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Discussion Starter #1
Since the DSG is technically a manual transmission, are there any potential damage issues by putting it in neutral when going down a long hill? I know this can be hard on a true AT, but what about ours? What about when going back into gear? Best to put it in "D" or bring the RPMs back up a little bit first?
 

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Since the internals are like a manual transmission, I don't see why not. Just keep in mind that it may not be legal to coast in gear. In addition, depending on how steep the hill is, it may be worth it to leave it in gear to minimize fuel use. When you put it in neutral the engine is using fuel to idle. If the hill is steep enough so that you can maintain speed with the foot completely off the accelerator, the engine is using no fuel. Everything else in between is up to the driver.

Don't know if it's best to rev match before putting it back into D or if it will do it automatically. Want to try and find out?
 

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Well, I'm now confused.

I was curious on this same matter, so I decided to test the coasting down long hills in "nuetral" vs "normal foot off gas method", and I get better MPG's with the "nuetral" method :dunno
 

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It also depends on how steep the hill is.
Considering that diesel is like a lubricant for the engine during its operation, the engine probably need to spray a minimal amount of diesel fuel per cycle to keep its cylinders well lubricated. While the amount of fuel needed per cycle may be smaller than the amount to idle the engine, considering the higher RPMs involved while coasting down a steep enough hill, the minimal amount of fuel at higher RPM may just be greater than the amount of fuel needed at idle for the same distance.

So, the above statement about it depending on how steep the hill is correct, however, it isn't as simple as finding as steep of a hill as possible to coast down from. IMO, more than likely, the amount of fuel used coasting down in gear in most circumstances will be higher than it is in neutral.
 

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I'll buy that.

However, practically all modern car engines use no fuel whatsoever if your foot is completely off the accelerator pedal and you are coasting down in gear. The momentum turns over the engine. No TDI uses fuel when your foot is completely off. This can be verified with VCDS. You can also hear the difference in sound.

The engine has oil sprayers and oil splashing around that keep the cylinders lubricated. Only the fuel system is dependent on fuel for lubrication.
 

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My logic (which is not always sound:eek) tells me that an engine uses more fuel when the RPM's are between 2-3k than when it is below 1k while coasting in neutral. Am I missing something here...by just taking your foot off the gas pedal, the RPM's are much higher than when in neutral.

So what I am asking is....does the RPM's have anything to do with fuel consumption while coasting, or is it just gravity keeping the RPM's high?

Thanks in advance for the education.
 
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Just gravity. If you roll down a really steep hill with your foot off the gas pedal, the car could be off and the RPM would be the same. If you leave it in gear and let it coast to almost idle, you'll feel the engine kick back in and start giving fuel so that the engine doesn't die.
 

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Answered while I was typing. Since the wheels are turning when you're going downhill, it's turning the engine. If you take it out of gear when coasting, the wheels aren't turning the engine. You'll coast farther because energy isn't being used to turn over the engine but because the engine must now use fuel to idle, you are using fuel.

I'm sure there is a forumla that tells you when to coast in gear and when to coast out of gear but it's over my head. Coasting out of gear is illegal in many areas anyways and I really only coast out of gear when I hear a funny noise and want to isolate it to engine rpm or car speed.
 

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Considering that diesel is like a lubricant for the engine during its operation, the engine probably need to spray a minimal amount of diesel fuel per cycle to keep its cylinders well lubricated. While the amount of fuel needed per cycle may be smaller than the amount to idle the engine, considering the higher RPMs involved while coasting down a steep enough hill, the minimal amount of fuel at higher RPM may just be greater than the amount of fuel needed at idle for the same distance.

So, the above statement about it depending on how steep the hill is correct, however, it isn't as simple as finding as steep of a hill as possible to coast down from. IMO, more than likely, the amount of fuel used coasting down in gear in most circumstances will be higher than it is in neutral.
The engine does not need Diesel fuel to keep the cylinders lubricated, that is what the engine oil and oil control rings.

