Why does my truck overheat when I turn the heater on?

I own a 1989 ford f150 standard cab 2wd 4.9L 5-spd manual. When ever I turn my heater on (winter) and it starts blowing hot air, my temperature gauge starts moving towards hot. I find this very strange because I'm pretty sure it's supposed to take heat away from the engine.

Hmm usually the heater helps cool it down that's strange I'll research it and let you know if I find anything

Actually, the heater pulls its heat from the Heater Core, which is what hot coolant (antifreeze) goes through. I suppose it can remove some of the heat, but definitely not all. I would start by checking your coolant level, it could be that it's too low and when it starts pulling through the heater core, it causes less to cool down the engine.

Hi so get it checked out by a Ford dealer.

The most likely culprit is that you are somewhat low on your coolant level. Let me explain:

When your heater is turned off the coolant "bypasses" the heater core and does not circulate into the core. This is so that unecessary heat is not added into the interior when the heater control is switched to the cold setting.

Now, when you turn the heater control up to the red or hot area… The hot water is then allowed to circulated into the heater core. The heater core and piping does consume a small bit of coolant and if the overall level is just a bit low this can create a level that is low enough to create an air pocket in the system. The air pocket will typically lodge at the thermostat as this is a "high point" in the system. In turn the thermostat is designed for liquid contact and does not properly respond to air contact. As a result the air pocket will prevent the thermostat from opening and thus your engine temperature will rise.

I have seen this exact same symptom many times and so far each and every time I have seen an engine temp go up when the heater is used it has been because of a slightly low coolant level. Start off by checking your coolant level.

Keep in mind that your radiator has a coolant reservoir with a hose that connects near the radiator cap. Some refer to this as an "expansion tank". Make sure that your expansion tank is okay and properly hooked up. As the coolant expands in the system the excess is "pushed" or migrated out to the expansion tank and held there for the time being. Later on when the system cools and the coolant retracts… Stored coolant is then pulled from the expansion tank back into the radiator.

You know how you can boil water in a pan and it will increase the level until if floods over? Same concept here and so this expansion tank holds the water which expanded beyond capacity and then returns it back as the system cools back down and retracts. If your expansion tank is not hooked up or leaking then the expanded coolant is pushed to the tank and "lost" and can't be returned back. This in turn can lead to a slightly low coolant level.

So

1. Check your coolant level
2. Check out your expansion tank and connecting hose

3. NOTE: DO NOT attempt to open the radiator cap when the system is hot as hot and pressured liquid may spew out and harm you! If you are in doubt as to there being pressure in the system you can "squeeze" the upper radiator hose. If the rubber upper radiator hose will squeeze very easy then most likely there's no substantial pressure in the system. IF the upper radiator hose is HARD to squeeze or seems firm DO NOT open the radiator cap until the engine cools off!

If your coolant level is a bit low and the expansion tank looks good I would recommend doing a "pressure" test to determine if or where there might be a coolant leak somewhere in the system.

It's not overheating because your heater is on.