Cruise in Neutral to a stop.

Messages
1,251
Location
Austin, TX
I know this has been advised against, because deceleration in gear creates more vacuum in the manifold, which is the power source for the brakes. It is probably very dangerous to do this coming down from a hill. Besides this, is there any other adverse effect of doing this? Would this cause accelerated wear to a manual gear box?
 
Messages
1,904
Location
Canada
The practice is illegal in most places. Also, it causes modern fuel injected cars to burn more fuel (idle power setting), instead of the fuel being zero'ed out during deceleration. Yes doing it a lot causes increased oil consumption, but any additional engine or component wear is negligible.
 
Messages
2,387
Location
Chicago area
You will use LESS, not more fuel in modern cars. The O2 sensor doesn't magically stop working in neutral, and the car is turning less RPMs. The vacuum is high in neutral, the brakes will work fine. Oil cunsumption would go DOWN, not up, with less RPMs and vacuum. As long as the input shaft it turning, the oil will still splash and lube the trans OK.
 
Messages
239
Location
California
quote:
Originally posted by pitzel: Also, it causes modern fuel injected cars to burn more fuel (idle power setting), instead of the fuel being zero'ed out during deceleration.
This is only true in a few modern direct injection (VW calls it FSI) engines. Otherwise you burn less gas in neutral.
 
Messages
34,199
Location
South Jersey
Just trying to clarify here, mechtech are you saying that oil consumption will be less if you put the car in neutral and come to a stop vs down shifting? I do both. Most of the time I approach a red light too fast and I'm forced to put the car in neutral and stop. I down shift to as low as 3rd gear but thats it. If the vacuum is higher in neutral, wouldn't oil consumption go up?
 

Jonny Z

Thread starter
Messages
1,251
Location
Austin, TX
quote:
Originally posted by jtantare:
quote:
Originally posted by pitzel: Also, it causes modern fuel injected cars to burn more fuel (idle power setting), instead of the fuel being zero'ed out during deceleration.
This is only true in a few modern direct injection (VW calls it FSI) engines. Otherwise you burn less gas in neutral.

Anyone knows for a typical EFI car, what is the fuel useage of coasting in gear vs idle?
 
Messages
10,905
Location
Nokesville, VA
Both a "modern" 1991 Ford Escort and my old 1995 Contour would shut the injectors down when coasting in gear. I don't know about the 1996 Contour but that's because the scantool I have won't display fuel injector pulsewidth from OBD-II cars. I suspect it's the same...it seems kinda dumb to continue injecting fuel into an engine which is closed-throttle decelerating, both from a fuel economy and an emissions standpoint. Here is what the book "Ford Fuel Injection & Electronic Engine Control" by Charles O. Probst, SAE which covers "modern" 1988-1993 Ford vehicles has to say: "Deceleration injector pulse times become shorter. When pulse times are less than 2 milliseconds and RPM is greater than 1500, control shuts off the injectors. In some engines, the oxygen sensor continues to switch voltages, calling for closed-loop operation to maintain ideal air-fuel ratio; in others, the mixture goes lean and the system operates open-loop. From shut-off, the control module resumes normal injection pulses: o As engine speed decreases to 1200RPM, sooner if ECT signals indicate cold engine o As you step on the accelerator" Here is what it has to say about MECS engines (the Mazda fuel injected engine that Ford used): "Deceleration fuel is cut off from the injectors above 2200RPM".
 
Messages
10,905
Location
Nokesville, VA
Then there's this from EFI Systems: Things Old and New Motor Service, Oct, 2001 by Phil Fournier Which is google-cached here (it seems to be missing from findarticles.com): the short link for better viewing It says that even throttle-body injected systems use deceleration fuel cutoff: "TBI utilizes nearly all of the computer functions of port injection and only differs in that it sprays fuel on top of the throttle plates rather than at the back of the intake valve, and therefore has a wet, rather than dry, intake manifold. This creates a less-than ideal situation for emissions control, especially on deceleration when high vacuum causes fuel absorbed by the walls of the intake to boil out and enrich the mixture, making for CO and HG emissions. But deceleration fuel cut off mode, where the computer shuts down the injector based on signals from the MAP sensor, rpm sensor, and vehicle speed sensor, reduced this effect sufficiently to keep TBI alive in truck applications well into the 1 990s." [ September 07, 2005, 08:12 AM: Message edited by: Gary Allan ]
 
Messages
2,387
Location
Chicago area
There is higher intake vacuum when slowing down in gear than in neutral. So there is then the potential for SLIGHTLY higher oil consumption. Unless your engine is broken, and an oil burner, you will never tell a difference in oil consumption, either way. I downshift to be in the proper gear - never to brake. I have standard 4-500 HP brakes [like most cars], and 10 HP engine brakes. I put her in neutral and coast a lot - I drive as far as I can see.
 
