Just how important is idle speed

[steppinthrax]

I was talking to a couple of Nissan owners. A lot of the older nissans and some newer cars on the road allows for the adjustment of idle speed. On the nissans you adjust idle speed by adjusting the screw on the IAC (idle air control). The OEM specs are for 700 rpms on idle and no load on the engine in neutral. Most of them said they use their in car tach to do the adjustment and don't bother making it acurate. The in car tach increments are at 500 rpm and you have to kind of guess where 700 is. Some even say as long as it's below 1000 rpm you should be fine. A lot of them adjust it a little high just because it feels comfortable (ride).

I imagine if the idle speed is higher then normal you would waste gas. I also think if you have to adjust idle speed does that mean some componet on your car is out of wack and your essentially compensating for it (i.e. intake leak). Any thoughts on this?

 

[Tarrant64]

I can't see how a minor adjustment to idle speed would waste enough gas to be noticable.

Why people can't just leave their cars at their default settings and not mess with it. I suggest leaving it as bc chances are there are a lot of rumors on big differences between 500rpm, 700rpm, and 800rpm at idle. The only time I've EVER been worried about my idle is when it varies(drops down to 100 and back up again to whatever, or even to zero almost stalling). Then I start worrying about other things.

 

[imported_Truenofan]

well the thing they're talking about is that it isn't automatic, its the same on my 7, has a little screw to adjust idle speed. different seasons, different density air, different idle speed. think of those cars with fuel injection but still require carburetter kind of adjustments. it has to be changed depending on season, i set mine below 800ish and its fine, sometimes i have to slightly engage the clutch to drop the idle lower than 1000 for some reason, it likes to stick up there even if i have it adjusted really low. other than that it really doesn't matter honestly as long as its running, it wont use up that much more fuel if it noticeable at all.

 

[Zenmervolt]

Originally posted by: steppinthrax
I imagine if the idle speed is higher than normal you would waste gas. I also think if you have to adjust idle speed that means some component on your car is out of whack and you're essentially compensating for it (e.g. intake leak). Any thoughts on this?

The difference between a 700 RPM idle and an 800 RPM (or even 1,000 RPM) idle isn't going to waste enough gasoline to be noticeable. Also, idle speed is not given, ever, as a set number. It's typically listed as something like "700 RPM +/- 50 RPM" because it's not terribly important for it to be a specific number.

On newer cars, you're right. You should really never have to adjust the idle. On older cars, however, it was/is fairly common to simply have the idle set screw vibrate out of adjustment over time. It used to be one of the things that was commonly checked and adjusted with an oil change.

ZV

 

[Zenmervolt]

Originally posted by: Truenofan
well the thing they're talking about is that it isn't automatic, its the same on my 7, has a little screw to adjust idle speed. different seasons, different density air, different idle speed. think of those cars with fuel injection but still require carburetter kind of adjustments. it has to be changed depending on season, i set mine below 800ish and its fine, sometimes i have to slightly engage the clutch to drop the idle lower than 1000 for some reason, it likes to stick up there even if i have it adjusted really low. other than that it really doesn't matter honestly as long as its running, it wont use up that much more fuel if it noticeable at all.

Any electronic fuel injection system should maintain a stable idle regardless of temperature without you having to make compensatory adjustments to the idle speed set screw. If you are constantly having to adjust the idle set screw to accommodate a change in seasons, you have a fault in your EFI system that needs to be repaired.

ZV