Simon's new Xantia V6 and Leaf blog

[CitroJim]

I've always found the bottle a pain with the XUD engines, I just fill it up and run it until the thermostat opens then top it up. In the past when I used the bottle I still found they needed topping up after the first run so I ditched it.

My findings and method exactly James

[Mandrake]

Ok so this afternoon I bled the system again - twice.

First time I did cold, working my way from the lowest bleed point to the highest, the lower three dribbled immediately with no air, but by the time I got to the highest point (heater matrix elbow) nothing came out, and on checking the expansion vessel I realised the level was right down near the bottom. Whether that was due to losses through the previous 3 bleed valves (I did let a reasonable amount out, especially through the radiator bleed valve, that flowed quickly before I could get it closed again) or whether the level was low after boiling some coolant out yesterday (and then contracting when cooling over night) I'm not sure. Or maybe releasing some trapped air yesterday caused the level to drop.

So I topped it up again and it started running out the matrix elbow bleed point, and naturally I dropped the cap (after about 20 failed attempts to start the cap on its thread) and had to go searching under the car for it in the damp and wet. Fortunately it was found and finally went on.

I then went for a drive to get it nice and hot with no problems noticed, and then came home to bleed while hot, this time the level was fine. (so maybe the level was a bit low to begin with the first time)

I gave the fan low speed a good check over to make sure that it was working, which it was, and I was able to let it idle until the fans came on to slow speed with the cap off without any bubbling whatsover - the coolant was calm and flat with just a little bit of vapour rising off it, as it should be. I then went for another drive to try to provoke it into playing up again - which it wouldn't.

That all sounds like an air-lock, probably in the head, that would cause localised boiling, just a tiny bit of which can produce enough steam to create pressure and eject coolant when the cap is released.

Simon, I did say when you were asking about adding Forte, that you should bleed the system (and properly). It could always be something else, of course, but part draining a Citroen cooling system and not bleeding it thoroughly is not the best.

With only one litre removed I thought I would have got away with it, but apparently not! If I had to guess I would say an air-lock started in the heater matrix pipe and stayed there for a few days then for no apparent reason got lodged in the head.

Also, a genuine 90-95C is really nothing to be worried about. One of the 2.5 thermostats doesn't open until 88C, and fully open until 100C ! 70-80C sounds much more of a problem - positively luke-warm.

Not really - the V6 dashboard thermostat is known to read 10-20 degrees lower than "actual" temperature due to its sensor location, which effectively measures the return flow from the radiator. So if the thermostat is open and you are moving at speed or the fans are on the reading will routinely drop down to 70-80 even when the real temperature measured by a Lexia is about 93 degrees, because it is measuring the cooled water coming back from the radiator. This is because the sensor that the ECU uses is located near the thermostat housing and gets a true reading.

I've checked via Lexia recently and the thermostat starts to open at about 88 degrees, low fans come on at 93 and high fans at 97 degrees, so there is nothing wrong with the thermostat or fan activation unless it has given up the ghost in the last few weeks.

It's not the absolute temperature reading of 95 that worried me, it's the fact that it was completely out of character (it has never gone that high before) and that I know the dashboard gauge always reads lower than "true", so a reading of 95 means actual temperature was probably 105-115, and if it boiled a 50/50 mix of coolant with the cap released it must have been at least 108.

So I can only conclude that it was an air-lock in the system that took 4 days to find its way to somewhere it could do some harm, or possibly there is an intermittent fault with the low fan speed (which would also fit the symptoms) but checking the fans fairly carefully they seem to be working OK on both low and high, so I don't think it's that, especially if it was a coincidence!

[CitroJim]

Fingers crossed all will be OK now Simon... My recent experience(s) with the cooling fans on Gabriel suggest that regular testing of them is an imperative!

With a petrol engine you can just wait for it to get hot enough after a run and on idle... Not so simple on a diesel - I end up disconnecting the Bitron sensor!

Flicking the aircon on should also get the fans running at low speed... Another good confidence test.

[white exec]

Glad you got it sorted, Simon. Think you are probably right in that an airlock eventually migrated to the head, where it would cause a hot-spot.

I did not know the V6 gauge sensor was placed in such an odd position. Why, for heaven's sake? Normally, the gauge sensor is located in the water output 'tank' from the head/block, along with the other coolant sensors. Wierd. Can't understand how such a 'cool' gauge reading could be regarded as useful . . . even if predicatible, once you know what's going on!

