Startup Idle Speed (?)

On my 2008 RAV4, and many of the other newer vehicles I've driven, there is some sort of auto-rev when starting the engine that seems like a waste of fuel to me. Am I missing something?

Reply to
croy
Loading thread data ...

Yes.

Reply to
Jeff Strickland

I read somewhere it's because of emission requirements, it heats the catalytic converter up quicker. google it and let us know what you find.

Reply to
Fatter Than Ever Moe

What did the dealer say when you asked about this?

Reply to
JoeSpareBedroom

That means Joe has no clue...

Reply to
Jeff Strickland

Nor do you, based on your response to the OP. However, I know exactly why some vehicles are set up to behave the way he described.

What I want to know is what he heard from the dealer. You have no problem with that, but you will pretend that you do.

Reply to
JoeSpareBedroom

THEN OFFER THE GUY SOME EXPLANATION, ASSHOLE!

Jesus H. Christ, Joe!

Nothing changes by knowing that the dealership says the same thing as you, or that he hasn't even asked them.

You have absolutely no intention of saying anything of any value to the OP. Nothing. You know nothing. You say anything.

Reply to
Jeff Strickland

Yes, you are missing something.

Starting a cold engine requires a bit of choke to hold back air and push in gas. Or, push more gas and the same air.

Fuel Injection uses a system where the fuel pump delivers a constant pressure, even when only a fraction of that is needed. Any extra fuel is sent back to the gas tank by way of the Fuel Pressure Regulator. The regulator works by vacuum, high vacuum at idle sends more fuel back to the tank, low vacuum closes the regulator so the fuel is delivered to the injectors.

What you see on start up is the result of this, and it's desireable. When the engine is cold, and there's little vacum, there is extra gas delivered to the injectors than is actually needed. The engine starts, races a little, the vacuum kicks in to close the regulator, the idle speed drops, and life in the universe returns to normal.

If you are worried about wasted gasoline, go put air in your tires and let up on the gas pedal a little. The two biggest factors in wasted gas are low inflation pressures in tires, and a heavy gas foot. Wasted gas happens more on the sprints from 0 to 60 in 4 seconds than cruising at 80 for an hour. When you mash the gas to start, slam the brake to stop, you use lots of gas. Arguably (the argument I am making) more than the difference in gas needed to cruise at 80 instead of 70, or 60. Proof of this is that you get better gas mileage at a steady constant speed -- no matter what the speed -- than you get from mashing the pedals to change from stop to go, and vice versa.

So, cruise at 80 if you please, but take your time transitioning from 0 to whatever, and spend more time and distance in the transition from whatever back to 0. And, pit some air in your tires.

Reply to
Jeff Strickland

Why didn't YOU offer the guy some explanation in your initial response?

Reply to
JoeSpareBedroom

Umm... you are comparing fuel pressure to the air as a normal gauge would do, AKA PSIG.

The injectors don't fire into the atmosphere however, they fire into the intake manifold so if you hooked a differential pressure gauge to the fuel rail and the intake vacuum you would see the pressure remain about the same. There are aftermarket FPRs that have a slope to them however where the change in manifold pressure doesn't cause a 1:1 change in fuel pressure.

This is why the FPR for TBI doesn't have a vacuum line it just has a balance port on top, it is firing above the throttle blades hence the FPR needs to sense the pressure above the throttle blades.

Also on "returnless" systems the FPR is in or near the fuel tank and there isn't a vacuum line run back there so the fuel pressure is fixed referenced to the atmosphere and varies referenced to the intake manifold.

Reply to
Daniel who wants to know

He asked, "am I missing something?"

The answer is, "Yes." The answer is not, "what did the dealer say?"

While the "Yes," answer is short on information, it is correct. The, "What did the dealer say?" answer not only lacks any useful information (Yes is useful, even if short), it says that the person that offered it hasn't got a clue.

Reply to
Jeff Strickland

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.