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Old 06-12-2008, 09:47 AM   #19 (permalink)
EcoModding Lurker
Join Date: Apr 2008
Location: Nashua, New Hampshire
Posts: 53

The Crimson Jellybean - '08 Toyota Yaris
90 day: 41.8 mpg (US)

White's Elephant (in Black) - '02 Jeep Wrangler Sahara
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Originally Posted by Blue07CivicEX View Post
I have been doing it ever since assuming it was good for something but it seems revving the engine as you slow down would use more fuel even if you're not on the throttle.
It depends on the vehicle, but some have something called DFCO, decelleration fuel cut off. When the computer senses the car is decellerating and in gear, it will shut off the injectors. That means you are burning no fuel while in that state, however, you do get the resistance of spinning the engine. Apparently some cars even play with the valves in this state to reduce engine braking loses. From what I've read, the reason it exists is to prevent unburnt gasoline from reaching the catalytic converter, however hypermilers can use it to their advantage by deciding when to use DFCO and went to coast out of gear.

Originally Posted by Blue07CivicEX View Post
would I be better off to pop the car out of gear then just a quick clutch to put it back in or will holding the clutch in for the period of the glide be safe on the clutch/tranny?
Thats a bit of a point of debate, but I do know that while your foot is on the clutch, the throw-out bearing is engaged and spinning. That bearing is intended to hold the clutch face away from the flywheel/pressure plate in the moments while you shift, not for extended sessions. Personally, I prefer to shift to neutral and disengage the clutch (release the pedal) so that while I may be spinning the input shaft of the transmission, I am at least using bearing surfaces designed to be used full time and which are most likely bathed in lubricating fluid, unlike many throw-out bearing designs which are simply sealed bearings since their duty cycle is typically low. Some cars seem to be fine with holding in the clutch, some seem to eat throw-out bearings. I do know I prefer not to have to replace them, which involves pulling the tranny most times.

As for the original topic of bump starting, can anyone comment on what Twerp said? My manual also advises against "severe catalytic converter damage" if you start the engine by engaging the clutch. Is the danger the combustion of accumulated fuel vapor ahead of the cat? I've done this for years only to now hear that it could be hazardous to my vehicle (of course, I started doing it on a 69 VW which was probably a tad more tolerant of abuse)

EDIT: By the way, one of my favorite "stupid Metro tricks" was that I could push start the car by opening the door, kicking off like a skate board until it moved, and then smoothly letting out the clutch
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