Lets say I drive at 55 mph at 1500 rpm for one hour on a level grade.

Now lets say Im driving down a grade for an hour in 1st gear at 3000 rpm and my foot is not on the accelerator. Im just letting the engine do the braking to slow the truck down and it runs at 3000 rpm in 1st gear.

Which scenario consumes the least amount of fuel?

 

The second one...thats one long freakin hill

 

So whats going on then, is it running lean?

I've had this question in my head a few times too.

 

I would say no its not running lean. Gravity pushing you down the hill becomes the power source. If your engine was coupled via a manual transmission you could go down the same hill with the engine off,clutch released, transmission in a specific gear and still run at approx. 3000 rpm.

 

At 3000 rpm, going up or down a hill, it would seem the same amount of air passes through the motor to me. So what happens with the gas?

 

OK, let me be more specific. It is an automatic transmisson and the engine is on in both cases.

 

i'm a bit confused

 

Thats what I want to know. I don't have my foot on the gas going down the hill but the slope of the hill causes the engine to increase in rpm so are the fuel injectors still injecting fuel?

 

The injectors are turned off. You are commanding idle with your foot off the accelerator. The PCM sees this and closes off the injectors. The potential energy that is stored up in the truck becasue it is up on top of the hill is transformed to kinetic energy when you are going down the hill. That kinetic energy is what is turning the engine.

Now answer me this; How did you get up the hill in the first place. That is where you used more gas than the level surface travel. Nothing is free.

JMC

 

NO. The same ammount doesn't pass through the motor. It's not necessairly running lean because the throttle body is closed, so it's not getting as much air.

 

So the pistons keep increasing the vacuum at 3000 rpm and the throttle body is strong enough to survive it?

 

I assume a vacuum forms in the cylinders and that no/very little air is passing by the oxygen sensors and thru the cats out the exhaust then?

 

A complete vacuum isn't but -14 psi. (I think) That's a complete vacuum.

 

Your burning twice as much fuel during a 1500rpm excel vrs. a 3000rpm decel.

It seems that way to me anyway.

 

Injector pulse is based on TPS readings, so they are not open because of rpm.