Coasting Downhill - RPM's Spike?

[DAVEM35X]

Hi Folks,

I have a question about my 07 M35x:

When I'm coasting dowhill on a long stretch of road, the RPM's spike when I get 2/3 down the hill. I am not on the gas at all, just coasting. Is this normal or do I have a transmission issue brewing? Or, is this the rev-matching feature?

Thanks for your replies and input!
Dave

[candymanfb]

In my car, it is an attempt of the engine to maintain the current speed. I believe the car downshifts - the reason for the higher RPM reading.
Our transmissions are meant to take a beating - I doubt that there is a problem.

[djpeak10]

Your automatic transmission downshifts thats all. Most automatics do that. Completley normal

[szh]

Agreed ... your transmission is down-shifting to let the engine do some braking.

By the way, do NOT put the car into neutral on long down-slopes. This may seem like a simple thing to do to save gas, but it actually can cause problems if the ATF is not being moved by the transmission pump while the wheels are still turning at a good clip!

Best to simply let your foot off the gas - the mileage will go up regardless.

Z

[The00Dustin]

Agreed ... your transmission is down-shifting to let the engine do some braking.

By the way, do NOT put the car into neutral on long down-slopes. This may seem like a simple thing to do to save gas, but it actually can cause problems if the ATF is not being moved by the transmission pump while the wheels are still turning at a good clip!

Best to simply let your foot off the gas - the mileage will go up regardless.

Z

I thought general consensus was the car shouldn't be rolled much in neutral while the engine was off. Is the transmission pump seriously connected in such a way that it doesn't pump when the engine is running if the car is in neutral?

[Double E]

Wheels are turning, yes, but rolling in neutral is no problem.
Roll with engine off in Neutral, probably not an instant disaster but I would not know either.

Engine running, I coast in neutral downhill with no issues and one of the posts on this topic from about 18 months ago was in agreement that there's no problem with it. I believe the source was credible.

[szh]

With engine off, letting the drive wheels spin is a definite disaster. That is why, when towing, a flat bed is best ... or make sure to get the drive wheels on a dolly and go at low speeds. I suppose you could lift from the rear too, but that might cause the bumper to get damaged if the belts on the tow truck are not properly set. With the AWD X models, a flat bed is safest for sure.

In regards to going downhill in neutral with the engine on, now that you guys mention it, I remember seeing posts that neutral would be okay, so my comment above is probably incorrect - my apologies. However, regardless of this, since the mileage is high enough (my car shows 99 mpg going downhill when I let my foot off the gas without putting it in neutral ), I'd say not not to do it still.

Z

[myother45isalesbaer]

I am old school. We were taught never to coast in neutral down hill. This also a safety issue as you are not getting the assistance of the engine in braking. In many states this is in fact illegal if you get caught doing what they call "free wheeling". In fact, if its a very steep, long hill I will downshift into a lower gear just to let the engine take on some of the braking load.

[The00Dustin]

I am old school. We were taught never to coast in neutral down hill. This also a safety issue as you are not getting the assistance of the engine in braking. In many states this is in fact illegal if you get caught doing what they call "free wheeling". In fact, if its a very steep, long hill I will downshift into a lower gear just to let the engine take on some of the braking load.

On a very steep long hill, I will shift to a lower gear. However, IMHO, the engine braking in our Ms in high gear is excessive with a capital EXCESSIVE, if a person wants to engine brake that much, they should be shifting down to begin with. Moreover, even if still enforced, I assume those laws are probably antiquated and more for standard transmissions anyway, but the opposite could actually be true. Regardless, on a hill where my 99 Park Avenue Ultra would coast up to 70MPH from 50MPH even after 180K miles (to be fair, I think GM transmissions may be designed to coast in high gear where most other brands are not), my 2006 M45 would coast from 60MPH to 55MPH, even before 45K miles, so to go down all but the steepest hills, the options are try to feather the accelerator to induce coasting or use neutral instead. I will say that flushing the J-Type transmission fluid out and replacing it with S-Type transmission fluid did help, but not substantially.

[myother45isalesbaer]

I am old school. We were taught never to coast in neutral down hill. This also a safety issue as you are not getting the assistance of the engine in braking. In many states this is in fact illegal if you get caught doing what they call "free wheeling". In fact, if its a very steep, long hill I will downshift into a lower gear just to let the engine take on some of the braking load.

