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Discussion Starter · #1 ·
I was wondering if 'coasting' down hill in neutral and then putting it back in drive would harm our R/T 5.7's transmission.
I realize the inherent danger in not having it in gear but if you're working the auto stick similar to having a manual, safety can be handled.
When in neutral coasting down hill with no traffic really lowers the rpm's and saves a ton of gas. I'm just wondering if it is hurting the transmission.
Most transmission guys I've asked have said 'absolutely not' but maybe they are trying to drum up some business...
Any one have any thoughts? Thanks
 

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it should not hurt the trans to do this I am not a trans specialist, I am a mechanic and have done it to all kinds of other trans so I do not believe it will IMO
 

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Probably wouldn't do any harm. Must be some really really long hills to be able to save a ton of gas by coasting in neutral. I personally doubt that you'd see a difference when calculating the mileage against a tank. Sure, the instantaneous fuel mileage will be better, but when averaged across 300 miles or so, Im betting you won't see a real difference.

That is, unless you drive downhill both ways....

Mike
 

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Discussion Starter · #4 ·
Probably wouldn't do any harm. Must be some really really long hills to be able to save a ton of gas by coasting in neutral. I personally doubt that you'd see a difference when calculating the mileage against a tank. Sure, the instantaneous fuel mileage will be better, but when averaged across 300 miles or so, Im betting you won't see a real difference.

That is, unless you drive downhill both ways....

Mike
Hmm, downhill both ways? I'll have to try that....:umbrella:
 

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It's really unnecessary, but you are the pilot of your own car. ;)

If you just keep it in gear going down a hill, it will just enter assisted-engine braking mode in conjunction with mds. You're only powering 4 cyl at idle-ish fuel schedules, and it is largely the wheels and transmission turning the engine + accessories at this point. If you are engine braking down a significant grade, you may even enter fuel cutoff mode, in which case you are essentially using zero fuel, even while the engine is still turning at a nontrivial rpm. If you could scan it realtime, these scenarios may be logging periods of as good as 50/80/infinite mpg (depending on the particular condition). I'm gonna guess that is comparable or superior to what you would be burning at idle rpm while on a free coast. ;)

Never underestimate how much potential energy is being released in a 4000 lb vehicle going down a long hill. Might as well tap that energy by keeping the transmission engaged and feeding it back to the engine. Mind you, you aren't just powering the engine as you roll down the hill, but you are also powering all of the parasitics (pumps, A/C, alternator, and such).

I no longer have big hills around me in my new locale, but it would have been interesting to see what kind of gal/hr readings I was getting going down a long grade vs. sitting idle at a stop in neutral. That would have been a dead simple way to quantify relative fuel consumption in those 2 scenarios.
 

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Ten
You are doing just the opposite of what you thought you were doing - saving gas. The injectors are designed to turn off while in coast until a predetermined RPM is reached and then turn back on. So, if you throw the shifter up a notch to N the RPM come down to idle, you coast with your injectors keeping it idling with the fule you thought you were saving.
They turn the injector off on coast for emissions purposes. A closed throttle and fuel makes for extremely rich hydrocarbon readings. On some vehicles you can tune into or feel when this happens.
 

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Like the previous posts, when in decel mode, the injectors go into lean operation and cut off until a predetermined rpm level (avoid stalling, or surging).

Plus there's the wear and tear from engaging clutches while in motion and the engine is at idle speed. Best to leave the 'auto in gear and let it function as designed.

Pretty much anything built since the 80s has the feedback loops and engine controllers that manage fuel systems, etc.
 

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I can't recall where I read it, but coasting is not recommended. It makes the PCM think that the car is in some type of mechanical distress.
 

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I can't recall where I read it, but coasting is not recommended. It makes the PCM think that the car is in some type of mechanical distress.
i posted on another thread about a guy who was trying to find a clunk or a pulling sensation in his car and mentioned to get going and put it in neutral to help find it and someone else chimed in saying it's not recommended in the manual as trans damage will occur.
 

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2 words will happen. "LIMP MODE" If you coast in N it will go into limp mode. The car will not move until you come to a complete stop and turn the car off. Restart and all is well.
 

