Coasting in neutral

Joined
Jan 2, 2004
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Don’t do it. In many cars, the TCC solenoid is still energized and the engine will windmill coasting down hill. There will be no fuel supplied until you step on the gas or apply the brakes.

Also, in some cars, the oil pump in the trans won’t spin in N, and you’ll maybe grenade it if there’s a lack of lubricant.
 
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The engine will shut down if the ECU shuts off all gas to the engine. What happens, at least in BMWs, is the ECU (DME in BMWspeak) runs a very lean mixture, usually about 16.5:1 air/fuel mix. The optimal mix is 14.6:1 air/fuel mix by mass.
The torque converter clutch locks which causes the wheels to continue driving the engine despite there being no fuel while coasting. The engine isn’t making any power, but it’s not “shut down” since the ECU is still in full control of it. Some manufacturers use a “lean cruise” mode, but that’s while cruising at speed.
 
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What you think you are gaining is minimal and results can be catastrophic when trans internals have to suddenly engage with car rolling at highway cruise speeds. It’s just playing around with your car. It is yours so do as you like. You asked, we respond.
 
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The torque converter clutch locks which causes the wheels to continue driving the engine despite there being no fuel while coasting. The engine isn’t making any power, but it’s not “shut down” since the ECU is still in full control of it. Some manufacturers use a “lean cruise” mode, but that’s while cruising at speed.
My car has a manual transmission.
 
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With a manual transmission, the engine continues to use a small amount of fuel (whatever is required to keep the engine idling) when coasting in N.
When coasting in gear, the fuel injectors actually shut off, and the car uses no fuel at all.

But it'll also decelerate more coasting in gear compared to neutral. Driving the engine
requires power. That's probably why many modern automatic cars are coasting in neutral,
at least in 'Eco mode' from factory (Euro spec at least). NA spec cars may be different.


Drop a manual transmission into neutral and release the gas, and your engine automatically returns to idle, where the injectors must pulse to keep it running. Worse mpg than leaving it in gear.

Not necessarily. You're ignoring the fact situations exist when coasting in neutral
either keeps speed (slight decline) or provides the desired slight deceleration when
approaching traffic lights or some different reason to stop.


Btw., hasn't this horse been beaten to death many times?
.
 
Joined
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Also, in some cars, the oil pump in the trans won’t spin in N, and you’ll maybe grenade it if there’s a lack of lubricant.
That's why you pull the driveshaft to flat-tow or dolly a RWD automatic vehicle. I guess the pump would work if the engine is running, but that could depend on the specific model.

The discussion has expanded a bit from the OP's question about coasting down a long grade at 85 mph in neutral
 
Joined
Sep 23, 2008
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Does the auto trans lube when coasting in N?
I think most do, as for some vehicles you can flat tow them with the engine running in N.
I used to coast in N many times a day with my 1995 Neon for 100k miles, and autocross it, and did some track days. No problems with that 3spd auto atleast. My longest coasting was only 2 miles though. The real advantage of coasting of N is not having the engine braking slow you down so you need to used the gas again. With the Neon the engine was at 2800rpm 60mph so engine braking is quite significant. On my Outback its at 1600 rpm rolling down a hill at 60mph so engine braking is much less so I leave it in D.
Also my scangauge shows total fuel cut only on my light trucks at low rpm. None of my cars have shown true fuel cutoff at 60mph in top gear? If I down shift to 3rd at 60mph then it shows full fuel cut at 4000+rpm but of course doing that provides alot of engine braking, too much for my hills.
 
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As stated before several times: Modern fuel injected cars completely SHUT OFF fuel delivery when coasting in gear. Going to neutral and forcing the engine to idle ADDS fuel to That downhill trip. You may add speed in neutral but you don’t save gas.
Any idea if 1980s ECU does this too ?
 
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