Heater Fault

[Warren16891]

Hello, I have an X Reg Ford Fiesta 1.8 diesel. The heating on it isn't working properly. I do a lot of motorway miles, and when driving at around 70mph the heating is okay on fan settings 1&2 (not great though.) Above that the temperature Guage drops slightly to about 1/3 and the air becomes cool. Also, when I reach a city and slow to anything below 50mph the same thing happens even on the low fan speeds. I've checked my coolant level and that seems to be fine (just below the maximum line.) Any ideas on what is causing this? It was fine last winter and I've only discovered it in the recent colder weather. Thanks for your help!

[Martin350]

Could be the heater control valve, quite a common part to fail on a lot of Fords.

They are usually somewhere around the middle of the bulkhead, sometimes with a cover over them, and look like this;

[ch427]

I would guess at a coolant circulation issue, have you tried changing the thermostat?

[Warren16891]

No ive not tried anything yet to be honest. I didnt really have a clue where to start. I thought the thermostat regulated the temperature in the engine, and seeing as how it wasnt overheating (far from it) i thought that the thermostat wouldnt be the issue. Im sure im wrong, but that was the logic lol.

So you think replacing that could be the issue? Thanks both for replying to this!

[S0 What]

I'd say it was the stat personly, when the valve goes the heater stays stuck on hot or cold.

[mattyc69]

You mention you lose the heater when you slow down so I am thinking possible water pump issue. Try leaving the car in neutral and revving it and see if the heater works then. Could be a worn pump that at low revs is just not pushing the water around. The car is not overheating because ambient temp is low.

Another possible is the the thermostat is not closing properly so your engine is never getting up to working temperature because the water is circulating even from cold.

[ch427]

I had a similar problem years ago in a 1993 escort with the same engine. Changed the stat first and then found a small coolant return hose to the header tank blocked.
Once these were done it was better but never 100%, my only guess is the water pump was the issue but i couldnt be bothered to change it as i was selling the car.