From the California Vehicle Code:

http://dmv.ca.gov/pubs/vctop/d11/vc21710.htm

My recollection from high school drivers ed. is that California considers there to be a danger in stalling while coasting in idle and the driver would suddenly be without power assist on the brakes and steering.

Regarding starting with accessories on, try an experiment turn the lights on before you press the Start/Stop button. Do the lights come on while the engine is cranking, or after the engine has started? See if you can tell with the A/C and radio. My impression is that the OBC does not turn accessories on until the engine is running. The exception is when you hit Start/Stop without the clutch in and just turn on accessories.

I would follow your wife's advice on coasting, and have her participate in the experiment with the accessories.

 

I think they are talking only of deceleration. It that case, the momentum of the car is turning the engine over.

 

OK, so when I let off the gas and am engine braking, there is actually no combustion going on in the cylinders at all?

 

There is no vacuum in the cylinders on any form of deceleration, the intake and exhaust valves are still opening and closing.

You all are way overthinking this.......the fuel on/off is controlled by the computer and is affected by many, many things including temperature, road speed, engine rpm and so on. The difference in fuel use is microscopic and easily offset by one smart acceleration from a stop light.

A diesel engine is different, it has no throttle; it's always wide open to the air so any comparisons between diesels and gas engines on compression braking do not hold water.

Some localities do have laws about coasting in neutral, but I think they're referring to long downhills, not local stop and start conditions.

I am a mechanic (well, I spent 30 years of my life as one and still do all my own work if that qualifies) and coasting in neutral does not hurt a manual transmission. It could hurt an automatic as the pump that supplies all the lubrication and functioning oil pressure is driven by the output shaft (on most auto trans) , so I never recommend coasting with an automatic.

Trying to shift from dead idle into a gear while still rolling could wear the synchros depending on conditions, but then that's what they're for, and matching revs would mitigate it somewhat.

Starting with accessories on does not load the battery, the accessories are not powered when the engine is cranking, they are bypassed.

And please, the only thing you should ever slap your wife with is a big wet kiss!

Hope this helps........

 

when under engine braking, with high enough RPMS (the cut off is below 2000 RPM though) and when it's at normal operating temp (during start up it still runs the engine when it's cold to get it to normal operating temp).

But it really depends on the particlulars as to if you save gas or not. If I use neutral going down hill from my house, I get much better gas mileage than if I engine brake. The engine braking will slow me to the point where I have to give it a bit of gas and that more than makes up for the gas savings from the injector shut off. I've increased my gas mileage about 10% by using a combination of neutral coasting and less agressive acceleration duing most driving.

Matt

 

When I am in a car with a driver that continually selects Neutral when braking or when coming to a stop etc I absolutely HATE it.

I grew up with the mantra that one should always be in an appropriate gear.

That means that as I slow I drop down the box and make absolutely sure that if I need to get out of the way quickly, the move will not be preceded with the indecision over 2nd vs third, or indeed the fumble to find an appropriate cog.

This mantra was taught in the UK up until fairly recently, where they now teach 'block changing', this however absolutely precludes coasting in neutral, you are always in 'A gear' but because you block your downshifts, you may be in the wrong gear occasionally. This block-shifting also teaches you to find gears 'on demand' but reduces the 'get out of the way quick without thinking too much' advantage that us old gits have.

I have had heard of cars stalling while coasting, and I have seen first hand (ie from the passenger seat) drivers that coast in neutral attempting to find an appropriate cog as the red-lights that they where approaching turn to green.

Will it do any harm ?

As long as your engine does not stall, I doubt it, will it save gas - I doubt that it would be noticeable, is it good practice - in my opinion no, it is a horrible habit and scares passengers.....

 

this was recently addressed in r&t. again, the law is a bit old

from: http://www.roadandtrack.com/article....rticle_id=6578

Does Coasting Cost?
I drive a manual-transmission car. I'd been taught that when going downhill, one should use the same gear as one would select going up that same hill. This is supposed to help the driver maintain control and reduce brake wear.

I often drive a long stretch of highway down which my car will coast quite happily in neutral at an average of 50 mph. With ABS, Electronic Stability Control and other electronic gizmos, does putting the car in gear really improve control? Are there other safety, fuel-consumption or mechanical considerations I should be aware of regarding putting the car in gear versus leaving it in neutral and coasting?

