Temperature gauge drops when heaters are on

KORNFREEK

MEMBER
2010 FORD FOCUS
1.6L
2WD
MANUAL
47,000 MILES

When I turn my heaters on my temperature gauge falls, sometimes down to the 60° marker. This can happen even on longer journeys where the engine has been working harder or on short journeys. It never rises above 90° or normal range. I want to know if this is normal or if it is something I should be concerned about. If the fans are on cold it does not happen. It only happens when using the heaters.

1 Reply

MOTOR MASTER

MECHANIC
279 POSTS

Hello my name is Dave.

It sounds like you have a faulty thermostat that is preventing the engine from maintaining proper operating temperature. I would recommend replacing the thermostat which on your car it is part of the thermostat housing assembly. I have attached a diagram showing you where it is and how it goes together.
Here are the instructions on replacing it:

Removal and Installation

Note: The thermostat and thermostat housing are serviced as an assembly.

With the vehicle in neutral, position it on a hoist. For additional information, refer to Section 100-02 .
Drain the cooling system. For additional information, refer to Cooling System Draining, Filling and Bleeding in this section.
Remove the accessory drive belt. For additional information, refer to Section 303-05 .
Remove the 2 nuts and position aside the power steering tube.
To install, tighten to 10 Nm (89 lb-in).
Remove the cooling fan motor and shroud. For additional information, refer to Cooling Fan Motor and Shroud in this section.
Remove the 2 bolts, the stud bolt and position aside the A/C compressor.
To install, tighten to 25 Nm (18 lb-ft).
Remove the 3 bolts and reposition the thermostat housing to gain access to the hose clamps.
Remove and discard the gasket.
To install, tighten the bolts to 10 Nm (89 lb-in).
Disconnect the radiator and heater hoses from the thermostat housing.
Note: Lubricate the thermostat housing O-ring with clean engine coolant.

To install, reverse the removal procedure.
Fill and bleed the cooling system. For additional information, refer to Cooling System Draining, Filling and Bleeding in this section.

If you don't have access to a vehicle lift you can use a jack and jack stands, I have also attached a link to our tutorial on flushing and bleeding a cooling system.

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

And here is a link to our tutorial on safely jacking a car up to work on it:

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

Please keep us up to date on your progress and thank you for using 2CarPros!

Image (Click to make bigger)