The Function and Malfunction of the Diesel Thermostat

[HarryM1BYT]

Originally Posted by hoops

help please, have read lots of threads and i am really confused my water temp is reaching the 9 oclock mark but its freezing in the car even on hi unless i drive at over 60mph but when i hit traffic driving at 35mph back to just cold air blowing inside. it was great before, started to misbehave over the last five week. is it the thermostact or pump or inside temperture thingy.
also a lot of the abreviatted words i do not understand is there a thread for idiots so i coild just look up what ecu,fbh,oem etc means and any other mechanic speak then more could be added just a thought.
i open the bonnet and look and think OMG!!

First thing to do, is check out the dash diagnostics to find out the actual running temperature, because the dash needle tells lies.

Cool running is using down to the thermostat sticking open/ opening early.

If all above OK, it could be the heater matrix which is choked up.

[thundercat]

That's the Name of the Thermostat I have just fitted to the top hose of my CDTI. The part number is TH01489, I removed the top hose completely to fit this, first I boiled a kettle and poured it over the end of the hose that fits onto the engine (to soften it up), then I slowly worked the stat into the hose pushing it about two and a half inches down. With new Stainless Steel Jubilee clips fitted replaced hose and coolant ( very little came out when hose was removed). ran engine for 5 mins with Rad cap remover to get rid of any air locks
The hardest part was removing the old SS Clip on the hose near the engine, I used a baby Hacksaw but be careful as you can cut through the hose its self.
As yet have not been for a long run to see if its ok but as it was running at below 60c before I did this mod it can't be any worse.
Just a footnote, I noticed that once the coolant was out of the expansion tank it revealed high and low water marks. I have been over filling my expansion tank.

[Hogwarzt]

Originally Posted by thundercat

That's the Name of the Thermostat I have just fitted to the top hose of my CDTI. The part number is TH01489, I removed the top hose completely to fit this, first I boiled a kettle and poured it over the end of the hose that fits onto the engine (to soften it up), then I slowly worked the stat into the hose pushing it about two and a half inches down. With new Stainless Steel Jubilee clips fitted replaced hose and coolant ( very little came out when hose was removed). ran engine for 5 mins with Rad cap remover to get rid of any air locks
The hardest part was removing the old SS Clip on the hose near the engine, I used a baby Hacksaw but be careful as you can cut through the hose its self.
As yet have not been for a long run to see if its ok but as it was running at below 60c before I did this mod it can't be any worse.
Just a footnote, I noticed that once the coolant was out of the expansion tank it revealed high and low water marks. I have been over filling my expansion tank.

Just bear in mind, I fitted the inline stat and then found my expansion cap failed and had to be replaced. sinced then (well over a year) all ok.

[FrenchMike]

Hi Guys,

I have got a faulty thermostat from a French forum member and have had the
opportunity to make some measurements ;
I guess that will interest some of you .

At 50 degrees ,firmly closed and at 90 degrees fully open:

Here are the results:

50 deg
50.jpg



70 deg
70.jpg

80 deg
80.jpg

85 deg
85.jpg

88 deg
88.jpg


A question arises ,is this progressive opening normal or not ????

All comments are welcome .

Mike

[HarryM1BYT]

Originally Posted by FrenchMike

Hi Guys,

I have got a faulty thermostat from a French forum member and have had the
opportunity to make some measurements ;
I guess that will interest some of you .

At 50 degrees ,firmly closed and at 90 degrees fully open:
Mike

The water doesn't quite cover enough of the stat element, so perhaps had it been fully covered, it might have been fully open at around 60C?

[crofts]

Well I must thank you guys for the fantastic effort you have put into trying to solve this. Particularly admire all the photos & diagrams, you must have spent considerable time on this task.
I am sure other members will join me in thanking you

[FrenchMike]

Originally Posted by HarryM1BYT

The water doesn't quite cover enough of the stat element, so perhaps had it been fully covered, it might have been fully open at around 60C?

No Harry ,i did other sunken tests from 60 to 90 deg ,same results :

60 deg
60degr.jpg

90 deg
90degr.jpg


Mike

[Mike Noc]

Mike two questions;

When cold is there any leakthrough at all?

At what temperature does the stat start to open? Think it was Harry who suggested putting some cotton in the stat when hot and let it cool down, and then you can suspend the stat in water, gradually heat it, and as it starts to open it will fall into the pan.

Mike

[HarryM1BYT]

Originally Posted by Mike Noc

Mike two questions;

When cold is there any leakthrough at all?

At what temperature does the stat start to open? Think it was Harry who suggested putting some cotton in the stat when hot and let it cool down, and then you can suspend the stat in water, gradually heat it, and as it starts to open it will fall into the pan.

Mike

That was probably me - It was an old trick which ensured the all metal stats avoid direct contact with the pan.

So what we seem to be seeing is a gradual opening over quite a wide range of temperatures, rather than a what I would expect - a much narrower band of shut to wide open?

[wuzerk]

Originally Posted by T-Cut

Yes, where's Jules when you want him. He said he'd replaced hundreds of these and had not found a single one failed 'open'.



TC

So far I have never seen a report on here of a 'stat which failed open or closed on the diesels. I believe that the reason it fails to maintain the designed temperature is because the spring loses its strength allowing the valve to open too early. I know exactly when mine failed and that was after the first time I ever really thrashed the engine up a long hill thus reaching the highest temperature it had reached under my ownership (and compressing the spring the most)
The reading dropped below normal (9 oclock) straight afterwards.