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Hi Noz,

I'm pleased to hear you've got the problem solved. I see my explanation regarding the stat hasn't convinced you though!

I've got a question for you. There's a secondary 'stat on the 'coolant outlet tank' - can you tell me which of the inlets / outlets the 'stat controls please?

Many thanks

Hi all,

Having re-read the posts above again in more detail I have the following comments:

The 'secondary' stat on the 'coolant outlet tank' is purely for the intercooler and nothing else. Whilst the body of the stat is mounted on the coolant outlet tank there is no hydraulic connection whatsoever. The bulb for this stat sticks into the stream of water coming out of the cylinder head but the front hose connections are isolated form the rear. You can see the inside of the coolant outlet tank from my post above called "thermostats.pdf". I know its not completely clear from the photo but the water circulating throught the small stat never comes into contact with the water coming out of the cylinder head. I appreciate that the small stat being mounted on the outlet tank but with no hydraulic connection is confusing but thats just the way it is. Just to confirm, there are two connections on the small stat. The larger connection is the inlet and the small connection is the outlet. Why they are different sizes is a mystery !

You made a comment in an earlier post about the rads doing lots of cooling even when the main stat is closed. I have to disagree on this one. Maybe the previous paragraph will change your mind. When the water appears out of the cylinder head there are only 3 places it can go:
To the heater matrix
Some will vent to the header tank (but very little. The hole for the vent on the coolant outlet tank is only 3mm dia.)
The large radiator

The water going to the radiators can only go to two places if the main stat is closed:
The vent on the top/offside corner of the radiator
The route from half way down the main rad, through the small rad and then through the intercooler.

Both of these routes join together before going through the small stat. If the small stat was to open then some flow would indeed pass through the radiator(s). However, the pipe sizes involved in this circuit are very small. Compare this to the large radiator hose connection size and the heater matrix hose connection size. The amount of water which flows through this circuit is relatively speaking very small.

Although I'm not convinced as you say, The fact that replacing the main stat has 'cured' my problem ie I now have a hot bottom hose, suggests that the design works. I just cant get my head around the heat source for the stat bulb.

As an analogy take a domestic radiator as an example. It is designed to operate at a certain flow with an entering water temperature of 82DegC and a leaving water temperature of 71DegC, an 11 Degree delta (these are very traditional heatig water temperatures). At these conditions the heat given off by the radiator will be X kW. Now consider the TRV(Thermostatic Radiator Valve). When it senses that the room is approaching the correct temperature the TRV will begin to throttle the water flow. The inlet temperature will remain steady at 82DegC. As the flow rate decreases the outlet water temperature begins to fall. Although the temperature delta widens the total heat output falls which might be the reverse of what you think. The reason is because the flow rate falls faster than the temperature delta widens. The result is a falling kW output. The 'average' surface temperature will also appear to fall. At very low flow rates the inlet temperature will still be the same and the outlet temperature will approach the air temperature in the room ie 20DegC. The point to be made here is that a low outlet temperature is not a sign of an efficient heat exchanger (radiator). It simply suggests that the water flow rate is (very much) less than its design flow rate. To determine if a heat exchanger is behaving efficiently requires flow and temperature measurement on both sides (water/air) and some calculations to determine its actual operating conditions. The above responses and relationships are not linear.

The return water from the radiator will remain cool until the flow rate of the water passing through the radiator increases significantly. The main stat does not start to open until 85DegC.

Hope the rant doesn't put anyone off.


noz cool.gif
Hi Noz

Still interested in why your running temperatures are a few degrees higher than most of us get with a 2.5TD.


HI XMexc,

I'm a bit concerned too although I have to say she's behaving impeccably just now. I think the next test is to check the calibration of the gauge on the dash. Should be easy enough, just put the sensor in a boiling kettle and make sure it reads 100DegC on the dial. I'll make the assumption that if it has drifted then it will only be the zero which has moved and not the span.

I'll keep you posted.


noz cool.gif
Hi Noz,

Thanks for the info - I've been pouring over the schematic again a few times. Knowing what the small stat does certainly helps!

You made a comment in an earlier post about the rads doing lots of cooling even when the main stat is closed. I have to disagree on this one

Well I know what you mean - but I still can't see any other explanation as they seem to do sufficient cooling at least 1) in your case with a faulty main stat to prevent steam coming out the header tank and 2) in my case with both fans running constantly to prevent the main 'stat from even opening. I imagine that this situation may put extra strain on the water pump as it's presumably working that much harder to create flow through the small pipes only.