On a very steep long hill, I will shift to a lower gear. However, IMHO, the engine braking in our Ms in high gear is excessive with a capital EXCESSIVE, if a person wants to engine brake that much, they should be shifting down to begin with. Moreover, even if still enforced, I assume those laws are probably antiquated and more for standard transmissions anyway, but the opposite could actually be true. Regardless, on a hill where my 99 Park Avenue Ultra would coast up to 70MPH from 50MPH even after 180K miles (to be fair, I think GM transmissions may be designed to coast in high gear where most other brands are not), my 2006 M45 would coast from 60MPH to 55MPH, even before 45K miles, so to go down all but the steepest hills, the options are try to feather the accelerator to induce coasting or use neutral instead. I will say that flushing the J-Type transmission fluid out and replacing it with S-Type transmission fluid did help, but not substantially.

Thats interesting. My 2004 M45 tends to just go faster. I don't think it downshifts when you take your foot off the gas going down hill. I have to do it manually. The old automatics wouldn't downshift either when allowed to coast downhill. I am not familiar with the newer ones. Freewheeling is an old truckers term. They used to go to neutral and lose control as the speed got higher than the brakes could handle. I still don't recommend freewheeling in any type of vechicle on steep downhill long runs. I'd rather have that motor slow me up a bit. If I want to go faster I can always hit the gas a little. I also like to know that my transmission is still engaged and if I need to go slower I can drop it down another gear. Just a safety issue. I like to be in control of the car and not let it get turned the other way around and me fighting it to get stopped in an emergency situation.

[The00Dustin]

Thats interesting. My 2004 M45 tends to just go faster. I don't think it downshifts when you take your foot off the gas going down hill. I have to do it manually. The old automatics wouldn't downshift either when allowed to coast downhill. I am not familiar with the newer ones. Freewheeling is an old truckers term. They used to go to neutral and lose control as the speed got higher than the brakes could handle. I still don't recommend freewheeling in any type of vechicle on steep downhill long runs. I'd rather have that motor slow me up a bit. If I want to go faster I can always hit the gas a little. I also like to know that my transmission is still engaged and if I need to go slower I can drop it down another gear. Just a safety issue. I like to be in control of the car and not let it get turned the other way around and me fighting it to get stopped in an emergency situation.

Yeah, I forgot you had a 2004, I imagine they changed this in the 2006 model, I wouldn't be surprised if they changed it back later on, like say when they went to the 7-speed transmission. It doesn't shift down automatically, though, it engine brakes that much in 5th (high) gear. I definitely engine brake with downshift in mountains, I also engine brake with downshift if there's someone in front of me, but I hate that the car engine brakes that much when I'm not telling it to. When the cruise control is engaged, you can feel it jerking on the way down hills as it taps the gas to get up to cruise speed and releases it and immediately starts losing speed to the engine braking (not to maintain speed, just because 5th has that much drag). When I said feather the gas, I meant that it doesn't engine brake if the throttle isn't at its origin, so you don't necessarily even actually accelerate in order to coast more freely, but there is very little play and the cruise control doesn't even use this "feature" (I'm assuming it's by design since you can also feather the accelerator to temporarily coast to a lower speed when adaptive cruise is on).

[greggebhardt]

On my new M, when I let my foot off the gas, there seems to be little to no engine braking. Even if you use the shifter to bump it down a gear or two, with the RPM sync function, you get little braking effect.

Not sure if this is good or not but other vehicles if my foot is off the gas for a while, I actually could feel the engine braking.

[myother45isalesbaer]

Really.....I don't think I would like that. And as Dustin says his will do it on is own. If I manually shift down, foot off the gas, it will engine brake. Don't know why Infiniti would take that away. I have to ask.....how do you like the M56. Is there anything you don't like. I wish I had the cash to layout for one. Maybe next year.

[greggebhardt]

Really.....I don't think I would like that. And as Dustin says his will do it on is own. If I manually shift down, foot off the gas, it will engine brake. Don't know why Infiniti would take that away. I have to ask.....how do you like the M56. Is there anything you don't like. I wish I had the cash to layout for one. Maybe next year.

I like the M56s very much and have few complaints except the tires are slightly noisy in my opinion. New set of tires should take care of that.

The 420 hp is intoxicating and it will be hard to be happy with anything less