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Discussion Starter · #11 ·
2 words will happen. "LIMP MODE" If you coast in N it will go into limp mode. The car will not move until you come to a complete stop and turn the car off. Restart and all is well.
Wow!! Thanks, I haven't experienced that and don't want too. No more coasting for me. Thanks to all...:wave:
 

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Discussion Starter · #13 ·
... will take more of a beating :fight:
Actually I was only doing it coasting down a long hill, then slipping into D with MDS and then with auto stick gear 5 if the MDS started sucking and gulping.
I had planned another controlled MPG test using this method but after hearing from those in the know, I surely won't.
I never coasted in N to a stop so I guess that's why I never experienced any problems. Just tried it about a dozen times and wondered if it would work. I'll leave this to some one else - I want to take care of my car, another 1 mpg isn't worth it.
Thanks again :)
 

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As a casual exploratory reading, the engine computer appears to report about 0.6 to 1 gal/hr at a warmed idle in N. Does that sound about right? Naturally, cold-start engine reports much higher due to the higher initial rpm and rich mix (1.4 to 1.6 gal/hr?).

As I said before, I no longer live near any big hills to test, but on the short up and down "mounds" I sampled on the hwy, it seemed the best figures I was catching were around 2-2.5 gal/hr on the downside while rocking mds. This was corresponding to maybe 30-40 mpg? I've seen on a long, aggressive grade, mpg figures like 50/70/cut-off. So I can imagine that must be getting close to the 0.6-1.0 gal/hr figures that were typical at idle. (I guess fuel cut-off would inherently have to be less than idle, eh?)

It's a wonderful, wonderful thing (remembering those days in CA) when you've gone almost 60 miles from the high desert down to the metro area (in-gear at all times), and the needle has barely budged to the first click since you just topped-off. ;) Too bad, it can't be like that in every direction, eh?
 

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I remember finding out about the "don't play with neutral coasting in the city" the second night of owning our Challenger... you know those pesky civics's buzzing next to you and you just want to shut them up with some Hemi V-8 revving.

First time coasted/revved I noticed once back in Drive the motor stumbled and acted sick - I was like what the heck is up here. It's in the car manual about coasting in neutral and the bad stuff that can happen since the trans pump isn't keeping up with the internals.
 

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STONEWHITE R/T hit the nail on the head.

Designs vary and I'm not an expert on how these transmissions work, but I have also heard that coasting in neutral on long grades can be harmful to an automatic transmission because the fluid pressure at idle is non-existent or inadequate. Possibly the reason for the following warning from the manual:

CAUTION!
Coasting the vehicle or driving for any other reason with the shift lever in NEUTRAL can result in transmission damage.
....there is also this blurb:

Do not coast in NEUTRAL and never turn off the ignition to coast down a hill. These are unsafe practices that limit your response to changing traffic or road conditions. You might lose control of the vehicle and have an accident.
 

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As a casual exploratory reading, the engine computer appears to report about 0.6 to 1 gal/hr at a warmed idle in N. Does that sound about right? Naturally, cold-start engine reports much higher due to the higher initial rpm and rich mix (1.4 to 1.6 gal/hr?).

As I said before, I no longer live near any big hills to test, but on the short up and down "mounds" I sampled on the hwy, it seemed the best figures I was catching were around 2-2.5 gal/hr on the downside while rocking mds. This was corresponding to maybe 30-40 mpg? I've seen on a long, aggressive grade, mpg figures like 50/70/cut-off. So I can imagine that must be getting close to the 0.6-1.0 gal/hr figures that were typical at idle. (I guess fuel cut-off would inherently have to be less than idle, eh?)

It's a wonderful, wonderful thing (remembering those days in CA) when you've gone almost 60 miles from the high desert down to the metro area (in-gear at all times), and the needle has barely budged to the first click since you just topped-off. ;) Too bad, it can't be like that in every direction, eh?
Sounds like someone has been down the Cajon Pass a few times.
 

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No doubt! ;)
 

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Great thread with important info, thank you! No more coasting for me either. Coming from a manual transmission background it's just felt more normal, but I was unaware that the injectors were designed to shut down in a coasting/no throttle situation.

Love this kind of info.
 

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Coasting an automatic with engine off is a surefire way to ruin one.

There's no fluid being pumped (torque converter & main input shaft supply the internal pump) in the tranny with engine off. That's why a vehicle is towed with rear wheels off the ground or on a dolly.

Only a few automatics can be flat-towed with all 4 wheels on the ground (Saturns, CR-V) because the trannies are designed for for that kind of use. The W5A580 / NAG1 isn't one of those.
 
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