How to over-ride computer RPM while coasting?

[Fat Charlie]

Originally Posted by Wolf_Tm

why do you have to go downhill in Neutral and not gear in?

Engine braking. Unless you want something to slow you down, get out of gear.

[Wolf_Tm]

Originally Posted by Fat Charlie

Engine braking. Unless you want something to slow you down, get out of gear.

So are we talking about downhills you're going to do in Neutral and control the speed with brakes?

[Ecky]

Originally Posted by Wolf_Tm

So are we talking about downhills you're going to do in Neutral and control the speed with brakes?

If you need to use brakes, you might as well coast in gear to control speed, because you can slow down while using zero fuel by coasting in gear. However, any time you can coast, but don't need to use your brakes, you're better off coasting in neutral.

For a shallow hill, I'll coast in neutral down the other side.

For approaching a redlight or a steep hill, where I'll end up significantly over the speed limit, I'll use coasting in gear and take advantage of DFCO and engine braking.

[Fat Charlie]

Originally Posted by Wolf_Tm

So are we talking about downhills you're going to do in Neutral and control the speed with brakes?

No,

Originally Posted by Fat Charlie

Engine braking. Unless you want something to slow you down, get out of gear.

And if you do want something to slow you down, stay in gear to use the engine as a brake. The normal brakes are really your last pick.

[Wolf_Tm]

Originally Posted by Ecky

If you need to use brakes, you might as well coast in gear to control speed, because you can slow down while using zero fuel by coasting in gear. However, any time you can coast, but don't need to use your brakes, you're better off coasting in neutral.

For a shallow hill, I'll coast in neutral down the other side.

For approaching a redlight or a steep hill, where I'll end up significantly over the speed limit, I'll use coasting in gear and take advantage of DFCO and engine braking.

Exactly what I was asking myself, why don't use a long enough gear a use the decel fuel cut.
Ok so you are talking about some very rare (here) situations of a loooong shallow hill.
Thanks

[Fat Charlie]

I live in New Hampshire. We've got a couple of mountains here, and engine braking gives me all I need.

But I don't need it on most downhills.

[csnyder]

Originally Posted by Fat Charlie

Engine braking. Unless you want something to slow you down, get out of gear.

Not only that, there is a very significant fuel savings and the name of this group is "eco modder"

[jaysittinback]

With the TPS showing "closed" the IAC valve is most responsible for high idling, not increased injector output. It would have to get very lean before the O2 sensor would tell the ECU add fuel. I notice other cars not returning to idle. Did anybody check fuel rate or injection while encountering that condition, because I still wonder what all the ECU does for outputs in that condition..?

[hackish]

Originally Posted by jaysittinback

With the TPS showing "closed" the IAC valve is most responsible for high idling, not increased injector output. It would have to get very lean before the O2 sensor would tell the ECU add fuel. I notice other cars not returning to idle. Did anybody check fuel rate or injection while encountering that condition, because I still wonder what all the ECU does for outputs in that condition..?

The majority of new cars have electronic throttle control (ETC). This means there is no IACV so when the accelerator position sensor (APS) reads full off the throttle controls idle. Some cars control the throttle position based on the bottom 5% or so of APS travel. So if you hit decel fuel cut (DFCO) but are still on the pedal a bit it opens the throttle a bit more and reduces engine braking while using no fuel.

Modern PCMs are not perfect but manufacturers have done a _LOT_ to improve fuel economy in the last 5 years. Many of the ideas I see floating around here are based on incomplete understanding and ultimately result in significantly increased emissions.

As for the quoted post above, fuel follows torque. So if you are at high idle the ECU must request more torque to achieve the higher idle speed. More RPM also means more injections per minute. More fuel used. It will not go significantly lean before the closed loop control (O2 sensor) triggers addition of fuel. This is necessary as a lean mixture is going to put like 5-10x more pollution out the pipe.

The days of sluggish slow responding O2 control are gone in the late 1990's and they would never pass modern emissions standards.

-Michael