Idle RPM question

[JustaDriver]

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?

[theholycow]

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

[suspendedhatch]

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.

[JustaDriver]

OK, thanks for the reassurance!

[VetteOwner]

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.

[ziddey]

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?

[itjstagame]

Originally Posted by ziddey

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?

I would say probably yes. Most IACV are placed right near the intake after any MAF or anything else so they usually can't calculate air being added. They 'know' based on how much signal they give and therefore how 'open' the valve is, but no direct meter I don't think. So depending on how you disabled it, maybe it's assuming it's open and there's excess air and still increasing fuel as if you were running at 800RPM.

You best bet would probably be to get someone to flash the chip to a lower RPM, I think you'll only hurt yourself trying to slowly disable/figure out a working system.

[1993CivicVX]

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.

[theholycow]

Originally Posted by 1993CivicVX

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?

[suspendedhatch]

Originally Posted by theholycow

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?

Actually I explained it already. The VX has a strategy that takes into account VSS and uses the IACV to hold the idle a few RPM higher above a certain speed. Then when your speed drops below a threshold, you'll notice the RPM smoothly drop to 600.

No VX and no D15Z1 was ever mated to an auto from the factory.