Idle speed increase if heater on

Yes when in drive....enough to "tug" me if not paying attention to my brakes. And yes once it is warmed up too.

I'll try to pay some attention to it over the next few days and confirm.

If you have ever felt a vacuum leak on a older car/truck then that is exactly what it feels like. A sudden tiny rise in R's that tries to push you forward.

 

Mine quits after the engine/coolant temp is at normal operating temp. By the time I get to work it no longer hi-idles. My commute is about 12 miles. When I first got the truck my commute was only about 3 miles and same story: it quit hi-idling by the time I got to work. I always warm up the rig for a few minutes before driving so it had a head start warming up on that short 3 mile commute. I am assuming it quits hi-idling because of engine temp; however, I do not know this is the reason, might be something else going on.

 

IMO this is not about normal high idling on a cold engine. This is about 300 RPM surges during the entire day of city driving IF the heater fan is on.

Obviously not going to feel it outside of city driving.

 

When I say cold I mean anything under normal operating temp, it can take a while to get there. I've not had this happen all day. I will take more notes when it happens again, it has not been very cold so this has only happened one time this fall and I run the heater every morning.

 

Make a short video, maybe we can learn something new.

 

Not sure if serious. When it's real cold and engine is not up to temp, I turn on heater and revs increase by about 200. You don't need a video to imagine this.

 

Yes take a video, then folks can try to interpret themselves, someone might understand it better with the video.

Once I start in the morning, it might high idle for about 10 seconds, then clearly the rpms drop, and then I drive away. The engine or heat is not warm yet, that takes about 1.5 miles. I’ve let vehicles “warm” up for a few minutes, and I still get heat just about 1.5 miles from my drive way.

 

My 4runner did this again this am, it was 14 degrees F. It is difficult to notice because the truck normally hi-idles for a bit when cold. What I did: start and let run for about 5 minutes or until the idle settles down to about 900 or 1k, temp gauge has just barely moved at this point. Turn on heater, no ac. Idle increases about 200 revs. Turn heater off and revs return to 900 or 1000. I tried again as the temp gauge moved a little and same thing. Once I arrived at work the temp was at normal operating temp and turning the heater off and on had no effect in revs.

What makes this difficult to notice is the truck normally high-idles when cold so if you just start up, turn on heater and go you would never notice.

 

Here's my two cents, probably not worth much more than that. The purpose of the blower is to transfer heat from the engine to the cabin, so turning it on can marginally reduce the engine temp and pull up the idle speed a bit until the engine is in its happy place. The temp gauge isn't a precise enough instrument to reveal such changes that can depend on threshold effects in a fairly tight range of degrees.

 

The engine speed increases immediately when the heater is turned on, there's no time for the coolant temp to drop and the engine react to that. If the truck is attempting to keep the engine warm it is doing it via some instant logic switch. Maybe: IF heater on AND outside temp less that 15 AND engine not up to temp THEN increase engine idle revs when heater is turned on. This could be the truck attempting to produce heat for the cabin more quickly or attempting to get the engine hot for the sake of the engine. Dunno. That it stops doing this after the engine/coolant is up to temp tells me it is probably supposed to be doing it.

Anyone else try it?

 

I got a chance to try this today as we finally got a cold snap. I have an Off-Road trim with the manual hvac.

Outside temp was -2 degrees F. Had an OBD reader on that gave me radiator coolant temp. A minute or two after starting (from dead cold), it went from high-idle to low idle. Once it went into low idle, I started drive to try to replicate this.

At every stop sign, I tested for the rpm increase while stopped and in Drive. I noticed the RPMs went up two hash marks on the gauge anytime I turned on the hvac fan with the AC off. It didn’t matter if the temp dial was all the way hot, or all the way cold (last notch before AC automatically kicks on). The only thing necessary for the RPM to increase was to turn the fan speed from off to the first “on” setting.

Once I reached a coolant temp somewhere between 160-170, turning on the fan no longer caused an RPM increase.

This seems to be part of the logic of the system.

 

Thanks for the post, I am going to try this. I thought the temp control dial had to be set to heat for this to happen. I don't remember if I have.tested it by turning on the fan while on the cold setting.

 

Related ish: Do they make a product where you can adjust the idling rpm? I would like to be able to leave it at 1k in the winter so it warms up good

 

In would have figured the temp dial would need to be in heat range as well. But apparently the logic doesn’t need to be that complex.

It actually wouldn’t surprise me if the only two conditions to raise the rpm are coolant temp and fan on/off, regardless of outside temp.

 

You are right, turning on the fan raises the engine revs regardless of the heat setting, I tried this yesterday when it was about 5 degrees F outside temp. I question the second part of your reply here since this does not seem to happen when it's above about 20 degrees F. I don't know the exact temp threshold, I have seen this happen at 14 degrees F and lower.