On the 'header bottle', even though it's not mandatory on many engines (it is on the 2.5), it does encourage air to be displaced, avoids the header tank going empty, and allows bled-out air bubbles to be easily seen.


Normally half-full of coolant during bleeding. A bit of old radiator hose at the bottom fits snugly into the header tank neck, and blocks off the overflow hole.

[CitroJim]

Good tip about a bit of old rad hose on the bottom of the bottle Chris

[Mandrake]

Glad you got it sorted, Simon. Think you are probably right in that an airlock eventually migrated to the head, where it would cause a hot-spot.

Here's hoping. It was absolutely fine again on the way to work this morning.

I did not know the V6 gauge sensor was placed in such an odd position. Why, for heaven's sake? Normally, the gauge sensor is located in the water output 'tank' from the head/block, along with the other coolant sensors. Wierd. Can't understand how such a 'cool' gauge reading could be regarded as useful . . . even if predicatible, once you know what's going on!

It doesn't just measure return flow from the radiator but is a blend of return flow and some other direct circulation - the sensor is in the back of the rear bank head near the firewall, while the ECU sensor is near the front beside the thermostat housing.

I think one reason they might do it (other than trying to find room to mount three different temperature sensors - ECU, fan sensor and gauge sensor!) is that it gives more variation in reading based on the cooling load of the engine. What I mean by that is that once warmed up the ECU sensor at the front tends to report a very constant (and true) temperature of around 90-95 degrees regardless of whether you are cruising gently or working the engine hard, thanks to the regulation of the thermostat and the fans.

So long as the amount of heat put out of the engine is within the capability of the cooling system to dispose of the temperature doesn't shift much, therefore a gauge measuring at that point wouldn't really show any change as you were driving leaving you in the dark as to whether the cooling system was coping with ease or just barely managing to keep up with demand. It's not until the ability of the cooling system was exceeded and the engine temperature did actually start going up into the unregulated region would the gauge shift, and by that point you're already getting yourself into trouble.

By measuring the blended return flow from the radiator the temperature reading gives an indication of how hard the radiator is working - if the water coming back from the radiator is a lot cooler than engine temperature the radiator is doing a good job and coping well, if the water returning from the radiator is nearly as hot as the water that went from the engine to the radiator, then the radiator is struggling and therefore the gauge reading will go way up.

So the gauge reading whilst generally reading quite a bit lower than "true" temperature will modulate a lot with the heat load of the cooling system and give a nice proportional indication of how well the cooling system is coping under current conditions. An example would be if the low fan speed failed - the gauge temperature reading would go way up on what it would normally be, and quickly, since it would detect the much hotter return flow from the radiator.

Either that or they just put the sensor in a bad place and didn't care - take your pick! The moral of the story though, is on this engine when checking the opening temperature of the thermostat and the cut in and cut out temperature of the low and high fan speeds, it should be compared with the engine temperature reading taken from a Lexia, not from the gauge on the dash.

[xantos]

Just to hijack this tread a bit...

Would Forte seal expansion bottle and degassing tank?

[white exec]

You're not supposed to ask questions like that.
Bit like The Meaning of Life (which we know is "42").

I'm guessing that the stuff is designed to go off (seal) in the presence of air, so at the coolant-air interface in both those places, possibly yes.

Maybe, in the presence of very high humidity (inside the cooling system) it won't.

Perhaps someone should call Forte technical and ask . . .

[Mandrake]

I'm guessing that the stuff is designed to go off (seal) in the presence of air, so at the coolant-air interface in both those places, possibly yes.

I wondered that, but wouldn't that mean it would go off in the surface of the coolant expansion chamber, where there is plenty of air ? I've not seen any rubbery scum floating on top of the water...

I suspect something a little more clever than exposure to air is going on. I'm still impressed that the heater matrix stopped leaking immediately when I used it - even on the initial test drive that would have been required to mix it around and heat it up it didn't leak! (how ?)

[Stickyfinger]

It will not work as those joints open and close a significant amount by pressure applied. The heater matrix type leak is mainly due to corrosion or a thin track caused by the plastic part cracking due to going hard.
Forte will not seal a rubber hose for the same reason.

[elma]

Thats news to me and very handy information, might save me wasting some time and cash in the future.

