I have read in a couple posts that when coasting in 6th gear the fuel injectors are shut off and use no gas. I am having a hard time believing this any explanations or is this just forum here say?

Thanks

 

It doesn't matter which gear, the injectors will shut down when coasting in gear above a certain RPM. This works because at that point the wheels are turning the final gear, which is connected to the transmission, which is connected to the flywheel via the clutch. The crankshaft has to turn, otherwise you have bigger issues. :D

 

You can even test it yourself. Get the car up to speed and then take your foot off the gas, leaving the transmission in gear (make sure there's no one behind you, of course.)

As the car slows down and the engine RPM decays down, you'll feel a slight shudder/jolt when the engine speed gets down to about 1100 or 1200 RPM. That's the fuel injectors kicking back on. The shudder is more noticeable in higher gears, since the RPM will be decaying more slowly than in lower gears.

 

I haven't noticed the shudder, but if you show your momentary fuel consumption on the tach display while coasting downhill, you'll see the figure rise to 99.9. It may actually shut off, I don't recall. But that's your engine using no fuel while coasting.

Spridget

 

Thanks

I know my mileage is better when coasting in 6th but didn't realize the injectors went off. I am up to 38+ mpg right now hoping for 40 in my 2008 mcs.

 

since the mid-80's on many cars w/ now way out of date CIS-E. Much easier to do actually with individually actuated injectors that current cars have than the old mechanical systems. Had that back to my 85 Audi 5000 5 speed (stick), which read up to 200mpg before the display went to dashes meaning fuel was shut off entirely; manual documented the same (as the Mini one may).

Car in gear still does have motor-based decel drag from the compression cycles that reduces speed; not clear if clutch in w/ motor idling might actually give better overall mpgs by keeping vehicle speed up longer under the right conditions--slope and safety both. Presumably very minor though unless entered in an ultimate mileage contest where every last drop and 100' can count.

 

Saving a few pennies on fuel by placing un-necessary wear on an expensive to replace clutch plate and throw-out bearing doesn't make much sense.

 

I have noticed this and wondered why I'd feel that little jolt. Now I know why. Thanks

 

Since I had my JCW tuning kit installed, I don't often see the instantaneous mileage go to 99.9. I use to see it all the time before the tuning kit was installed. Not sure what it means...

 

Only had 185,000 (miles, not km) on the original Audi drivetrain last I saw it. Admittedly a different car.

 

no offence guys and possibly girls but i find that hard to believe that the injectors shut off. my 1990 honda gives a little jolt when the RPMS are low when coasting. im pretty sure thats because your rpms are getting closer to the idle rpms and may possibly be the crank having a little more trouble spinning the weight. as for the gas mileage gauge going up its because your engine is reading 21 for vacumn and your gas mileage is based off of how much vacumn your engine has. correct me if im wrong but thats my theory

 

What would be the reason for keeping the injectors on while coasting in gear with no throttle applied? At that point, the engine isn't driving the wheels - the wheels are driving the engine.

 

This doesn't put any wear on the clutch. You're keeping it in gear, with the clutch disc fully engaged to the flywheel. The engine braking comes from using the wheels to turn the engine, while not injecting fuel.

And yes, the fuel injectors are not injecting any fuel while engine braking. If you were to descend down a long mountain road with the car in gear, coasting and not touching the gas pedal, you would use 0 fuel.

BMWs have done this ever since they first went fuel injected in the 80s with Bosch Motronic.

 

You need to read more carefully...

 

Re-read what you were talking about, got it.

 

If you step on the clutch in this case, the engine will burn some fuel to keep idling. If you leave the clutch engaged, the wheels turn the engine and the injectors cut off completely until the speed drops to/below idle.

Keith

 

the engine drag caused by the continued compression strokes will slow the car relatively quickly on less than pretty steep grades and thus require gas to speed back up more. That engine drag makes this apparent fuel shut off not some free mileage godsend in many everyday cases. Hard to say which might theoretically save more.

Does definitely save some gas if you realize the fuel cut off is there and start the coasting (fuel shut off) well before a stop ahead or a freeway exit or such, assuming it doesn't impede traffic.