Strange thermostat behaviour on 2.5, too hot & now too cold!

[endorfin]

As previously mentioned I recently replaced the main thermostat on my 2.5 after I had some slight overheating issues (nothing more serious than reaching temp of 100 deg at very high speed i.e. 100mph or working hard) However the new thermostat made things worse. It turned out to be a faulty stat that was stuck closed. I forgot to do a test in boiling water before fitting!

Back to motor factors for another, this one had a different design with the spring on the outside. Having learnt from the last faulty stat I placed it in boiling water and it opened up nicely. Fitted stat, refilled with coolant and bled the system. On test drive temp climbed in a timely manner and stayed on 90 degrees at high speed, sometimes even falling slightly below 90. Whilst stationary stayed at 90 deg with fans on.

All sorted. Or so I thought!

Drove back to Brighton today, started car from cold and it seemed to take a very long time to reach 90 deg, the temp gauge would rise, then fall slightly, rise again the fall slightly, a bit like 3 steps forward 2 back. I got onto the motorway and booted it and temp would not go higher than 80 degrees even at high speed, a lot of the time it would be on 65 & 70 degrees which I know is far too cool to be normal. Arriving into town and driving at a slower speed the temp rised to 90 deg but then curiously went slightly over, say about 93 degrees before settling back to 90.

Any ideas on whats going on here!! Since it worked fine when 1st fitted I'm thinking maybe the thermostat is stuck OPEN this time??

Maybe I should just buy a genuine Citroen one from Citroen? Any idea how much these are?

This post has been edited by endorfin on May 08, 2011 07:45 pm

[endorfin]

Price for the stat from Citroen is only a couple of quid more than a non OE one!

Wish I'd checked before buying cheapo ones. Will fit it tomorrow and see if this solves the problem.

[noz]

Hi endorfin,

There's a lot to go at in your post. Taking them a step at a time;

You say that you replaced the Main Stat. Can we just be clear, when you say the main stat it is a substantial affair with a two stage thermostat built into a cast aluminium housing with a 40mm pipe spigot on one end and a 3-bolt housing at the other. The entire assembly is bought as a single piece and costs £34 from Citroen. I have yet to find a non-Citroen part for this. I bought one last week from Citroen, that's how I know. The main thermostat is fitted to the flange on the intake to the water pump. The hose attached to the 40mm spigot is the one coming from the bottom of the radiator. Are we talking about the same stat?

You mention that "this one had a different design with the spring on the outside". Given my description above, this makes no sense unless we're not talking about the same stat.

The main stat is in fact two stats placed one inside the other. The smaller one will open first a couple of degrees before the main one. The reason for this is that stats are very non-linear devices when percentage opening is graphed against percentage flow. The first few percent of opening results in a very large increase in flow. The relationship is not linear. This makes temperature control very difficult. Citroen have obvioulsy tried to overcome this traditional mechanical deficiency by staging two differently sized thermostats in series. Although not perfect, the resulting opening/flow curve will be much more linear than the traditional relationship curve thereby providing a much smoother and more responsive action to temperature increases or decreases. When you put the stat in water did you see the two stages moving independently to each other?

I have just fitted two new radiators and the main stat last weekend. The temperature generally sits at about 89 now rising to 93 or thereabouts under heavy load (including towing the caravan). Previously it rose to about 100 under heavy load.

I connected the Lexia after fitting the rads and stat and took it out for a run. The air temperature sensor (located downstream of the turbo and intercooler just before the intake manifold) read 20DegC throughout the journey. I therefore concluded that the smaller stat mounted on the outlet tank which controls the intercooler temperature must have been faulty. The intercooler should heat the air when it is lower than 65DegC and cool the air when it is higher than 65DegC. The air temperature should therefore read close to 65DegC most of the time. I intend to buy a new one shortly. These are much more common and are indeed available from 3rd party factors.

Please let me know how the above relates to your recent replacement works.

Cheers

noz

[noz]

ps

Everything you wanted to know about the 2.5TD cooling system including pictures of the main stat.

http://club-xm.com/forum/index.php?showtopic=788&hl=


cheers

noz

[endorfin]

It is indeed the main stat I am talking about, this is the one that you need to undo the header tank to get to. Its curved and you clip a single large rubber hose to it.

The original main stat that I removed had the spring inside the curve with the copper sensor protruding on the outside, it had 2 rubber seals that when I placed in hot water both only opened slightly which I attributed to my slight overheating problem (100 deg under load)

Main stats are available from motor factors, including Partco, I got the replacement from a small local MF for £23 and it was identical to the original I removed albeit without the gasket. The gasket was an additional £5 odd from Citroen. Foolishly I didn't pre test this one before fitting and I had serious overheating problems, temps of 100 deg at low speeds, I had to drive home at 50mph on the motorway to keep the temp down. On removing this stat I tested it in water and only the outer seal would open slightly with the inner staying closed. This explained the overheating.

I returned this to the local MF and they could only get me a next day replacement from a different supplier at a higher price of £28 which when it arrived was of a different colour being grey and the spring was on the outside sort of wrapped around the protruding copper sensor (if this makes sense, I will upload photos when I've removed it) I tested this in boiling water & it only has one metal opening like a traditional stat and it opened nice & wide. However this one runs too cool, takes a very long time to get to operating temp (35 miles)

I had already come to the conclusion that you've outlined and this confirms it, I need a working 2 stage stat and the replacement from Citroen is awaiting me at the parts dept but it won't be until Friday before the gasket arrives. This'll be my 3rd gasket!

Hopefully this new stat will solve this problem. In the meantime I don't see any harm in running the car cooler than usual?

With regard to the small stat, I've already got a replacement ready to go but unfortunately I'm having trouble undoing the top bolt which appears to be stuck fast, don't want to push too hard for fear of shearing it off.

This post has been edited by endorfin on May 10, 2011 08:10 pm