Low coolant temperature

My wife’s 2010 Peugeot 308 1.6 Allure has developed a problem with the heater where the temperature gauge only goes to 70° and eventually drops to zero, and the heater blows cool to low temperature even after a run. A diagnostic check at a local Halfords showed no problem with the cooling system, but a further check at a local Peugeot dealer identified the thermostat as faulty.

The thermostat was replaced by a local garage, but the symptoms persist. Occasionally the heater blows hot air for a while, but never for long. There are no water leaks and the system has been vented several times.

Can you give me any advice on this issue? Tony Cook

As the system has been checked and vented, I am assuming the cooling system is topped up to the correct level. You have had the thermostat replaced and this has made little difference. Hopefully, the new thermostat is not faulty, causing the same symptoms.

The confusing part about your problem is that the temperature gauge also reads low, dropping to zero. If it were not for this, I would suggest it is simply a circulation problem – possibly a blocked heater matrix. In that case, I would recommend removing the heater hoses from the engine side of the heater and flushing the matrix through using a hose.

Because the gauge is also reading low, I am concerned that it is possible that the engine does still have an air-lock

within the system. Bleeding out the cooling system on your 308 requires a couple of special tools (parts no 4520-T and 4222-T); these are a filling cylinder and adapter pipe. Basically, the filling cylinder is used to give a head of water to the system when bleeding the system out via the two bleed screws located on the engine (one on the rear water outlet and the other at the rear centre of the cylinder-head). Once the coolant begins to flow steadily from the bleed screws, they should be shut off while maintaining a head of water in the filling cylinder. The engine should be started and run at between 1500-2000rpm until the cooling fan has come on and gone off, then come on again. The engine should then be turned off and left to cool, before removing the filling cylinder and levelling the reservoir. It should be possible to adapt an empty two-litre plastic bottle to act as the reservoir by cutting off the base and using tape or similar to seal the neck of the bottle into the filler of the reservoir while the system is bled out.

Finally, there is a small bleed hose coming from the top of the radiator to the coolant reservoir and I would ensure that this is clear and allowing the air in the radiator to escape.