Idle speed increase if heater on

I have noticed that the engine idles faster when the engine is cold and when outside temp is around 10 or 15 degrees F. If I turn off the heat the idle slows down to normal. If I turn the heat back on, the idle speed goes back up to a few hundred revs higher than normal. Once the engine is warm and the temp gauge is normal the engine idle speed no longer increases when I turn on the heater. I don't know at exactly what outside air temp this occurs, I am estimating when I say around 10 or 15 degrees F. This is a 2020 SR5 and it has done this since new. Anyone else notice this?

 

That sounds....normal. The engine is working harder to warm up, thus the higher idle. Then when it gets up to normal temp, the idle goes back down.

 

If by "off" you mean the entire HVAC system, then it could just be the AC compressor affect?

 

This. If the heater setting is on any of the options that involve the defrost, the A/C compressor will kick on, raising the rpms.

 

I've never heard of this...I do know the engine idle is higher at start, but eventually goes down. It does not go up or down when you turn the heater on or off.

 

No AC, no defrost. Just straight heater only. Turning on the heater does not raise rpm once the engine is warm (radiator temp normal op temp). This was in the OP.

 

Have you ever tried turning on the heater when the engine is cold and it is 10 degrees outside? I know it will high idle briefly after startup, this is not what I am talking about.

 

Only when A/C compressor turns with the heat-which is most of the time. Turn heat on, hit A/C button to turn compressor off.

 

I tested your theory today when I started my 4R to go to work (the temp, however, was not in the teens, but the twenties). No change to the idle when I turned on the heater. Your situation is a mystery, it seems. I have never heard of this happening.

 

Does the A/C light on the control knob on your 4R turn on when you turn on the defrost?

 

I have auto climate-no knob.

 

Got it. Mine is an Off Road.

 

Ok. 20's is too warm, try when it is 0 or 10. The change in revs is subtle so it would be easy to miss, only 200 rpm or so. Without thinking too hard I assumed the car was attempting to heat up more quickly if the heater is on. Once engine is warm turning on heater does not affect revs. Strange.

 

Why wouldn’t an engine have to idle higher when you are putting more demand on it by turning on the heat?

 

Speaking from a manual climate control standpoint - I do believe there's an interior temperature sensor that will automatically turn on recirculate when you turn on the A/C and the interior is warm enough. This functions even if "Max" isn't chosen on the temp dial. It's possible that when the same sensor picks up a cold enough interior and heat is turned on, the engine will idle higher to pressurize coolant so it will warm faster.

When adding heat to the interior a valve will open and allow coolant to flow through the heater core. Heat is leeched from the heater core to warm the interior. Very simple principle with maybe the exception of my theory above.

 

I would LOVE an explanation as to how the engine is "working harder to warm up" and how that happens.

 

I love you man. Never stop being you.

 

Turning on the fan?

And we also drive a Rav4 Prime, and when turning on the heat, it uses more battery or kicks on the engine.

 

We're talking about heat, no? Heat is literally leeched from hot engine coolant to warm the interior. This does not add electrical load even if we try to account for what the fan draws.

Your RAV4 primarily heats the interior with an electrical element since engine temperatures can vary in a plug-in hybrid.

 

If the fan load is causing a RPM dip, battery likely is toast. We need a video of this.

 

So no explanation for that ridiculous post - right?

 

I believe we are talking about heating the interior of the vehicle yes. And to do so, the fan, which requires energy, is turned on to move the warm air into the vehicle from the heating core, or what ever it’s called, that is part of the cooling system. So that needed energy comes from some where.

 

yes the fan or blower motor does require electricity to run but a 4Runner isn’t adjusting idle for a motor that draws such little amperage.

 

But there is an increase in the electric load required. And where does this energy come from? It’s either stored somewhere, such as the battery, or it’s coming from a producing source, such as an alternator. I would think that if any of those elements aren’t doing their job, then the 4Runner will act differently. I’m thinking it’s a very simple principle that energy has to come from somewhere.

 

It does-but not enough to cause RPM dip unless charging system is not working properly. The blower fan and blend door actuators take little to run.

 

I edited to say if one isn’t doing its job. Sorry for the typo. ADHD agents me all the time.

 

I havent seen you mention if you have auto climate control or manual hvac. If it’s auto climate, then the
system will use AC whenever it’s logic tells it it’s necessary. Asking for heat when it’s cold, im sure it’s programmed to use the AC along with heat to maintain inside air dry to prevent fogging interior. Ac kicks on, rpms rise. It may turn off when the interior temp rises, if that’s what it’s logic says to do.

If you have manual hvac, and rpm rises, and the AC light is off, I’d suspect what has already been mentioned…bad battery or charging system. A charging phone on the USB would probably draw similar to the hvac fan, so I’d check if charging your phone causes an increase in rpm as well.

But honestly, it kind of sounds like what an auto climate control system would do.

FYI some cars, even with manual hvac, don’t tell you when the AC is on. We have an old Mazda tribute with physically connected manual controls that automatically turns on with the defrost setting. Ac light doesnt light up, even though it has a light-up button for AC. I think the AC light on the manual hvac 4runners is honest though, based on seeing the rpm drop when its on defrost and I turn off the AC.

 

I don't know if I have manual HVAC or auto, it's a 2020 SR5. Does the model and year help determine which system I have?

 

To put it in very rudimentary words, whenever the HVAC system gets engaged, either by input or on it's own, the ecu will increase the rpm a bit to prevent stalling, compensating for extra load. Higher rpm means engine is working harder.
Just like when you turn your steering wheel, the rpm increases, again to compensate for extra load.
Now if you were looking for a very scientific explanation to this, I am sure Google will provide you one.

 

Do you set a temperature number like you would at home? If so, that is auto.

If it works like you’re back in the 1980s with a blue/red dial and another dial for the fan, that is manual.