Also if all parameters are met when decelerating in gear, fuel is one hundred percent shutoff. Most manufactures call it something like DFCO (deceleration fuel cutoff). If DFCO is active when decelerating you will use zero fuel as compared to coasting with the car in neutral and engine running.

My logic (which is not always sound:eek) tells me that an engine uses more fuel when the RPM's are between 2-3k than when it is below 1k while coasting in neutral. Am I missing something here...by just taking your foot off the gas pedal, the RPM's are much higher than when in neutral.

So what I am asking is....does the RPM's have anything to do with fuel consumption while coasting, or is it just gravity keeping the RPM's high?

Thanks in advance for the education.
As I stated earlier if DFCO is active you will use zero fuel no matter how high the rpms are. Once conditions are to deactivate DFCO then fuel usage begins again.
 

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Try manual shifting into 6th gear.

Since the DSG is technically a manual transmission, are there any potential damage issues by putting it in neutral when going down a long hill? I know this can be hard on a true AT, but what about ours? What about when going back into gear? Best to put it in "D" or bring the RPMs back up a little bit first?
If you want to "coast" down the hill without the DSG automatically downshifting try using the manual shift mode and leave it in 6th gear. The DSG will downshift eventually even in manual mode but if you're moving fast enough, it should coast in high gear.
 

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Discussion Starter #13
Even coasting in 6th is still more resistance than neutral.
 

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Read the Manual

With the DSG transmission, it does not slow you down when coasting downhill in gear! The Owners Manual tells you all about this. I frequently let off the throttle before the top of a hill and coast over the top when I want to drift slowly down in speed. This transmission is great in bumper to bumper traffic, where I rarely use the brake pedal!
 

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Compression Braking or Not ?

Does the dsg transmission provide compression braking or not? Or in all gears but 6th? If it does brake the car going downhill, there is a fine line between the acceleration you might gain ( hence some free distance until the car coasts to/or resumes cruising speed ) and the fuel burned at idle. Can the idle rpm's be adjusted down to squeeze the fuel? At what expense / hazard?
Bill
 

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Since the internals are like a manual transmission, I don't see why not. Just keep in mind that it may not be legal to coast in gear. In addition, depending on how steep the hill is, it may be worth it to leave it in gear to minimize fuel use. When you put it in neutral the engine is using fuel to idle. If the hill is steep enough so that you can maintain speed with the foot completely off the accelerator, the engine is using no fuel. Everything else in between is up to the driver.

Don't know if it's best to rev match before putting it back into D or if it will do it automatically. Want to try and find out?
The DSG is pretty smart transmission and its fairly hard to outthink it. I find the "leave it in gear and lift off the accelerator completely" the best option but you have to keep in mind that as the car slows the transmission change down and it automaticaly revs the engine to do this. I have found the best driving mode is to always start in "sport" mode, apply a moderate amount of throttle and let the engine rev freely to about 2500 to 3000 rpm uniti it changes up. Once I've reached road speed I want I change into "Drive" and cruise in top gear. I judge when to start to slow down (eg traffic lights), when I can, so that I can lift off completely then drop back into "sport" mode as the intersection gets closer. The transmission changes down several times and I use minimal breaking to stop. Using this method on long runs I can get down to 4.9l / 100 km (48 miles per US Gal) from my 2.0 TDI. I haven't tried coasting with the DSG but gave up on that as a fuel saving method when carby's got replaced by fuel injection for the very reasons cover in this post. Plus it's too dangerous anyway, you simply don't have full control of the car should an emergency situation arise.

I'm was always taught that diesel engines perform best at revs and not labouring to hard they don't like puttering around at idle speeds for long periods of time as they soot up. Nor do they like labouring up a hill in too higher gear. These more modern diesels may be different but given I'm halfway through cleaning out "sooted" up turbo on one perhaps not. I've driven a lot of trucks and it was always run at 3/4 throttle under load in a gear that if you put your foot down it would accelerate. Driving a DSG in "D" all the time doesn't do that its constantly looking for the highest gear, consequently I use "S" a lot and its doesn't seem to have hurt my fuel economy one bit. Perhaps if the prevoius owner had done the same I would be cleaning a turbo out now!
 
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