R&T Reply:
Being in gear does improve control in that it provides engine braking and power may be applied as quickly as possible should something go awry.

Decades ago when gearing was lower, brakes less capable, tire blowouts common and shifting more difficult, protecting against downhill runaways was a real concern. Coasting was considered bad form and legally prohibited. Today, with the universal use of overdrive top gears, powerful, fade-resistant brakes, easy shifting and rare tire blowouts, the need for "control" is arguably reduced. You'll find the legal prohibitions remain, however.

You're correct in that improved chassis and electronic aids have made modern cars easier to handle at the higher speeds a runaway might see, so as a practical concern coasting is a safer option than ever. Of course, the driver must exercise intelligence when choosing where to coast; high-speed driving should be done in gear as it may require rapid maximum braking (compression and brake) or power inputs.

Technically, if not legally, there is little reason not to coast at low to moderate speeds. Manual transmissions don't mind it, and as long as the clutch pedal is not held down, there isn't prolonged throw-out bearing contact, either.

Coasting is a boon to fuel economy, for sure, but staying in gear might be even better. That's because the engine is supplied with idle-speed fuel whenever the transmission is in neutral. However, if left in gear, the computer will likely turn the fuel injectors completely off for momentarily infinite fuel economy. The injectors will eventually re-open to maintain at least engine idle speed, though, so if the gradient is shallow, then coasting in neutral will garner the least fuel consumption.

 

DING DING DING! Winner. You should always be in gear, unless coming to a stop.

Its irrelevant how much gas you save if you're not driving safe.

 

i like this thread. its funny. wife, husband, puppy, clown, whatever...

 

You wanted to hear from a professional mechanic, who works on all vehicles, including MINIs....
I asked my professional mechanic, who also happens to be my husband, and the only one who is allowed to work on my car and here is what he says....

First of all he dealt with the question of the battery. When starting your car, as soon as the key reaches the start position, all your accessories are cut out of the battery and all battery power is used to turn your starter. When the key is released to the 'run' position, accessories are allowed to come back on line. Your starter is your biggest amp draw on your battery, and will always require a certain amount of amps to turn. After starting, the alternator energizes and starts charging between 13.8-14.5 volts to replenish your battery. When accessories are left on they draw from the battery bus bar and not the alternator. With a low battery, after start, immediately acc. start drawing from a low battery, the alternator must work harder to replenish the battery. This creates more heat from the alternator, and we all know that heat is the biggest destroyer of electronics, and electrical components. With a weak battery, such as one 3-5 years old, the alternator has to work continually harder to maintain the battery and accessory loads after start. Maintaining a proper battery and lessening the workload of the alternator will lengthen alternator and battery life. Driving at night creates a larger electrical load on the alternator and battery than if you were driving during the day, as does a weak battery. Typical battery life is 5 years.
As long as your battery is maintained and passes a load test, it does not really matter if the accessories are left on or turned off. One way to tell if your battery is on its way out is to place your hand on the alternator after you have started your engine, if the alternator gets very warm, after just a few minutes of running, with accessories on, I would suspect a battery that is not up to par, or alternator failure is imminent. Have them checked by a mechanic. Next...

 

Key? What's that? This is the era of the Fob.

 

Coasting vs. driving with transmission engaged:
This, is also a preference...
There is no harm done to your transmission if you coast with your manual transmission in neutral, and your foot off of the clutch. Your fuel saving will be minimal however when doing so. If you think you are ahead of the game, however keep reading.
First off, if you let your car coast, in gear, or out of gear, with your foot on the clutch you are indeed wearing on your throw out bearing. This is the case even when stopped at a red light.
Here is the down side of coasting:
Your car is equipped with a Catalytic Converter (CAT) and O2 sensors. When you are driving under load your car is pulling in a certain volume of air. Your O2 sensors, engine coolant temperature sensor, and Throttle Position sensor (TP) are all working in conjunction with the computer to provide an optimum fuel/air flow to all your cylinders. When you reduce your car to an idle by putting your car in neutral, your fuel is momentarily shut off until the desired idle RPM is reached. With the throttle position at 0% (idle), air by-passes through the Idle Air Control (IAC) valve, and your computer will run your engine at idle RPM. (Idle RPMs are richer than if your car were running [tranny engaged]). The idle will begin the cooling off your O2 sensors as well as your CAT. When you continue through (put the car back into gear and engage your engine once more), your TP sensor will be asking the computer to readjust its mixture richer until the O2 and CAT temperatures stabilize.
During this period of coasting, your temps have fallen on the O2 sensors and CAT, and once you are moving again, the temperatures rise to previous levels. It is the rise and fall of temperatures that can cause cooling or thermal shock (same thing). This shock is what takes out the bearings in your supercharger and can cause premature failure of the O2 sensors and CAT. That is why it is always a good idea to allow for a brief cool down time (idle) prior to shutting your engine off after a hard run. This goes for supercharged, and turbos alike...and coopers still have to worry about their O2 sensors and CAT!
Drive the way you want, but it is better for all engines to allow your transmission to help with braking, not for saving fuel, clutches or brakes, but for the life of the above listed components.
Believe it or not, in the long run you will actually get better fuel mileage if you just drive your car, and not coast. Your car will love you better for it.

