C5 II runnig hot (overheating) when towing

[Romzi]

Hi all,
I own C5 II break, 2.0 HDI 100kw without FAP.
Everything is fine, except in my opinion, the engine is getting very hot too quick when towing relatively small caravan (old Adria with approx. full weight 850-900kg). On long hills on motorway and when hot outside (35 C), I was forced to shift down to 3rd gear and slowdown to about 70km/h to stop raising temperature gauge and prevent overheating and stop lights. It was just below the red mark. Temp. gauge dropped a little so we managed to climb those long hills od the motorway (South Croatia, Dalmatia). But the workload was low, maybe half of the power of the engine (maybe less). Oil temp was very high too. I cannot imagine towing "real" caravan with 1300 or even 1500 kilos.
Previously we towed with Xsara HDI 2.0 80kw and it was also quite hot when working hard, but still not so much.
Last year, radiator was changed and flushed the system, but the car was prone to overheat while towing before.
I am thinking about (half) faulty thermostat, but am confused whether is it mechanical or electronic.
What about water pump? Is it possible being faulty or even wrong one for this engine?
When solo, the engine is beahivng perfectly fine, maybe there is a little movement on temp gauge when pushing hard, but nothing special.
Help please. Thanks!
P.S. I have access to citroenservice, so i can post addional info. about the engine, i also have chinese Lexia clone, so i can check things if neccessary.

[Mandrake]

If you've changed the radiator but never replaced the thermostat I would replace the thermostat.

They can fail in a variety of ways including seizing partly open. In that case the car would run too cool when under a very light load, but would overheat under a heavy load.

Also have you checked that the radiator fans work on both low and high speeds ? On some cars the low fan speed can fail leaving only the high speed working which doesn't come on until a much higher temperature.

Fan problems aren't usually noticed at motorway speeds (since the natural airflow should be enough) but under towing conditions it may still cause difficulty.

[white exec]

Is the C5 manual or auto?

[Paul-R]

Have you heard the cooling fan running at all? These are prone to seizing up over the winter when they seldom come on.

[Romzi]

Is the C5 manual or auto?

Manual transmission.

[Romzi]

Have you heard the cooling fan running at all? These are prone to seizing up over the winter when they seldom come on.

The fan seems to be ok. I will still check it witj Lexia again.

[Romzi]

If you've changed the radiator but never replaced the thermostat I would replace the thermostat.

They can fail in a variety of ways including seizing partly open. In that case the car would run too cool when under a very light load, but would overheat under a heavy load.

Also have you checked that the radiator fans work on both low and high speeds ? On some cars the low fan speed can fail leaving only the high speed working which doesn't come on until a much higher temperature.

Fan problems aren't usually noticed at motorway speeds (since the natural airflow should be enough) but under towing conditions it may still cause difficulty.

Maybe do you know, is it mechanical or electric? Is it possible to (just for test purpuse) eliminate the thermostat? Tnx
Will report back.....

[Mandrake]

Maybe do you know, is it mechanical or electric? Is it possible to (just for test purpuse) eliminate the thermostat? Tnx
Will report back.....

I'm not familiar with that exact engine, but I'm talking about the coolant thermostat - nearly all engines still use a mechanical thermostat for the coolant flow.

I have heard of a few new models in the US using partially electronically assisted thermostats for coolant flow but I think this is rare and unlikely to be in an old Citroen...

Removing the thermostat as a test isn't really worth the hassle as if it turns out to be faulty you'd have to replace it anyway. They are cheap to buy so I would just replace the thermostat and see how it goes. Overheating the engine is potentially fatal so it's not worth the risk playing games with an overheating engine.

On most engines replacing the thermostat is relatively easy.

[white exec]

Not sure about C5 II, but pulling the connector from the (? brown) temp sensor will show whether the fans are operational. Someone here should be anle to confirm.

Otherwise, take the car out for a run to get up to temp, park up, and with engine still running see whether the fans cut in - first at low speed, then after a while at high speed. This may take 15-30 mins to happen. Check what the temp gauge reads during the test.

Agree with Simon: just renew the thermostat (and its housing gasket, if it has one) if it's more than several years old. After doing this, thoroughly bleed the cooling system to remove any trapped air.

[Romzi]

Ok, some info.
I performed a few tests with Lexia.
The fan is operational and is blowing like a tornado
I performed global test and this faults came out:

Don't really know if it is important or not, but the second one does worry me, it's just about cooling circuit Does anyone know more about it?
I also checked operation of t-stat, in some degree it is definitely working, it starts leting coolant to the radiator at aprox. 82C. But the temperature does not rise any further when idling. I whill replace it and after that perform some test with a caravan or heavy trailer attached and also with lexia plugged in.

[GiveMeABreak]

Ignore the first one, I don’t know anyone that doesn’t have the glow plugs not supplied fault.

If you go into the second fault, and drill down, you’ll get some further inf. let us know or take a screenshot of it for us.

[white exec]

JOOI, what is the cooling circuit by-pass electrovalve?

[GiveMeABreak]

I think it's the electrovalve that diverts the coolant, returning it back to the engine and bypassing the radiator until the correct temp is reached.

[white exec]

Interesting. Might be wide of mark here, but for C5-II DW10 138, until RP 10338, this item is shown on the water outlet:
1338 E1 'Proport water solenoid'
in place of normal thermostat? or as well as?
After that RP, it reverts to just a normal mechanical thermostat.

[MikeT]

I don't think it's a coincidence the car's overheating and there's a reported fault with a coolant bypass valve.
I tried to google what this bypass valve does and read one explanation that it's to bypass the thermostat when the engine is under heavy load.