Hi all,
I'm having issues with my C3 picasso 1.6 bluehdi. Firstly I noticed that the cabin temp wasn't getting hot, just tepid. I asked a mechanic about it and he suggested more than likely the thermostat stuck open. I have changed the thermostat housing with intergrated stat and flushed out the heater matrix through the pipes in the bulk head. I have bled the system thoroughly. I am getting a little more heat but not much. I have a basic plug in Bluetooth diagnostic thingy, I plugged that in and kept an eye on the coolant temp. After taking it for a run I got the temp upto about 80ish but on tick over it drops to about 64 degrees (no fan kicking in), then the cabin heating runs cool again. I have now exhausted my knowledge of coolant systems. Can anyone offer any other possible things to check for?
Any help greatly appreciated!
That sounds like symptoms I had when I had an airlock in the system. Normal bleeding didn't cure it but using a funnel in the rad topped up well above the highest part of the system and then running the engine at 3000rpm with the heater on hot and full fan produced a strong stream of bubbles for a good minute or so. After that the heater performed fine. You can see my tale at https://frenchcarforum.co.uk/forum/view ... 89#p828389
I used to be indecisive, now I'm not so sure!
I used to ride on two wheels, but now I need all four!
mickthemaverick wrote: 03 Jan 2026, 18:41
That sounds like symptoms I had when I had an airlock in the system. Normal bleeding didn't cure it but using a funnel in the rad topped up well above the highest part of the system and then running the engine at 3000rpm with the heater on hot and full fan produced a strong stream of bubbles for a good minute or so. After that the heater performed fine. You can see my tale at https://frenchcarforum.co.uk/forum/view ... 89#p828389
Hi Mick, thanks for the info! Unfortunately I dont think my rad has a top cap to it. I believe that the only way to top up is through the expansion cylinder .
I assume that if you say expansion cylinder, you mean expansion tank. If the level in it is between max and min after reaching the temperature several times, then the system is fine, but the heater matrix may be clogged. And that is easy to say and difficult to verify. In fact, when the engine temperature is in order, the heating is turned on, then you should feel heat on the hoses towards the cabin.
Mihael_M wrote: 04 Jan 2026, 09:47
I assume that if you say expansion cylinder, you mean expansion tank. If the level in it is between max and min after reaching the temperature several times, then the system is fine, but the heater matrix may be clogged. And that is easy to say and difficult to verify. In fact, when the engine temperature is in order, the heating is turned on, then you should feel heat on the hoses towards the cabin.
Hi Mihael,
Yes sorry that's what I meant. I flushed the heater matrix through and got clean water running through it. The pipes going through the bulk head both seem to get hot when the engine is warm. When I rev the car a lot the heaters do get warm, however when it's on tick over, they go back to being tepid.
As the matrix seems clear and the hoses are both hot then you should start looking at the airflow. Are the control flaps working correctly? Have you replaced the cabin filter recently? Check whether any vent gets warmer air through than another?
I used to be indecisive, now I'm not so sure!
I used to ride on two wheels, but now I need all four!
mickthemaverick wrote: 04 Jan 2026, 10:45
As the matrix seems clear and the hoses are both hot then you should start looking at the airflow. Are the control flaps working correctly? Have you replaced the cabin filter recently? Check whether any vent gets warmer air through than another?
I believe they are working properly (although not entirely sure how to check properly) as when I change the temp to cold, it does go icy cold, especially with the outside temp being around freezing at the minute. I am not sure what you mean by cabin filter? Is that the pollen filter?
All vents seem to be equal in temp.
I would check the thermostat again. It should not start to open until its rated temperature and once that is reached the radiator top hose should start to get warm. When it goes below its rated temperature it should be closed. I have replaced a faulty thermostat only to find the new one was also faulty. If you find your new one is faulty you should be able to return to the dealer for a refund, assuming it is a new one since I just had a thought possibly you fit a used one off such as ebay since new ones are expensive.
To get all the air out the funnel that Mick has used would be used in place of the expansion tank cap. I made a tool out of drain pipe and an old tank cap as noted in this 2017 post copied from the Citroen Owners Club site.
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In the Haynes manuals and the Citroen service box procedures it is recommended to use special tools 4520-T and 4222-T when filling the coolant since the max coolant level in the expansion tank is below the air bleed screws. All the tools do is raise the level of the coolant above the bleed screws so the air can be pushed out.
