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Hello everyone, I am wondering what RPM your Civics idle at when you are coasting. Mine idles perfectly at ~800 RPM while sitting still regardless of loads, but when I am rolling it idles at 1K-1200 RPM and when I turn the AC on the idle increases to 1600-1800 RPM and I am wondering if this is normal. The high idle while coasting with the AC on has been driving me crazy and after checking everything and still having this issue, I am starting to wonder if that's just normal and there's nothing I can do about it. Thank you in advance!

 
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No one knows what RPM your car idles at when you are rolling in neutral or with the clutch in?

 
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vacuum leaks

 
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Good suggestion but it does not have vacuum leaks, I already checked several times including the brake booster and PCV valve. What's your idle speed while coasting? I have read that on some cars a slightly higher idle while rolling is normal and intentional.

 
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i keep it in gear, its safer and better gas mileaga

 
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It is. Like 1k rpm is normal.

This. Whenever I slow down to a stop I stay in the top gear until like 1k-1.5k rpm then push the clutch in. Unless I'm downshifting but I don't always do that.

 
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Cool, thank you for an actual answer to my question! I wonder why the ECU is programmed to increase the idle while rolling. Either way I don't like it but with a stock ECU there's not much I can do about it.

That's what I do too when I have to stop. As a hypermiler my preferred method of slowing down is to shift to neutral well in advance and let it coast down, but when there is too much traffic for that to be practical I either stay in gear until I feel fuel delivery resume (1200 RPM or so) then push the clutch or downshift through the gears.

Whether coasting or engine braking is more efficient certainly depends on the situation. Staying in gear is preferred when slowing down is the goal since the engine uses no fuel at all when coasting in gear above idle and the engine braking helps you slow down, but the drag from rotating the unpowered engine slows the vehicle down much faster than if in neutral so you can't coast as far in gear or pick up as much speed down hills. I get my best mileage using neutral whenever possible and only engine braking when I need to slow down. My MPG records suggest that covering a longer distance in neutral for almost free (idle fuel consumption) is far better than covering a much shorter distance for free (decel fuel cutoff), but your mileage may vary.

As for safety, I would argue that being in 5th gear isn't a whole lot better than being in neutral if the driver needs to quickly accelerate. Obviously when travelling down large hills brake fade is more likely without engine braking, but only an idiot would coast in neutral just to ride the brakes.

 
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That is absolutely correct. But as I said, the drag created by spinning the unpowered engine slows the car down significantly just like using the brakes. That's why it's called engine braking. That's great when you want to slow down, but not when your goal is to keep rolling.

One example of how this is useful is when slowing down for turns. When no one is behind me if I shift to neutral early, I can coast for, say, 1/2 a mile while getting, say, 200 MPG and be at the desired speed by the time I get to the corner without needing to brake, engine braking or friction brakes. But if I stay in gear, I need to stay on the gas much longer before I can let off since I can't coast nearly as far and then all that energy gets converted to heat spinning the engine over.

Another way I save fuel by coasting is picking up speed going down hills to get a head start climbing the next hill without having to use the throttle. Let's use one hill by my house as an example. If I'm at 40 MPH at the top and I stay in 5th gear on the way down, I maintain my speed and I'm down to 35 MPH by the time I get to the next hill due to engine braking. But if I coast in neutral, I pick up speed and get to the next hill at 50 MPH and can coast about half way up with NO throttle at all before getting down to the same 35 MPH.

But whatever the reason, it works for me. I stay in gear my entire drive, I can get 40-43 MPG over the whole tank. However, if I coast whenever possible rather than staying in gear and engine braking, I can get 46-49 MPG. Feel free to do your own testing and report back what you find. If done properly, I think the results may surprise you.

 
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Mostly for power steering I think. Better pump performance with the engine speed slightly higher. Alternator load as well, especially when there are high electrical loads.

If you're constantly shifting to neutral to coast for turns you'll be increasing wear on the clutch and probably synchros as well.

 
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That makes sense. I really wish I could get rid of the high idle mode somehow since I don't even have a power steering pump anymore (EPS). I would be curious to find out if Hondas with factory EPS like the EP3 and 8th gen SI idle high while coasting too. My friend's Acura ILX drops down to normal idle while coasting, but it's 12 years newer and automatic.

That's a good point and certainly something to consider. I always blip the throttle before clutching and going back into gear to reduce synchro and clutch wear and I got pretty good at rev matching (usually within 50-100 RPM or so) so I wouldn't think I'm causing that much extra wear. And if I have to replace the clutch a tad sooner that's not a big deal to me.

 
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It’s also because it’s drive-by-wire. They have much better idle control than transitional cable drive. It’s such an awful system though. It lags terribly (at least with the manual but I’ve heard equal complaints with the auto)

 
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On hydraulic there is a PSP switch (power steering pressure). 0V when the wheel is straight, 12V at full lock. I think on the EPS rack it's called PSW but does a similar thing. Tells the engine when the steering is loaded which increases idle. Even if you disable this I'm not sure that will lower the idle.

 
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That's true but I don't think that's the full reason. Clearly my car is capable of idling at ~750 RPM while rolling since it does it when it's stationary, my ECU is apparently programmed to hold the idle higher while rolling. I have managed to reduce the rolling idle by adjusting the IACV closed more, the idle was around 950 or so while coasting for my entire trip to work this morning instead of 1K-1400. Hopefully that lasts. Although I would still prefer for the idle to drop all the way down while coasting, at least it runs smoothly at 950 ish RPM. At around 1K-1100 RPM like where it liked to hang before the idle is annoyingly rough.