I'm not convinced that my system's working correctly either of course - and I don't like the fact that the system's (apparently) not getting hot enough to open the main stat in normal circumstances (particularly with the aircon switched on). I share your concern about the main 'stat and I'm going to have a go at changing it as I've got a new one here in any case and I'll let you know what effect that has (if any). It could well be that my main 'stat is just starting to play up - as that might explain things. Now that your car is working again can you tell me what difference switching the aircon on makes (if any) please?

It's certainly a wierd cooling system. Although I agree it's an unconventional design I still stand by my original explanation for the heat source for the main 'stat bulb. I do think though that it probably opens and closes much more frequently than the 'stat in a conventional system and that may lead to premature failure (although it does seem to be more substantial than a conventional 'stat). I think you're absolutely right to urge everyone to test the 'stats in their own cars as I can see this has the potential to be the source of a lot of 2.5TD overheating problems.

No doubt we'll get to the bottom of it.


Hi XMexc,

Thanks for the information re the location of the aircon rad. I've found it now!



RE : your split hose.

Had the same one go on mine, (if its the one with Y connection)

Mine did not split but blew up like a balloon, I used a split coke bottle and ties wraps as a temp get you home repair. I wonder if its another componant thats prone to failure

RE: Running Temp

Current 2.5 with AC 90-95 Deg
Previous 2.5 No AC 90-95 Deg (This was witha recon engine less that 5K)
Sick 2.1 with AC 90-95

I just thought this was normal for XM dervs

I have just checked (and subsequently changed) my main stat also. Having read this topic I noticed my temp. was much higher than normal at about 100deg. hence the check. Sure enough, just as described above.

Citroen themselves could not get one for a few days so after some ringing around found a source:
EURO CAR PARTS (020 8391 9191 in Chessington is my nearest)
- only £18.80 and they got it in overnight.
Be warned, it does not come with a gasket which is a bit strange. I had to make my own from old fashioned gasket paper.

Now the temp. is firmly down in the 85-90 range. The car is with air-con.(not that it works right now!)
Sorry, I'm still on the trail of this one.

Another conundrum. When you come to a halt on a warm sunny day and switch off the engine the water stops travelling round the engine. The stored heat in the cylinder liners and cylinder head is absorbed by the water and the temperature rises. Convection takes that heat upwards and out through the coolant outlet tank where the temperature sensor is slocated. The sensor sees the rise in temperature and brings on the cooling fans. However, the hot water present at the coolant outlet tank will never find its way to the radiator without the engine running. So the water in the coolant outlet tank doesn't get cooled no matter how long the fans run. How do the fans know when to switch off? It must be a timer because if the fans ran on until the temperature came down then they would flatten the battery.

A new thermostat has made no difference to my symptoms. Running temperatures are the same as they were before.

I still think the design is flawed.


noz cool.gif
Hi Noz,

Sorry to hear you've still not got right to the bottom of this (yet).

I think you're right - the fans are probably on a timer. Having said that it wouldn't take all that long for the (small volume of) coolant around the sensor in the header tank to cool down either. (I appreciate the short term effect of convection but the heat source is gone when the engine is stopped.)

My current car (twin airbag version) behaves very differantly to my previous 2.5TD (single air bag version) although there was little difference in age. There were obviously some changes made to the electrics at the time of the Feb '96 revision. My previous car used to switch the fans on one after the other and they used to remain on for a while (at high speed) after I'd switched the engine off (as you describe). The current car only ever seems to switch both fans on together, never keeps them running after the engine is switched off (at least it hasn't so far) and runs both fans constantly whenever the aircon is engaged. I've never hear the fans running fast.

Earlier in the post I reported the following test results for my car (in April when the ambient temperature was 8-10 degrees):

1) With aircon switched ON

Normal run of 15 miles - 80-82 degrees (bottom hose COOL)
Stationery on tickover - 80-82 degrees (bottom hose COOL)

2) With aircon switched OFF

Normal run of 15 miles - 88 degrees (bottom hose HOT)
Stationery on tickover - 90 degrees (bottom hose HOT)

At the time I was concerned that with the aircon ON the car wasn't reaching normal operating temperature. I was also concerned by your own findings with the stat - and given the potential for it to cause problems decided I would fit a new one.

