Toledo 1.9tdi 110 heater and coolant issue

Last week my heater stopped getting hot so felt the hoses, top rad hose hot bottom hose cold, left heater going into bulkhead hot Right hose cold. Removed the heater hoses and flushed out with hose pipe but no blockage found. So then drove it hard to see if it overheated and it did so I thought change the thermostat. Done that, car sits at 90 as always til I drive it hard or drive for more than half an hour, then it rises, heater still freezing. Constantly tried changing heater controls from hot to cold and back again but no change.No coolant loss anywhere, could it be the temp sensor I keep reading about? If so, where is it? Expensive/easy to replace? Hope it's not water pump but no leaks anywhere...Please help..

there is usually a separate thermostat from the engine to the heater matrix. follow the bulkhead hoses back to find it

Ok, thanks Niall, so if there is a separate thermostat do you think this would stop water circulating giving the impression it's the water pump, just priced a water pump, only £25, just not sure if I can do the job if it needs one

How long and how many miles has the water pump been installed for ? sounds to me like it's poor circulation from the pump. Thermosyphon is enough to keep the engine temp ok while knocking around but not enough to cope when driven hard and not enough to circulate through the heater in any decent amount.
Not sure about the separate heater thermostat, the heater usually gets water straight from the engine to give you heat and windscreen defrost as soon as there is any heat in the engine.

Hi Niall, I've followed the bulkhead hoses back to the engine and the one that gets hot goes to a part on the side of the head with an electrical switch on it (temp sensor??). Then it goes to the top rad hose. Also the oil pipe from oil cooler goes into this part. The cold hose from bulkhead goes under this part and around the front of the head to the housing of the thermostat that I replaced. The main hose coming out of the thermostat housing goes to the bottom rad hose. There is no loss of coolant and no noise from the water pump which suggests to me it is ok, I wonder if it's the electrical switch mentioned earlier?. I have been advised to drain the rad from the bottom and run engine with some rad flush in, then re-fill the header tank slowly with the cap off and engine running to see if I can see water/coolant circulating into the header tank.

Hi Rob, no idea as I've only had the car for about a year, I will check the paperwork to see if it has been replaced in the past. Car has done 163,000 miles and goes really well still

Hi, the electrical part mentioned earlier is the temperature sensor I've just googled it and euro parts say it is green round 25mm diameter. This is what I can see above the hose mounting on side of the head. They are £17 so if I unplug it will it let water circulate around the engine thus telling me know I need a new sensor?. And thanks muchly guys for the advise.:D

The temp sensor may be for the fans to work, or a sensor to the dashboard / ECU . The dash should show 90 degrees when engine warm.

On older cars the thermostat used to be underneath the water temp sensor, usually an elbow. Older cars used to have a sensor on or near the radiator for the fans to work, a lot bigger than the one above the engine thermostat.

Anyway I digress and you don't need a history lesson !

I think the cheapest option is

1) replace the engine thermostat first, it may be sticky and the fans are cooling the engine enough to stop overheating, with the colder weather; perhaps.

The " sticky " thermostat will slow down water to the matrix, or not at all.

2) Replace expansion tank cap as this is pressurised

3) Failing that, then I would replace the water pump .......................... with the timing belt, as you are that far down & may be due for renewal anyway. You may find that you can get a metal propeller water pump as an upgrade, rather than a plastic one ; with other VAG cars this has helped for water circulation

Replace any coolant with G11 or G12 ............ usually stamped somewhere and do not mix in my humble opinion.

Also Pink coolant should not be mixed with Blue coolant, as will block your heater matrix , in a gooey blob

I think its a failed plastic pump impeller

Agreed as above ....