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Old 10-07-2008, 05:05 AM   #1
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Idle RPM question

While using P&G (neutral coasting) in my Taurus, I've noticed something odd. That is, the idle RPMs seem to depend on the speed I'm coasting. Coasting at 45-50, RPMs are around 1100 or so, and only drop down to 700-800 when the car is completely stopped.

Is this normal? If not, any idea what is wrong?
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Old 10-07-2008, 05:16 AM   #2
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That is normal for some automatic transmissions. Mine doesn't do that, though it does take some time to settle down to 550 rpm, whereas while parked it goes straight down.

Possible explanations:
- Neutral doesn't perfectly disengage the engine from the driveshaft, due to viscous power transmission through the fluid or bands/clutches not perfectly disengaging
- Computer keeping idle speed up to make sure transmission stays cool (or lubricated -- that's a whole other discussion), since the transmission pump runs off the input shaft
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Old 10-07-2008, 05:52 AM   #3
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Most engine management systems have a strategy that keeps you in High Idle above a certain speed so that when you come to a stop, your RPM's don't drop past idle causing a stall.
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Old 10-07-2008, 01:50 PM   #4
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OK, thanks for the reassurance!
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Old 10-07-2008, 05:13 PM   #5
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yea even some manual trannies do this, its just from the viscus oil in them that even tho its in neutral and not mechanically connected to the driveshaft, the fluid is still spinning in the same directions as if it were.
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Old 10-07-2008, 06:49 PM   #6
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sorry to threadjack a little, but on the topic of idle rpm...

How about lowering idle rpms for fuel economy? Stock, my car is around 850 or so. But I can get it down to 600 and still have it be smooth. However, I have a problem with setting the mixture. I can adjust the idle mix screw, but it seems like the system compensates over time, and I end up having a pig rich idle at low rpms, or it'll eventually have occasional issues with wanting to stall unless I have gas when idling. FWIW, I have a digi2 vw. I've got the idle air boost disabled so it won't add more air when idle drops below 800. Guess the system is trying to compensate with more fuel when rpm is below 800?
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Old 10-19-2008, 03:20 PM   #7
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My car idles between 900 and 1100 rpm while driving on the highway in neutral and about 700 or 800 at lower speeds in neutral. At a stop it idles at 500rpm. Honda Civic VX 5 speed manual.
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Old 10-19-2008, 04:02 PM   #8
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Quote:
Originally Posted by 1993CivicVX View Post
My car idles between 900 and 1100 rpm while driving on the highway in neutral and about 700 or 800 at lower speeds in neutral. At a stop it idles at 500rpm. Honda Civic VX 5 speed manual.
That breaks every explanation I can remember that's been proposed to explain why cars with automatics behave that way...and I can't think of any explanation for it at all. Can anyone else?

Was it originally an automatic, and still tuned as one?

Can you measure throttle position in each of the described conditions?
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Old 10-19-2008, 04:10 PM   #9
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Here's a guess as to why it can be safer to have a higher idle speed while rolling.

We can all agree that this Honda has a really low idle speed at a stop- maybe a little too low if the plugs and wires needed to be replaced...

Perhaps Honda engineers were worried about the engine dying when neutral coasting down a big hill- leaving the driver with only a few pumps of power brakes. So they figured they'd keep the idle bumped up a little the faster the car was rolling to try to avoid it dying and subsequently losing power braking, while coasting in neutral.

This is just a guess on my part- I'm not trying to start a rumor or anything...
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Old 10-19-2008, 04:40 PM   #10
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The hypothesis I came up with while coasting down the hill on the Mass Pike today (btw, go Red Sox!) was that if it stayed at 500 RPM it would jerk considerably more when re-engaging 5th gear as compared if it's at 1100RPM. Most people don't bother to Rev match. But maybe other VX owners can weigh in on this? What do other VXers idle at at 65mph in Neutral? The other question I had was the lowest RPM you can accelerate at. For my car it is 1000 RPM. 900RPM if I accelerate feather footed, but if I give it considerable throttle, then it lugs at 900rpm. I remember someone saying their VX was happy at 800 RPM in 5th gear. My car cannot cruise at this speed--it's not making enough power and it lugs. It can cruise at 950rpm just fine if it's flat and I don't accelerate much. But 1000 seems to be a good starting point for acceleration in my experience.
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