 

OK, for all you guys with the new fangled FOB...the start button is where it cuts out!

 

This explanation lines up with what I've always thought!

1) Don't coast: maintain control of your vehicle at all times by remaining in gear (down-shifting as/when required) until almost stopped.

MaxN mentioned that coasting "scares" your passengers. I can remember a friend who always did this and it was unsettling. I couldn't explain why, but now it seems quite clear. At a minimum, it's off-putting. And if it won't improve your gas mileage, then why bother, especially given the safety rationale. I've driven standard (stick) for 30 years and have always followed the "stay in control" approach.

2) Don't bother shutting down all the accessories unless your battery is on its last legs. Plain and simple. It's just a neurotic habit. If you have to do anything, adjust the volume so that you don't get blasted when you start up again the next morning!

 

I'm sorry moodylewd, but I respectfully disagree, you are not going to shorten the life of the 02 sensor if you coast or don't let your engine idle after a hard run............that's bunk.

Even if you were on the track, by the time you take your cool down lap and drive slowly into the paddock, you have more than cooled those components down, in fact only at a track like Daytona would you have your foot on the floor long enough to really get them hot, so unless you're driving at Daytona Speedway, do not worry about your cat or your 02 sensors.......They are made for conditions far harsher than anything we'll do to them on the street.

Keeping your car in gear as you slow down is preferable from a control perspective, but at city speeds as you approach a light, slipping it into neutral and coasting to a stop hurts nothing, not your fuel economy, your transmission or your clutch. And if your throwout bearing only lasted that long you'd be changing it every year. It's far harder on that bearing when you push the clutch in at 6,000+ rpm for a gearchange (spinning it up from 0 rpm to 6k + in a millisecond) than holding it down at idle for a few minutes. Neither hurts the bearing, it's what it's built for. These cars are not fragile.........

And you are not going to wear out your pads either, they're built for sustained stops from high speed, which is far harder on them than coasting to a light at 35 mph...........

The only time any of these issues arise is if you're coasting for miles and miles down a mountainside - now that's something you should not do!

I'm telling you, you guys are way overthinking this issue!

 

We change a lot of clutches, and I can't remember any car that had the throwout bearing give out before the actual clutch did. And I KNOW many of my customers sit at lights with the clutch in as a matter of habit.

--Dan
Mach V

 

I like the way I drive and while the foregoing has been interesting, I don't think I'll stop popping into neutral for the half block to the red light or sitting in neutral while I'm waiting on the light. It makes my left leg happy.

New question, however. What in the world is 'block shifting'?

 

I've always heard "Anyone who thinks they have won an argument is wrong." Who cares about coasting? Buy your wife dinner - she's worth it!

 

No combustion, but if the transmission is still engaged, the engine will still spin, and the intake and exhaust valves will still opperate just as usual. As the piston goes down and the intake valves open, it will suck in air... it compresses then decompresses without combustion, compresses again and the exhaust valves open, letting out the uncombusted air...

 

If you want to experiment with the fuel cutoff during engine braking, try this:

Get up to 30 MPH or so in second gear, and let off the gas (keep the car in gear). This actually cuts the fuel supply to the engine completely. As you slow down, watch the RPM needle and listen to the engine and get a feel for the vibrations.

