 10 hour ZX dash out job! Could do without that to be honest. Car is Jan 1995 so I imagine sensor for injection doesn't come into this particular equation. 1.9TD.

I'll give that Liqui-Moly Radiator Cleaner #2506 a shot in the first instance.
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Re: Thermostats - troubleshooting

Post by xantia_v6 »

The Xantia matrix is a 10 hour dash-out job, documented a few times on this site.

It could also be a problem with the air mixing flaps. You should be able to feel whether the hoses to and from the matrix behind the engine are both getting hot.
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Re: Thermostats - troubleshooting

Post by Rhothgar »

I wondered about the flaps as I have had the controller out of the dash before and wondered if I could have kinked the bowden cables! Hope not. Mind you changing one of those is probably less painful than a HM change.
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Re: Thermostats - troubleshooting

Post by Peter.N. »

If anyone wants to venture into 2.0. Hdi X7 thermostat replacement, which unfortunately is nothing like as easy as it used to be, I have recently done mine and have taken a lot of photo's. If anyone is interested I will do a guide.

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Re: Thermostats - troubleshooting

Post by Rhothgar »

Peter.N. wrote: 01 Jan 2020, 11:45 If anyone wants to venture into 2.0. Hdi X7 thermostat replacement, which unfortunately is nothing like as easy as it used to be, I have recently done mine and have taken a lot of photo's. If anyone is interested I will do a guide.

Peter
Sounds good but you're already putting me off buying a late naughtie's C5 Tourer which I keep toying with buying one day. The other thing that puts me off is them not having the 'proper' suspension. Mind you, that's not entirely a bad thing. Oh and the horror story steering rack that someone has had on here!
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Re: Thermostats - troubleshooting

Post by Peter.N. »

It is a pig of a job but with the knowledge I have gained and the special hose clip removing tool I could another one a lot quicker. I'm sure a younger and more dexterous person could do it even quicker.

I wouldn't be put off buying one now, the main problems seem to be the steering rack and front wipers, I have had both problems. The rack is not loosing a lot of fluid and the extra lubrication it makes the steering very light, so as long as it passes the MOT tomorrow, :shock: I wont bother to replace it. The wipers are intermittent, I hope they will be working tomorrow!

Apart from the thermostat nothing has failed on the silver one - I tell a lie the seat reclining switch is faulty but I'm sure I can fix that without to much of a problem, I will wait until the warmer weather, oh and the thermostat of course. I hadn't failed altogether but was stuck slightly open so in cold weather the water temperature would drop going down a long hill, I could have left it but do like things to work properly.

To my mind they are a superb car, one of the very few that is really smooth and quiet, even with steel springs and I wouldn't change. Whether they are worth the potential problems or not depends on how much ride comfort means to you, it means a great deal to me.

The engine and drive train are common to many vehicles and it seems to be very reliable, the thermostat failed at 104k which isn't bad, the other one has done 107k and the thermostat is fine, in fact I used it as the bench mark for the other one - that's how I knew it needed a new thermostat. Its useful to have two the same. 8-)

I would avoid the Executive, 1.6Hdi, auto and petrol as the potential for expensive problems is much greater. You can sometimes find the basic one with an up spec interior as with my silver one. To my mind the best of the bunch is the 2.0. Hdi estate. As you will have noticed I am very impressed. :-D

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Re: Thermostats - troubleshooting

Post by GiveMeABreak »

Rhothgar wrote: 06 Jan 2020, 10:13 Sounds good but you're already putting me off buying a late naughtie's C5 Tourer which I keep toying with buying one day. The other thing that puts me off is them not having the 'proper' suspension. Mind you, that's not entirely a bad thing. Oh and the horror story steering rack that someone has had on here!
What's your definition of 'proper'? There was a choice of Hydractive 3+ or Steel springs depending on Trim. Mostly Exclusives with Hydractive 3+.
Please Don't PM Me For Technical Help

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Re: Thermostats - troubleshooting

Post by Peter.N. »

I have run hydractive Citroen's since the days of the CX, around 30 years, but I think the steel sprung one comes a very close second, maybe even better on small imperfections i.e. concrete roads. Not so good over big bumps but you don't get to many of those.

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Re: Thermostats - troubleshooting

Post by Rhothgar »

GiveMeABreak wrote: 06 Jan 2020, 11:09 What's your definition of 'proper'? There was a choice of Hydractive 3+ or Steel springs depending on Trim. Mostly Exclusives with Hydractive 3+.
It can only be Hydramatic. Grease Lightning!

Ahem! Sorry. Hydrapneumatic for me. Interested to hear that both variants are available though and Peter's experience. Too many speed bumps around here to have a sprung steel car.

To me though, fundementally having coil suspension on a Citroen is like having a water-cooled Harley. It's just not quite 'right'.

In other news, the genuine thermostat arrived today. £9.50 on Ebay. Well happy with that. Fitted it and to my amazement the engine heats up so much quicker now. I went through my car fuel usage history and the highest last year was 47 mph. Last tank was 35mpg and two before we less than 40mpg.

I just would never have believed that such a tiny slither of light between the rubber inner valve and the body would make such a difference. I drove down to town (2 miles downhill with a few traffic lights and reasonably well moving traffic for the most part) and- gave the engine a good race for the final 200 yards before pulling up and temp was at 80 deg.

The other night I drove just over 4 miles and deliberately slowed to a virtual standstill at speed humps and revved it up a steep 100 yard incline just before home and it only got to 70 deg.

Fuel consumption should improve.

Heater obviously feels better but it peters out pretty quickly which possibly suggests reduced flow through the matrix. Will report back as the weather gets colder. That'll be the acid test.

As for proper bleeding, how do you guys who have had a Xantia manage to remove the schraeder cap off the heater matrix bleed without scalding yourselves and to allow enough water to bleed through?

The schraeder near the offside engine mount is probably the highest on the system. Are there any others?

BoL states pre-97 Xantia should have 89 deg 'stat! It had and now has 83 deg. Maybe this was to allow for poorer flow through earlier matrices.
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Re: Thermostats - troubleshooting

Post by white exec »

Use a short length of small-bore rubber hose pushed on to the schrader cap to unscrew and replace it.
Saves scalding fingers. Properly tighten by hand afterwards.
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Re: Thermostats - troubleshooting

Post by Rhothgar »

white exec wrote: 10 Jan 2020, 22:17 Use a short length of small-bore rubber hose pushed on to the schrader cap to unscrew and replace it.
Saves scalding fingers. Properly tighten by hand afterwards.
What a great solution!
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Re: Thermostats - troubleshooting

Post by GiveMeABreak »

He's full of Carlos Fandango Super Savvy Tips is our Chris. :-D
Please Don't PM Me For Technical Help

Marc
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