Question about Engine Braking

[TorqueEffect]

Just a question about engine braking on a Festy.

I always assumed the faster the engine was spinning, the more gas it was using so I always put the car in neutral when coasting down a steep hill. But after doing more research, I find that is not the case.

Now my question is this, is the B3 ECU sophisticated enough to cut fuel when the gas pedal is released but the car is still in gear? Or does it just drop to idle fuel consumption?

Now I have though I had heard something about it doing that in 5th gear on here as a part of slight engine brake for slowing down on the highway without using brakes, but that obviously won't cut the fuel when I engine brake by downshifting to 3rd.

[rmoltis]

So on my scanguage when I am coasting in gear.
The voltage reading from my o2 sensor drops to 0.
Which leads me to believe the fuel is cut off during decelleration.

But the real time mpg increases after I let it out of gear compared to coasting in gear.
This presumably is because coasting in gear requires enery to keep the drivetrain rotating
& higher resistance on the piston drawing intake air in from high vacuum of throttle plate being closed
Meaning your vehicle may not coast as far in gear compared to out of gear.

But as far as the 0 voltage o2 sensor readings in gear.
I'm assuming it cuts the fuel.
Because at idle I get the voltage fluctuations from lean to rich like o2 sensors do.
And its anywhere from .06-.95

But I do drive an aspire not a festy.

[TorqueEffect]

But I do drive an aspire not a festy.

What year Aspire? OBD1, or OBD2?
Because I am sure the OBD2 is a tad more sophisticated.

[rmoltis]

It's obdII.

So the way for you to find out would be to get o2 value readings while your idling.
Compared to when your coasting in gear/ and coasting out of gear.

But as with most vehicles.
The vacuum pressure is one of the main sources the engine uses to determine fuel needed

When the vacuum is highest the engine requires less fuel.
When the vacuum is lowest usually the throttle is wide open. Requiring more fuel.

And when engine braking.
No matter what rpm or gear you are in.
If the throttle plate is fully closed
It creates high vacuum.
Meaning it will use the least amount of fuel.

And say your in 3rd gear high rpms off throttle coasting
The vacuum pressure will be much higher than in low 5th gear.
due to the higher speed the piston is travelling.

So even if your car does use fuel it would be very minimal.
Due to the higher than normal vacuum pressure.
And I'm assuming that there is fuel cut built into most cars when the manifold vacuum reaches a certain point.
It's usually there for deceleration

[TorqueEffect]

Or I could just get a aftermarket vacuum gauge, and if the the vacuum spikes over what the vacuum would be at idle, then I can only assume the injectors are shut off when coasting in gear.

Since basically what I have read is, the more vacuum there is, the less fuel that is being used.

Yeah, I figured I would get a vacuum gauge to help me save some fuel anyway, plus I am a geek when it comes to seeing what the engine is doing.

[rmoltis]

Yes that is usually the case when it comes to manifold vacuum.
Also your fuel pressure regulator runs only off of manifold vacuum.
At high vacuum it supplies least fuel.
And at 0 vacuum it delivers most fuel.

For example when I had the vacuum gauge connected the other day.
My vehicle would idle at 20"hg.
And when I would open the throttle the vacuum would drop and the rpms would rise.
But as I closed throttle plate the vacuum rose above 24-26"hg during slowdown.
To return to 20"hg when back at idle speed.

I believe engines cut fuel during decelleration.
Because if fuel was present.
The engine wouldnt slow down.
It would continue rotating.