As the RPM needle drops through about 1100-1200 RPM, you'll feel/hear a very slight kick/shudder. That's the fuel coming back on to keep the engine running. The torque multiplication from the transmission in the lower gears helps to amplify the effect - if you try this experiment in 5th or 6th, you might not be able to feel the shudder.

So, if you have your foot off the gas at anything over about 1200 RPM, you're not using any gas at all (whether the car is in gear or not). But if you put the engine in neutral and coast, the engine falls below the 1200 RPM threshold pretty quickly, turning the gas back on that much sooner.

And for the "accessories on or off during start" issue, Moody nailed it on the last page - there's a "load shedding" relay on all cars made in at least the last 20-25 years that interrupts power to high-current accessories while the engine's cranking. Not all cars interrupt the same accessories, but these are some of the ones that are frequently part of the interrupt circuit:

1) Air conditioning compressor clutch
2) Headlamps
3) A/C fan
4) Defrosters / heated seats
5) Radio

 

Everyone is missing the point here, he's says he feels like slapping her!!!!!!!

Give her a slap and then see what her father says................. her lawyer too! You might be saving gas but that's all you'll be saving





























PS kiddin

 

I agree, don't slap her, just shake her really hard until she becomes disoriented. Kidding, of course.

On a serious note, when I start the engine (R56) with the CD playing, the CD doesn't pause while the engine is turning over.

 

Okay. I've read all the posts and there is probably some clarification needed. There are only a few select places that I really coast. Ususally on exit ramps as I decelerate and let the natural lay of the land slow me down saving both brakes and letting the engine run at idle instead of 2 or 3k RPMs. Yes I know there is minimal, almost negligible gas savings. At lights i usually sit in nuetral without the clutch as I can see no reason to engage the clutch /throwout bearing to just sit. I also shift into 1st a few seconds before the light changes so I am ready to go.

My point about accessories is that with all the cars I've had (Chevette, Isuzu Impulse, Ford Ranger, Honda Accord, Fiat Spider) I never had anything beyond typical battery issues and my wife neurotically shuts verything off before getting out and says I killing my car(s).

And about slapping her. I do not condone spousal abuse. Althogh i do understand it

 

Apparently these arguments occur quite frequently.

http://www.cartalk.com/content/colum...tember/05.html

http://www.cartalk.com/content/colum...8/July/12.html

 

Block shifting is the exact opposite of sequential shifting.

In Sequential shifting you go 1,2,3,4,5,6 then 6,5,4,3,2,1 back down

Block shifting misses out ratios as required.

For instance you might be on the freeway at 65mph in 6th and as you exit, rather than dropping to 5th, 4th, 3rd while braking for the 3rd-gear curve, you brake in 6th and remain in 6ths until your desired speed has been reached, you then block-shift to 3rd, then make the turn.

The idea is that you separate out the moves.

Brake, Shift, Turn are all independent moves.

However, in this model, at no time is the car in neutral, it is always in gear, just not always the correct one.

The other interesting thing about this methodology is that drivers that are taught this are inherently better at finding the correct gear for any given situation, because drivers (like me) that use the sequential method are only used to finding the next gear (5th from 4th or 4th from 5th for example), we find it really hard to '****** 3rd' in an emergency.

There are two other aspects to neutral coating that that have not really been touch on.

Firstly, your car has very, very different handling characteristics when you drive the front wheels in a corner to when you are coasting.

Try this

Find a corner, preferably on a race track and go into the bend and accelerate through it. Notice that as you accelerate the car seems to pull into the corner ?

Then try engine braking through the same corner - it will run slightly wide but this is easy to correct with the throttle.

Now, finally, coast into the bend - I would advise a lot less speed for this test - it is downright scary because you lose the ability to make a mid corner correction on the throttle, essentially you only have half of the control coasting around a bend. Also you have way more body roll - which is scary from the passenger seat.

Secondly, are you able to find gears on-demand ?

Can you estimate your speed and select the exactly appropriate gear every single time for any situation that may appear. can you reliably grab (say) 3rd gear at 50mph if you suddenly need to ? If you can, practice block changing and make sure that you are always in 'a' gear

Seriously, neutral coasting is a habit that any advanced driving class will beat out of you very, very quickly, you are in less control of your car and you are way less prepared for an emergency 'go' situation. Your wife is right, but maybe not for all of the right reasons......