I've now fitted the new stat when I flushed the system (it was pretty clean) and replaced the coolant/antifreeze. These are the latest results (in June when the ambient temperature was 20-22 degrees):

3) With aircon switched ON

Normal run of 15 miles - 88-90 degrees (bottom hose HOT)
Stationery on tickover - 88-90 degrees (bottom hose HOT)

4) With aircon switched OFF

Normal run of 15 miles - 88-90 degrees (bottom hose HOT)
Stationery on tickover - 88-92 degrees (bottom hose HOT)

This would all seem normal now. In fact I doubt there was anything wrong with the old stat (although I still think it was worth replacing). (I've still got the old one which I can test if necessary.) It seems to me that the low temperature I experienced previously with the aircon ON was because of the low ambient temperature. Presumably the system isn't designed for the aircon to be switched on when it's that cold! [I've been leaving it switched on all the time on the advice of my local Citroen agent - who said that modern aircon systems have very little effect on fuel consumption and it's better for the compressor - particularly the clutch mechanism if they are left on all the time. If anyone has a view on this please let me know - otherwise I'm going to revert to switching it on for cooling only.]

I'm not sure if any of this is useful to you Noz - but I said I would report after changing the stat. There seem to be a lot of folk watching this thread too and it may help them. The car has only done 61k miles now and has been main dealer serviced. Judging by how relatively clean the system was when flushed I'd say the coolant system is in good nick so it may be useful for comparison.

Just a final point - the running I did in the tests was normal country running at around 60mph. In everyday use I notice the temperature does sometimes climb over 90 degrees if I'm pushing the car - but even pushing it hard it never goes above 92 degrees.

Good luck

My Citroen specialist indy had told me to switch on the air con every fortnight at least, to avoid -the XM usual- possible sticking of the compressor clutch. I doubt it does it any good having it on all the time blink.gif ...but then I'm no expert rolleyes.gif . Other opinions??

Hi All
I'm in the process of dismantling the front of my car as the large rad sprung a leak. While I was at it, I decided to check the stats as my car has constantly ran over 90c.
The small stat that has a hose running to the heat exchanger is difficult to see if its opening or not.
The main stat inner opened OK when I poured hot water over it straight from the kettle, the outer barely budged.
I am going to order a pair, having gone to the trouble of dismantling the front of the car. The symptoms of the rubber being cracked and distorted on the main stat as the sample pictures shown earlier look just like mine.
Hopefully this should bring the temperature down.
Are these dealer only parts? If not, where else can I order from?
I just ordered a spare bottom radiator hose from the main dealers at just under £40.
(we're off to France in the summer hols- I don't want to get caught out).
Hi shoestring,

You found the same symptoms I had. The stat is in two sections and I couldn't ge tthe outer ring to move at all. When I searched I coouldn't find a replacement anywhere apart from Citroen. However, another club member told me3 recently he got one from GSF. Its worth a try. From memory I paid about £24 or so.


noz cool.gif

I just replaced both 'stats on my 2.5 and the difference in time to get to "normal" 90' and the fact that it stays there is phenominal....

GSF couldn't get them, don't have EuroParts (I don't think!) so I got them from CitUK - worth it though!
Hi Noz & Cheddars
It's been an expensive fortnight. I ended up getting them from Citroen main dealer. I replaced both stats complete with gaskets. The temperature has stabilized. It is now running at 85-90. How it will run with caravan on back, I've yet to try it out.
I also had a punctured main rad, a tiny pinhole, (manufacturers stamp 2004), so replaced with one which I bought from GSF.
Is there any way to do a good repair on radiators with a tiny hole?

While the coolant was drained off, I took the opportunity of taking out the heater matrix, as that was still partially blocked from the incident of the expansion tank internals disintigrating when I first bought the car. I managed to remove more debris from the matrix, until I got a good steady flow through it with a hose pipe attached.

So my conclusion is that if your stats have never been replaced do it as a matter of urgency. The pair came to £63.90 with gaskets.
Part numbers are:
C1336F9 Main stat 28.04+VAT
C133846 secondary stat 20.13+VAT
C134014 secondary gasket 2.31+VAT
C134021 main gasket 3.90+VAT

The main radiator came to 96.00+VAT.
I refilled with a mix of blue antifreeze at 33% with distilled water.

As my heat exchanger has never been connected whilst I've owned the car, this would be the next part I would like to replace, as some of you appear to as well.
The new price the dealer gave was 400 and something and they had one in stock at their other branch if anyone is interested at that price.

It's been a great feeling to get all these little jobs sorted in one go! smile.gif

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