[Mandrake]

Looks like the problem is not gone as I just had another big scare tonight!

This morning it was fine and on the way home tonight it was fine at first - I didn't exceed about 35mph all the way home and ambient temperature was about 4 degrees, over about 10 minutes it climbed to normal indicated temperature of about 80 and seemed fine for a while.

Then when we were only about a mile from home the temperature reading suddenly started rising steadily from the normal 80 to 90 then kept climbing to 95, and by the time we got home had hit 100! The last stretch of road I was literally coasting down hill and it still rose from 95 to 100. As soon as I arrived home I heard the fans blasting on high and popped the bonnet to discover a major clue - the radiator was absolutely stone cold to the touch with the fans blasting on full and the temperature gauge indicating 110!

Clearly no water at all flowing through the radiator... During the whole period of time that the temperature was rising unusually the cabin heater was working normally and was nice and hot and didn't really seem to change temperature so there was at least water flowing around the heater matrix normally.

I didn't want to open the cap and let it boil like last time so I turned the engine off and ran inside to find the Lexia so that I could get a temperature reading from the ECU sensor near the thermostat housing but the laptop was so slow to boot up and load Diagbox that it was 10 minutes by the time I had the Lexia up and running in which time the engine had cooled quite a bit - when I started the engine with the Lexia running it read 93 almost straight away which is perfectly normal, and the radiator started getting hot as soon as the engine was started, indicating flow through the radiator again. The fans also dropped back to low within about 20 seconds then went off after a while.

I tried opening the radiator bleed valve and water flowed out without any air - in the process I noticed the valve has a crack right through it:



I think this is just a red herring though because the o-ring that it pushes into the hole should still seal ? If not is it possible to get this bleed plug by itself ? Unfortunately I only recently threw away the old radiator...

Anyway, I don't think it's that but thought I would mention it. I also unscrewed the cap after it had cooled a bit and there was no problem with the level. I went for another drive to try to provoke it again while the Lexia was connected but no such luck. I did notice the temperature indicated by the Lexia was lower than expected when driving faster though - at 50mph the coolant temp was dropping to about 81-82 degrees, which I think is a bit lower than it should be, while at 30mph it was around 86-87 degrees, perhaps suggesting that the thermostat is sticking slightly open ? Or is this just to be expected on a 4 degree night ?

So what do people think it is ? Is there still an air lock going around the system, or is the thermostat intermittently sticking closed ? If its the thermostat why did letting the car sit for 10 minutes with the engine off get the radiator flowing again ? And for that matter why would it be fine and at normal temperature (with the thermostat presumably open) and then suddenly it is stuck shut and overheating ? Doesn't make sense...

Considering I have to drive this car on a daily basis and I'm not sure what is going on I'm getting quite concerned. I really don't understand why this has started after Forte was added - after 2 1/2 years of absolutely no problems with overheating. At least I know one thing it is not - the fans. The fact that they were blasting on high while the radiator was stone cold proves that, I also double checked the cut in and cut out temperature of the low fan speed afterwards (93 and 90 respectively) and it was absolutely fine.

[RichardW]

Not good, Simon...

Can't see your picture; but I can't see any separate bleed screws on the rad on service.citroen. Seems to me it's either water pump or thermostat. As you still had heating, my money is on the stat. If you can't get one you could always take it out, but then it will run a bit cool! New one is £28 from Citroen - used on C5, Xantia and XM, so probably available aftermarket. I would think most likely nothing to do with Forte, but it does contain a water pump 'conditioner' I think? Just possible that this has caused a seal or something in the stat to swell a bit, and if it was already a little on the sticky side it is now more on the sticky side.

[white exec]

Sounds like a (the?) thermostat was stuck shut, blocking off the radiator, but later broke free and opened up. I would replace it.
The radiator bleed plug, if the same as XM, is a miserable little thing, with a slim O-ring attached - the bayonet-fit plastic plug is easily damaged if made use of often. Might be better to fit a tiny upward-pointing tap, or a threaded plug.

[Mandrake]

Not good, Simon...

Can't see your picture; but I can't see any separate bleed screws on the rad on service.citroen.

Image link fixed. The radiator bleed tap shown is in the top corner nearest the battery. It works in a similar fashion to the drain plug at the bottom - twist 90 degrees then lift.