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-   -   At what point is it advantageous to shut the engine off while driving? (

Mazda_2.3 02-15-2009 12:00 PM

At what point is it advantageous to shut the engine off while driving?
Hey everyone, I'm Andrew and yeah this is my first post. I'm trying to figure out if and when I should shut my engine off while driving. At a long stop light? I always do. Coasting down a long down hill straight away? I often do. My real question is at what point does it become advantageous to shut the engine off? Someone once told me the engine uses more fuel to start up than you would use just sitting at idle but I don't think that is necessarily true. Any input?

MetroMPG 02-15-2009 12:06 PM

Your hunch is correct.

A modern fuel injected engine, fully warmed up and in good tune uses very little fuel to re-start. We're talking about an advantage gained after several seconds.

Might want to see:

You may want to take starter wear into consideration as well. (I don't worry about it - I haven't had a bad starter since I had a series of old VW Rabbits when I was going to school. Also, starters are usually easy to remove/repair/replace.)

gascort 02-15-2009 08:27 PM

Andrew, Welcome to EM!

Do you have a manual or automatic tranny?
If you have a manual, see:
Clutch Starting an Engine
If you have the same commute every day, you can determine pretty well where the lights change that are on timers. I clutch start at every light except for 1 or 2 where I know I'm going to have to wait for more than 10 seconds.

Mazda_2.3 02-16-2009 01:18 AM

Great info guys.

My car has a manual transmission paired with the 2.3L 4cyl w/ Variable Valve Timing. The car is all stock with the factory 17" rims. EPA gas mileage is 24/29. I've had the car about 3 weeks and am getting 28 or 29mpg. Not bad but am hoping to make it better. So far I've been coasting as much as possible, downhills and instead of braking. I also shut the engine off at long stops and am shifting pretty early. Unless I'm in fifth, I tend to keep it at around 1500 RPM. Doing 55mph in 5th the tach is at 2800(?), I think.

This is where I am now. Tomorrow I'll start shutting the engine off more often and will start figuring out how to add a kill switch that will allow my accessories to still have power.

Any wisdom or spectacular links from the more experienced folks out there? I'm an apprentice diesel mechanic so I can be a little savy at times. Hopefully any advice you guys can offer won't go in one ear and out the other as a result. Thanks for reading. . . I'll try to keep my posts shorter in the future.

MetroMPG 02-16-2009 09:12 AM

I've got a kill switch in my car - mounted on the shifter. Saves wear on the ignition barrel, and makes it more convenient to use, since I'm usually going for neutral at the same time I want to kill the engine anyway.

FYI, some ideas on how-to...



Have you got fuel economy instrumentation? That'll help more than anything else. It'll motivate you, show you what's working and pay for itself in fuel savings.

Also, in case you haven't seen it:

- 100+ hypermiling / ecodriving tips to increase gas mileage -

PaleMelanesian 02-16-2009 12:35 PM

My bro-in-law has a 3s hatch. Watch out for the electric power steering. It stays off, even after you restart the engine, UNTIL YOU DROP BELOW (about) 20 MPH. Then, most likely in the middle of a turn, it kicks back on. It's manageable if you are expecting it, scary if you're not.

Another source about idling vs shutting down:

Geonerd 02-16-2009 12:41 PM


IMO, routinely killing the engine just isn't worth it. Apart from starter and/or clutch wear, you are also beating up the ignition switch. The engine itself also experiences a little extra wear at every stop: The protective film of oil collapses as it slows to a stop and doesn't reform until the engine regains speed. Also, I find that the constant obsessive fiddle-farting to be distracting and tedious. A small engine like yours probably doesn't burn any more than 1/4 gal per hour at idle. Unless you're stuck at a loooong light, train crossing, etc., you're really not saving much.

Since you've got a manual transmission, I'd suggest you read up on specific fuel consumption and pulse/glide driving. The basic notion is that you try to run the engine in it's most efficient RPM/throttle envelope. This is usually found from ~20% to ~50% of redline, and between 40% and 80% of full throttle. (Don't get too close to flooring it, the car may richen the mixture if it thinks you REALLY want to accelerate.) Your variable valve timing will probably extend and distort this zone somewhat, depending on how the ECU is programmed. In practice, all this means is that you short shift and give it a fair amount of gas. This was worth almost 5 MPG city for me.

Actual BSFC (brake specific fuel consumption, or sometimes just SFC.) charts are, unfortunately, hard to find. You might ask the mechanics at the local dealership; sometimes the factory manuals have extra goodies that are not generally available.

PaleMelanesian 02-16-2009 12:46 PM

60+ mpg says it really IS worth it. ;)

Geonerd 02-16-2009 01:33 PM

In your opinion. :D If you're hardcore regarding FE, go for it! I do think that most people are looking to take a more balanced approach.

I do know that the average Joe watches hypermiler videos of dorks like Gerdes on TV, and just shakes his head. They see him doing insane things like taking freeway exits at unsafe speeds, creeping down the road with 10 cars bottlenecked behind him, slamming the car into gear the second it's started, pushing the car around parking lots, turning the engine off with the car in motion, etc. Joe Sixpack watches all this and thinks, "what an idiot." Joe leaves with the mistaken notion that FE driving is strictly for neurotics, and that there is no point in attempting to improve his own FE. This inability to connect to the general public is a shame, and is something the HM community needs to address.

I'm glad you're getting your 60MPG, but please don't assume that what you do is best for everyone.

MetroMPG 02-16-2009 01:44 PM

Geonerd - I'm not sure how you can be opposed to routinely switching off the engine, then advocate P&G in the next paragraph.


you're really not saving much.
You'd better alert the world's auto manufacturers, who are adding idle stop systems to more and more vehicles (non-hybrids) precisely because it saves a significant amount of fuel in urban driving.

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