I have a habit of going into neutral if I'm going down a long hill to save gas(it ain't cheap where I live!). I then read in the VW that this can damage your engine. Is this true? I have a 2001 Jetta GLX automatic VR6.
It can damage your brakes, which will cost you more than the gas you save, it can decrease your safety, which will cost you more but it will not damage your engine.
I might add that you are not going to be able to buy a cup of coffee with your annual savings doing this.
I'm not sure, but it may even be possible that you use more gas this way than by downshifting and letting the engine and brakes keep the car's speed in check. The reason I believe this may be true is that, when the throttle is shut off but the engine is being turned by the wheels, the fuel management system shuts off fuel flow to the engine. When the engine returns to idle, the fuel flow has to be restored, to remove the need to use the starter to get the engine running again.
So keeping in gear and allowing the engine to turn over at normal rpms with your foot off the throttle should use no gas at all, versus a small amount when the engine is idling with the transmission in neutral.
I'm not sure about the engine, but the transmission maybe.
For example, if you check your manual you'll probably see that it says in the towing section that an automatic should never be towed more than a certain number of miles with the car in neutral and drive wheels on the ground. This is because the hydraulic fluid in the transmission isn't being pumped around to lubricate things.
I'm not sure if that necessarily applies with the car's engine running in your case though (even though you're coasting in neutral your engine is running), but at any rate, automatics generally like to stay in gear and it's probably more wear on the transmission to be shifting in and out of drive.
Manual transmissions are another matter and they handle coasting no problem.
However there is the tradeoff in either case in that since you don't have engine compression braking that you might be using your brakes more, and it's even more up to you to watch your downhill speed too. I'm not even sure it saves gas that much either since even when in gear your throttle is closed (if your foot's off the gas).
I guess what I'm getting at is that I wouldn't do this coast-in-neutral thing with an automatic at all and even with a stick I'm not sure I'd bother either.
I've also noticed that in my GTI (where the temp. gauge is very reactive to changes in engine temperature, unlike new VWs where the temp gauge is pegged at 190F if the temperature is in a normal "range" of temperatures) that my engine cools off faster when going downhill in gear versus in neutral, probably because the water pump is turning faster and pumping coolant around better.
As long as in know, all of the recent cars doesn't need to be shifted to neutral to reduce gas. Older cars using carbureter (a device in disk shape that intake air and mix it with gas in specific ratio before pushed to the cylinder) dumps the gas mixture according to the engine rpm. But, recent cars using 'fuel injection system' (all the recent model since early 90's, i guess) do control the gas injection according to the acceleration. So, when you drive along the downhill and doesn't step on the gas pedal, no more gas will be supplied to the cylinder or very small amont of gas will goes in the cylinder.
You won't save gas this way. In overrun, modern cars cut off the fuel injection anyway. If you are using your brakes to limit the speed down a long hill they overheat. I'm not sure what changing from netutral to drive does to your ATM, but it is a safe bet it isn't good for it.
In short: you aren't saving fuel, you are abusing your baking system, and you may be damaging the transmission.
And, technically, fuel intake in a carbureted engine is not determined by RPM per se (i.e. as if there were some sort of gear or belt connection between the crank and the carb), but by the vacuum being generated by the engine.
IMHO, no, you won't harm the tranny. But, it isn't worth doing. The ECM and TCM know how much throttle you are giving (steping on the gas pedal or not), plus it knows engine load, car's speed, ect ect ect. When you are coasting down a hill, with your foot off the gas pedal, the ECM is going to cut your fuel supply since the engine is not under load.
I could see damage happening if you "misshift" though.
Similar question: is it bad to downshift an automatic transmission? if I am going down a long hill, it is bad to go into third gear, and coast (thereby slowing me down more than being in 4th/Overdrive). I know it helps slow me down more, but it is hard on the tranny?
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