Hi, I noticed that my fiero (2.8, 5-speed) idles at about 1500 RPM when I am coasting in neutral (which I often do down hills or approaching red lights). When I come to a complete stop, it settles down around 900 RPM. This is when it is warmed up. Other than that, I think it is operating normally. It always idles at about 1500 until it is all warmed up.

Is this normal? How does my car know it is still rolling?

I did use the search, and there are millions of idle-related posts but haven't seen this particular issue discussed. Also, I am pretty new at this so please forgive my ignorance.

My GT (automatic) will idle (in park or neutral) at 1500 until warmed up - once warm, the idle drops to 950.

It's normal. The ECM would throw a code if it was more than 50RPM off the correct level.

Yeah, it will always be higher until it's warmed up. If it reads an idle of 1100+, you will get an error code from the computer.

Yes even warmed up they will do that some 1500 sounds a little high, mine probably does around 1100 when I coast...
I would not worry about it as long as it idles down ok.

Id have to say normal also. Not sure what causes it to think long and hard before it comes down to 900? But it doesn't seem to drop down until your almost completly stopped.

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1986 black gt fast back 4 spd recently rescued. 1987 gt fastback 5 speed! Hopefully swapped before end of summer.

quote Originally posted by ozzy83: Maybe the ecm reads the speedo or a sensor on the tranny? To decide its still rolling and will not idle all the way down?.

I think you're right.

My V6 5 speed acts the same way. I think as long as it knows you're moving it will hold the throttle above idle. As soon as I stop is goes back down to normal idle.

[This message has been edited by fyrebird68 (edited 03-03-2010).]

My guess would be the O2 would prompt a fuel delivery, if you switched the engine from a load condition to idle, to prevent it from leaning out.

Thanks for all your input, guys. I think I am not going to worry about it. Agreed that possibly it is by design. I was just curious if anyone else noticed the high idle when coasting.

My Intrepid also does the same thing. As long as I'm rolling it will sit at 1K and drop to 500 when I stop.

As said, it probably to anticipate that your going to speed back up and they don't want it to go lean.

It's called "shift anticipation" and it's normal. The ECM sees you are moving and assumes a shift is pending and will float the RPM to make the shift smoother. If they let the RPM drop to idle (900) then the syncros have to do more work and the shift is more effort.

It's also "stall saver." If you snap the throttle closed really fast the IAC can't always back out fast enough to allow enough air in to keep the engine from dying.

To assist that they added in "throttle follower" - as you open the throttle the IAC moves in and as you reduce the throttle the IAC moves out to try and stay in a rough position to take over the idle air flow if you snap the throttle closed. There is a bit of code in the ECM that tries to keep the air coming in via the idle passage (IAC) as low as possible (so the throttle is the majority of the air flow) but still be in that rough position to take over if the throttle plate snaps closed. The IAC motion isn't extremely fast but your foot is.

So together they can assist the shifts and anticipate changes in the throttle position.

When you roll to a stop the VSS goes to zero and the ECM knows to let the RPM drop. Actually, I think the ECM is set to let it drop at around 10 MPH (I don't recall exactly). If you let it slowly roll to a stop, at some slow speed you should see the idle drop to 900 before you actually come to a stop.

[This message has been edited by TK (edited 03-03-2010).]