Here's the dilemna...

[BHarvey]

My car fully warmed at idle produces 650 rpm while stopped.
Coasting on the highway gives 830 rpm or so.

If I am sitting still the idle is 650 but even starting to roll in neutral in will bump up to 1000-1100. Or if I am at lower speeds from 10-40 it stay at 1000-1100 from a while before coming down to the 825 idle.

The highway rpm drop is decent and acceptable but this weird in town hangin at 1100 rpm is unacceptable.

The reason is that at idle speed of 650 it shows .19 gph and at 825 it gives .21 gph. When it does this idle up crap I am showing .3 gph or 50% less fuel economy....

What can be done about this?

[Piwoslaw]

My car does something similar: idle around 820 rpm when coasting, which drops to 740 rpm at standstill. I forgot what the consumption is then, but I'd also love to lower idle speed. I'm willing to bet that it is controlled by algorithms in the ECU, so changing it may not be easy, if at all possible.

[BHarvey]

That's what I am afraid of. This alone would probably put me over 40 mpg.
I'm on a record tank now according to the UG as it is reading 39.7 mpg.
It's complete BS, the car is already running with whatever acc's you have running and it thinks you need more idle speed....

[Piwoslaw]

I remember reading here somewhere that certain cars idle around 1000-12000rpm when rolling in neutral "to be ready" to accelerate. My guess is that may be the engineers decided that the only time a car is rolling in neutral is between gear changes, so it may be better to not allow the engine's rpm to fall too low since in a moment it will be in gear and will have to rev up to 2000 (or more) again.
Pretty dumb, so maybe there's a better explanation.

[cfg83]

BHarvey -

This is similar to what Piwoslaw said ...

I think that's the Idle Air Control valve (IAC) in action. It's a pintle valve that is controlled by the ECU/PCM to increase/decrease the idle as needed. I like to think of it as a computer controlled choke. When the engine is cold the IAC is used to raise the RPM until the engine warms up :

http://www.2carpros.com/articles/how...ol-valve-works

An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The cars computer or ECM (electronic control module) receives information from various sensors and will output signals to adjust the IAC motor in or out to adjust engine idle speed by controlling engine idle air. ...

At high MPH, the ECU/PCM decides to keep the RPM higher in preparation to rev-match with the next gear. At least that's what I think Piwoslaw is describing.

A proposed solution to the IAC is to intercept the IAC control wiring. The idea is to disable the IAC with a switch at driver discretion. You don't want to fully disable the IAC because a really cold engine is hard to drive. However, some people have tried this. They wait until they have a nice low idle and disconnect the IAC. From what I understand this should leave the IAC in it's current position.

You will find the IAC built into your throttle assembly.

Can someone chime in and correct me if/where I am in error?

CarloSW2

[Piwoslaw]

Cfg83, that should work on a gasser. Unfortunately, diesels lack a throttle, so engine speed is controlled by fuel only. In newer cars this means that the ECU has a few different sets of injector signals for idling, depending on a few parameters, like speed, coolant, intake air, and fuel temperatures, etc. In theory it may be possible to fake certain sensor signals, but that wouldn't be easy. I believe that Euromodder had his idle speed adjusted through the ECU's software.