You can make a simple tool for less than £5 using an old expansion tank cap (or a new one for around £4), about 12 inches of basin waste pipe 32 mm diameter (1.25 inches) and the triangular section rubber seal from a 32 mm fitting such as a basin U bend or pipe connector. 2 m of the push fit waste pipe is £1.29 from Wickes. To make the tool prise out the tank cap internal valve then cut out a hole in the cap so the 32 mm pipe can just fit through it (drilling/Stanley knife/filing work). Push the pipe through the cap and fit the seal on the pipe.
To use it, fill the coolant in the expansion tank to just below the max level, fit the pipe in the tank so it extends in to the coolant to just below the max level, push the seal down to the tank neck and then tighten the cap on the tank to give an air tight seal. Add coolant in the pipe to about 2/3 of its height (use a funnel to avoid spilling) - the pipe will fill but there will be an air pocket in the tank so the tank does not fill up. Then open/close the bleed screws in turn until any air is displaced. If needed more coolant can be added in the pipe. Once the air is out, undo the cap which will release a slight bit of air pressure and the coolant in the pipe will run in to the expansion tank and the pipe can be removed without any spillage - the tank level should be at max. Then fit the normal cap to the expansion tank, check for any leaks and then run the engine. This diy tool worked well when I replaced the thermostat housing on our C5.
Paul-R wrote: 04 Jan 2026, 11:05
The pollen/cabin filter is often overlooked as part of regular servicing. I think that would be my next (and easiest!) item to check.
PaulC5 wrote: 04 Jan 2026, 13:50
I would check the thermostat again. It should not start to open until its rated temperature and once that is reached the radiator top hose should start to get warm. When it goes below its rated temperature it should be closed. I have replaced a faulty thermostat only to find the new one was also faulty. If you find your new one is faulty you should be able to return to the dealer for a refund, assuming it is a new one since I just had a thought possibly you fit a used one off such as ebay since new ones are expensive.
To get all the air out the funnel that Mick has used would be used in place of the expansion tank cap. I made a tool out of drain pipe and an old tank cap as noted in this 2017 post copied from the Citroen Owners Club site.
-----------------------------------------------------------------------------
In the Haynes manuals and the Citroen service box procedures it is recommended to use special tools 4520-T and 4222-T when filling the coolant since the max coolant level in the expansion tank is below the air bleed screws. All the tools do is raise the level of the coolant above the bleed screws so the air can be pushed out.
You can make a simple tool for less than £5 using an old expansion tank cap (or a new one for around £4), about 12 inches of basin waste pipe 32 mm diameter (1.25 inches) and the triangular section rubber seal from a 32 mm fitting such as a basin U bend or pipe connector. 2 m of the push fit waste pipe is £1.29 from Wickes. To make the tool prise out the tank cap internal valve then cut out a hole in the cap so the 32 mm pipe can just fit through it (drilling/Stanley knife/filing work). Push the pipe through the cap and fit the seal on the pipe.
To use it, fill the coolant in the expansion tank to just below the max level, fit the pipe in the tank so it extends in to the coolant to just below the max level, push the seal down to the tank neck and then tighten the cap on the tank to give an air tight seal. Add coolant in the pipe to about 2/3 of its height (use a funnel to avoid spilling) - the pipe will fill but there will be an air pocket in the tank so the tank does not fill up. Then open/close the bleed screws in turn until any air is displaced. If needed more coolant can be added in the pipe. Once the air is out, undo the cap which will release a slight bit of air pressure and the coolant in the pipe will run in to the expansion tank and the pipe can be removed without any spillage - the tank level should be at max. Then fit the normal cap to the expansion tank, check for any leaks and then run the engine. This diy tool worked well when I replaced the thermostat housing on our C5.
Wow! That is incredibly informative! Thank you so much. How can I physically check whether the stat is faulty? (It was a new one, mid priced)
I have ordered a no spill funnel to try to bleed it further.
Before the first thermostat was replaced, it was necessary to establish its behavior, i.e. the movement of the temperature. This is done with a cheap ELM327. ??Now if it is suspected again, it should be done first. I don't know how it is with Citroën, but with Renault there are flaps that let air through electronically controlled.
If the flaps do not open (there are more left and right), then there is no heating. Usually one of them gets stuck.
How can I physically check whether the stat is faulty? (It was a new one, mid priced)
I have ordered a no spill funnel to try to bleed it further.
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Start the engine from cold and keep touching the radiator top hose to see when it starts to get warm. Should be at the thermostat rated value but if well before that then the thermostat is opening too soon. The cabin heater hoses should be getting warm as soon as the engine warms.
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