In addition to my 07 Prius, I also have an 88 Toyota 4Runner, which unfortunately is a gas pig. However, I prefer it in certain weather/road conditions. Lately, I have taken to shutting off the ignition while idling at red lights and also while coasting to lights/stop signs (when there are no curves as the power steering cuts out with the ignition). The mpg seems to have gone up significantly just from gauge observations.
So my question is this: Am I doing more damage to my automatic transmission (with the shifting in/out of neutral) and/or engine (208k miles) than is worth the fuel savings?
Thanks in advance for any advice you might be able to offer.

 

Chris,

The transmission should be OK but the starter motor will give out a lot sooner than normal. You will also increase the wear on the starter ring on the flywheel. Depending on the cost of the starter and whether or not you do your own repair work, you will more than likely wind up losing on this method of "saving".

Hybrids use either the integrated motor that helps drive the car or (in a mild hybrid) an oversize alternator to assist in starting. Some of the hybrids have a "backup" conventional starter but do not use them in normal day to day operations of starting the engine.

 

GeorgiaHybrid is right. Starter Motor, which is also the Generator, may fail a lot sooner.

Depending on the lubricating system design (eg. piston oil jets), the ICE may not engine oil to all critical parts. This could affect life of ICE. Besides lubricating the engine moving parts, the engine oil is critical in cooling the ICE.

Shutting off while coasting to a stop is VERY DANGEROUS! Besides the power steering being cut out, the BRAKE BOOSTER does not function. The ABS does not function. I will never switch off the engine.

 

Forgive me but I believe you are confused (or else I am).

The OP's 2007 Prius would already be shutting itself off automatically so near as I can tell gpb5200 is talking about shutting off the ignition on the 1988 Toyota 4-Runner with 208k miles on it. I doubt the 1988 4-Runner's starter does double duty as a generator....



I agree that doing a "FAS" can be "VERY DANGEROUS". The PS does cut out (unless it is electric, in which case it comes back after the reboot) and depending on the vehicle steering effort can be greatly increased. On some smaller lighter hybrid vehicles hypermilers will pull the PS fuse so that they NEVER have power steering. Once you are accustom to driving without it you don't miss it (on smaller lighter hybrid vehicles).

The ABS does indeed continue to work, after a reboot delay of a second or two. You are without ABS for the time it takes to reboot but not the entire time of the FAS. One of the reasons you turn your ignition back on during a FAS is to re-enable ABS.

Finally as you mentioned the brake booster stops generating vacuum. This can be a big deal for sure. If your canister does not have any leaks and if you only need to stop once or twice the booster retains enough vacuum to allow the power brakes to work normally. As long as the system is in good working order (no leaks) and as long as you only need to step on the pedal once or twice the brakes will work as normal. The dangerous part is if the system has a small leak or if you bleed the vacuum off by modulating the pedal. In such case your power brakes will be gone and pedal effort will increase dramatically. This is potentially very dangerous!

 

If you are in a competition to save gas then yes it might be worth it if it saves gas... I believe engines use quite a bit of gas to start up. I would imagine you use somewhere between a half gallon to a gallon of gas idling. So for every hour less you spend idling you save $1-2. So you might just save a nickel in gas... not worth it in my mind. Particularly if you are the type that doesn't pay attention to when the light turns green and as a result a dozen people burn up a gallon of gas waiting for you and later passing you.

 

As the others have pointed out, turning off the engine while moving is a very bad idea not only for safety but lubrication. Without looking at the transmission, we don't have a good way to tell what it will do internally. The worse thing would be any reverse flow through the filter, unlikely, it would 'clean the filter' but putting the debris back into the transmission gears. But at 208k miles, this might not be such a bad thing. Of course a running 4Runner makes a better trade-in on a Ford Escape or other hybrid SUV.

Now the problem with manual auto-stop is driver overload. A better system is a microprocessor controlled system:

• wheels stopped AND engine warmed AND brake applied - stop
• starter motor temperature - if the starter motor is over 100C, suppress the 'stop'. Burning out the starter motor in hot weather and heavy traffic is not a good idea.
• 12V DC battery state of charge - if the battery is getting weak, suppress the 'stop'. Having a dead battery in hot weather and heavy traffic is not a good idea.
• brake off OR wheel turning - engine start cycle
• engine start cycle under microprocessor control

An automated system would be much safer because the driver can pay attention to traffic, their primary duty. But I have no problem with an automated engine off system even for older vehicles provided it doesn't result in starter motor burn out (may need a heavy duty starter motor) or battery depletion (may need a newer, heavy duty battery.)

Bob Wilson

 

I agree that an automated system would be better.

I'd like to see newer car come out with an even more aggressive auto FAS.There is no point in running an engine and wasting gas to get down a hill. Fuel cut does not stop engine braking. It would be great if someone would come up with an automated high speed FAS so that a vehicle's engine could be off whenever it is not needed, regardless of speed. Clearly such a system would be difficult to adapt to older vehicles but it wouldn't be too hard to design into a new one...

 

If you're that interested in engine and fuel control, I'm surprised you don't have a stick. I've done a lot of that "very bad idea" lots and lots, and can tell you that strange things occur with the cooling system too; but if you do it right you'll help cool off an overheated car.

 

I agree that doing a "FAS" can be "VERY DANGEROUS". The PS does cut out (unless it is electric, in which case it comes back after the reboot) and depending on the vehicle steering effort can be greatly increased. On some smaller lighter hybrid vehicles hypermilers will pull the PS fuse so that they NEVER have power steering. Once you are accustom to driving without it you don't miss it (on smaller lighter hybrid vehicles).

The ABS does indeed continue to work, after a reboot delay of a second or two. You are without ABS for the time it takes to reboot but not the entire time of the FAS. One of the reasons you turn your ignition back on during a FAS is to re-enable ABS.