Question about standard car with ECU controlled injection: Does the ECU still inject some fuel if my engine has higher revs than idle speed and throttle is closed (foot off)? As much as I know (I've been working only with racing ECU's for racing cars), ECU shuts the injectors completely, and Air/Fuel ratio will show 30-35 on closed throttle (4.2%) at 10000 RPM going down to idle.

  • Situation: Approaching a roundabout or junction, etc.
  • Reason of asking: Fuel consumption and global environment-friendly driving behaviour :D

Would it save a fuel if we leave in gear when slowing down, or stick in neutral and let it roll? Years and years ago some drivers used to save fuel by accelerating to a cosmic speed and then switch neutral, and drive on idle :D But it was a carburetor. Agree with that.


3 Answers 3


So I have 2 vehicles I can confirm it is better to stay in gear.

1987 Dodge Daytona Shelby Z 2.2L MPFI intercooled turbo 5 speed stick. It has an in dash navigator with a live fuel usage readout. Leaving it in gear would consistently get better gas mileage than dumping it in neutral. Even coming down mountains, where I am actually accelerating in neutral, i would get better mileage leaving it in gear and staying on the throttle slightly.

1970 Chevrolet C20 Pickup 350, TH350, quadrajet. I am not sure how stock the carb is. The spring for the power piston on the primaries should be set that if you exceed a certain vacuum, it pulls the power piston closed. Can't really confirm from gas mileage, but I can hear it in the exhaust. When I am cruising, say 2000 RPM, barely on the gas, it is quiet and the A/F gauge reads really lean. At idle, I can hear the exhaust more and the A/F is somewhere around stoich.

Idling is inherently inefficient, since the engine has so little load. Once you get past idle vacuum, you can basically stop supplying fuel (or very little) until the vacuum drops.


Thank you everyone, the actual answer is, as 'I have no idea what I'm doing' have posted a link in comments, no fuel is being injected at all, so modern engines consumes fuel at idle speed, when you switch in neutral and keep rolling, but it consumes no fuel at all if you just take your foot off and leave in gear, using an engine as a break. Plus you don't wear your brake shoes :D That's all. Same as I had on racing engines, same technology on road engines. Thanks rpmerf for experiences :)

Carburetor engines still gives a fuel, that's why they are shooting fire out of exhaust.


How EFI Manages Injection

So, reading this article on fuel injection, it seems to answer your question.

The answer being - yes the ECU does still add fuel, it just does it differently. Personally I don't understand all of how ECU code works (cause I haven't studied it, I am a computer programmer tho), but I imagine they are separate, non-correlated items.

The ECU adjusts idle speed by enriching the fuel/air. This pushes faster, but less efficient. It does this because the motor is cold and is trying to increase the heat of the whole thing faster.

Likewise, when the throttle is pressed, the injectors are still being opened, but now they're being opened to match the airflow. The airflow is increased when the throttle is rolled. As far as I know, injection is always being handled by the ECU regardless of state (fast-idle, idle, throttle opened/opening, throttle closed/closing). This is the whole point of the Throttle Position Sensor (TPS).

Based on this cute and basic YouTube video, the injection has no strictly mechanical action other than throttle body.

This video was really interesting, but I bet it's a bit dated.

Driving Theory *

So, is it more economic to reach high speeds, then idle or roll in neutral? Technically, no. Even in a perfect, frictionless vacuum, it always takes the same amount of energy to produce the same amount of work.

In real world scenarios, every engine has different points of efficiency. Likewise, every drivetrain will have different points as will every body. Remember, the drivetrain is where all the surface friction comes from, the body so where all the air friction comes from, the the power plant is always fighting that.

Your motor may produce the best HP/gallon of fuel at (total random example here) say 5000 RPM, but at 5000 RPM in top gear you'll probably be going over 100 MPH. A lot of vehicles have their aerodynamic efficiency point around 80 MPH (though may around 60-70 MPH as well). So once you exceed that, the body starts fighting you more.

Now, say your engine sits at 5000 RPM at 4/5ths Throttle Pisition (TP). Your drivetrain is going to change things as you drive too. For example, holding 4/5 TP keeps you at 5000 RPM and 100 MPH on a flat surface. Well now what if you hit an incline? The car starts to slow and RPMs start dipping! Why? Cause the drivetrain (via the transmission) is exerting more force into the engine. On a 100% slope you'd have 50% of the weight of the vehicle added, plus tire friction, plus air resistance, plus, plus, plus... So the vehicle slows. More energy (combustion force) is needed to produce the same speed because more more resistance is involved. Does revving higher produce better MPG? Generally no. This is why cruise control is really ideal for relatively flat travel. Conversely on the other hand, you don't need as much throttle going down hill to keep the same speed!

I could keep rambling on, but the basic point is that there are too many variables to say it is or is not more fuel efficient to drive fast and then idle, but I will say that it's sketchy to put your car (especially w/ automatic transmissions) into neutral at highway speeds. Can an experienced driver do it? Sure. But it's not the safest practice.

I can say however that I found the most fuel efficient I could get with a 2010 Chrysler Town & Country was 85 MPH on flats, faster on ~60% grade downhills, and around 60/65 MPH on a ~60% grade uphill. That got me ~30 MPG if I remember correctly. That was on a road trip from Los Angeles to Kansas City (~1,500 miles).

*: All of the numbers used herein are just random numbers for simplicity. They are not based on any real world datasets unless otherwise specified. Also I'm using "drivetrain" to mean everything other than the power plant.

  • The link you provided gives zerro answers, the question is not about the idle, and surely not about driving at 100MPH and air flow stuff.. I love engineering, it is awesome, but the question is only about will it inject fuel on "coasting" at gear or comletely shut the injectors off. Sorry for this.. Jul 16, 2016 at 17:57

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