Messages
1,836
Location
Ky
Driving in neutral not only is uneeded but can be extremely dangerous. An auto in neutral would be much harder to avoid an accident because a persons natural defensive actions would most likely forget to put the car 'back' into gear to avoid said accident. In many situations,you have only a few tenths of a second to avoid an accident and putting a car back in gear is going to make it almost impossible to do anything. Popular Mechanics says to leave the car in gear. Here is the link to the article: http://www.popularmechanics.com/automotive/sub_care_clinic/1675807.html?page=2&c=y
 
Messages
34,199
Location
South Jersey
quote:
But perhaps most important, the pressure of the throwout bearing on the clutch’s pressure plate is putting force against the crankshaft’s thrust-bearing surface in the crankcase. Eventually, this thrust bearing can wear to the point at which the crankshaft walks in and out. This slight but pronounced fore-and-aft movement of the crankshaft can affect cam and ignition timing and wear the oil seals. Eventually this can precipitate a major bottom-end failure. A skillful driver can shift a manual transmission into Neutral while slowing down without even using the clutch. As far as saving gas? Not even a little.
I've never heard of hit being a bad thing (with a manual trans) putting it in neutral and relying more on the brakes. ?? [I dont know]
 
Messages
1,836
Location
Ky
quote:
Originally posted by mechtech: Driving in neutral is not extremely dangerous.
How did you come to such a conclusion? The diffrence between an accident and avoiding said accident is milliseconds,barely enough time to get a car in gear. You dont have time to react to an emergency situation.
 
Messages
16
Location
Beaverton, Oregon
The fact is that, if you leave the tranny in gear, the engine helps slow you down. Haven't you ever heard a large truck in "compression-braking" - you know, the loud "b-r-a-a-p" sound? This will reduce brake wear, also. And, for those of you with auto-tranny's, shifting into neutral while rolling (yes, some people do this) creates excessive fluid pressure which will destory the tranny.
 
Messages
18,449
Location
East of IGO
quote:
Originally posted by newfordrichy: The fact is that, if you leave the tranny in gear, the engine helps slow you down. Haven't you ever heard a large truck in "compression-braking" - you know, the loud "b-r-a-a-p" sound? This will reduce brake wear, also. And, for those of you with auto-tranny's, shifting into neutral while rolling (yes, some people do this) creates excessive fluid pressure which will destory the tranny.
How can shifting into neutral create excessive fluid pressure?
 
Messages
239
Location
California
I don't see what you have against rolling in neutral. If I'm rolling down a long hill on the freeway I would rather it in neutral or the clutch all the way and so I use up the energy stored up in the car. I've been able to roll in neutral for well over a mile while maintaining constant speed. I'm very familiar with the hilly freeways in my are. I know exactly what speed I'll have by the bottom of the hill, what gear to shift to even which lane will have the least amount of traffic (usually the slow lane). Even if the injectors were shut down while engine braking (which they are not) I would still burn more gas because it would slow me down too much. I need a very steep hill in order to maintain constant speed while engine braking. Automatic trasmissions have a pressure relief valve to maintain costant pressure even while rolling in neutral. Think about it. What happends when a tow truck is pulling a RWD car?
 
Messages
2,387
Location
Chicago area
Diesel engines in trucks [with very high compression], or using 'Jake' brakes, are another world than gas cars. Use your brakes, save your engine [unless you really like the sound of downshifting]. I can't concieve of an 'emergency situation' where being in neutral would hinder me at all.
 
Messages
39,805
Location
Pottstown, PA
All engines have compression braking (or lack of it)..what diesels don't have is vacuum ..hence the need to stop up the outlet. They don't have a throttle butterfly to slam shut. When someone says "compression braking" in a gasoline engine ..they mean vacuum braking. Compression braking is a misnomer.
 
Messages
738
Location
Suburban St. Louis
quote:
Originally posted by jtantare: .... Automatic trasmissions have a pressure relief valve to maintain costant pressure even while rolling in neutral. Think about it. What happends when a tow truck is pulling a RWD car?
Automatic transmissions do not circulate fluid unless the engine is running. The pump at the front must operate. In olden days, there was a rear pump driven by the wheels--no longer. If towing a rear drive car you must go slow, and not very far. Some 4WD vehicles can be flat-towed, e.g. behind a motor home, because there is a transfer case to put in neutral. Otherwise, they'd better pick up the rear or remove the drive shaft.
 
Top