Why does my car idle rough when I turn on the heater 

Having heat in your car during the winter months is a must. When temperatures reach low numbers it can be dangerous to be stuck in a car. Sometimes using accessories like heat can affect how your engine runs.

Why does my car idle rough when I turn on the heater? This happens when something like a blower motor is drawing too much power from your charging system. It could be an accessory that is bogging your engine down.

When you turn your heat on the blower motor spins and blows the heat towards the inside of the cabin. Your blower motor uses quite a bit of electricity to run. If your blower motor has gone bad and is using too much power then it can make your car idle rough.

On the other hand, your blower motor may be good but your alternator may not be producing an ample amount of electricity. Without enough electricity, your engine will bog done and idle irregularly.

What is a rough idle?

A rough idle refers to your car’s engine when it’s not operating correctly. In normal operation, your engine should maintain a certain RPM when the car is stopped. The engine should run at a constant speed. If it starts revving up and down by itself then that is considered a rough idle.

A rough idle can be due to multiple parts of the engine failing. Although, if it only happens when you turn your heat on then you know it’s probably related to your accessories or charging system.

What can cause a rough idle when turning on the heat?

Blower motor

A blower motor is useful when you use your heat or ac. When the blower motor is operating normally it shouldn’t cause any issues. Although, if your blower is older and going bad it can use too much electricity.

This excessive use of power can put a strain on your charging system. A rough idle can happen when there isn’t enough available electricity.

Bad Alternator

An alternator is what provides electricity to components when the engine is running. As the engine spins your belt the alternator spins and produces electricity. If your alternator is bad it may not be producing enough electricity.

If you turn on something like an interior light or accessory the alternator may not be capable enough to operate the components and it may cause a rough idle. An alternator goes bad over time. When it reaches its lifespan you may start to see things like dimming lights or a twitching speedometer.

When you turn your blower motor on it requires power from the alternator to keep running. If the alternator is failing when you turn on the blower motor then you can have a rough idle.

Running too many accesories

Running too many accessories in your car can cause a rough idle. Things like your radio, heat, amps, and more can cause your engine to have a rough idle. That is because each accessory uses a certain amount of electricity.

If there isn’t enough power available in the charging system your engine could get bogged down and have a rough idle. Especially adding extra things like sound amps, extra speakers, or cell phone chargers can cause an excessive draw of power.

How to fix a rough idle

Find the problem

Why does my car idle rough when I turn on the heater? Well, the first step in answering this question is tracking down the problem. You can test your blower motor to see if it is good. The easiest thing you can do is take your car to an AutoZone and have them test your charging system.

If after your charging system test you find out that your alternator is bad you can assume that there isn’t enough power which is causing your rough idle. After you get a second opinion and find that the alternator is indeed bad you can replace it and see if your problem still persists.

If you just have a rough idle that isn’t related to turning on an accessory like heat then you may want to see if you have any check engine lights. AutoZone is also able to run your computer and download the codes. These codes will tell you if there is a bad component in your car. After you find out what the bad part is you can replace it.

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