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Old 10-24-2008, 06:19 AM   #1 (permalink)
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Coasting in gear question

Apparently I'm not getting this whole coasting in gear thing, and the engine isn't using fuel. Someone explain this to me. If you coast in gear, the engine isn't using fuel, but you don't coast as far. That's the theory right. Does this apply to all newer cars? If so define 'newer'. Also, at what point does the engine being using fuel again, when you push the accelerator or the brake? Someone help me out.

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Old 10-24-2008, 07:20 AM   #2 (permalink)
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Most of the cars that do that start using fuel again when the rpms reach somewhere near the normal idle rpm. The only times you should use DFCO is if you have mis-judged the distance to the stop light and are coming in too hot or if you are going down a hill that is long enough to build up too much speed. In the case of the hill, you may even have to start downshifting.
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Old 10-24-2008, 09:01 AM   #3 (permalink)
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I'd have to say pretty much all OBDII cars have DFCO (decelleration fuel cutoff). The engine will begin using fuel again if you press the gas pedal, shift to neutral, or drop your rpms below the idle speed as Larry said. He is also right in saying it should really only be used when you've misjudged a coasting distance, or you know you need to stop.
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Old 10-24-2008, 09:02 AM   #4 (permalink)
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Ok. But is this the case with all newer cars? If not what should I look for to tell if my car's engine will shut off if I take my foot off the gas?
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Old 10-24-2008, 09:05 AM   #5 (permalink)
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You can tell very easily with a scangauge!

If you don't have one (and you should) you can tell by coasting down in gear. Let the RPMs drop below idle speed. You should be able to feel the engine try to keep itself at idle rpms.
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Old 10-24-2008, 09:14 AM   #6 (permalink)
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I don't have a scangauge. Is this the only way to tell, by feeling if the engine is trying to keep itself at idle rpms?
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Old 10-24-2008, 10:05 AM   #7 (permalink)
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Well with a manual, one test I heard about it is getting up to like 20 mph and taking your foot off the gas. Note how the car starts to slow down. Now "shut down" the engine with your key (still in gear, still moving). If you keep slowing down at a similar rate, you have DFCO. If it suddenly starts jerking to a stop...you don't. Be safe trying this if you do.

If I am engine breaking from a decent distance (but not far enough to simply coast in) I usually downshift at about 1500RPM because I believe at 1400RPM my engine starts getting fuel again.
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Old 10-24-2008, 10:19 AM   #8 (permalink)
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I know some recent (5 years old) Hyundais don't do DFCO, but those are the only ones I'm aware of. My 1996 Honda does DFCO.
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Old 10-24-2008, 01:33 PM   #9 (permalink)
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Your THM-4T65E transmission is a four speed auto with a lock-up torque converter clutch (TCC) that works on 2nd, 3rd and 4th gears. When you lift your foot at speed, the engine is driven by the rear wheels and the control unit shuts off the injector pump. See:
4T65E Transmission Info

This is a very reliable and mileage-efficient transmission. However, like ALL automatics, you can damage this transmission by coasting in neutral, regardless of whether the engine is off or on, because it is lubricated by a front engine-driven pump that requires higher engine rpms at speed. Therefore if you coast at speed in neutral at idle, the pump will not be receiving sufficient rpms from the engine to properly lubricate the transmission.

I know you didn't say you were coasting in neutral, but I have noticed that some people on other threads were confused by this subject so I thought I should mention it.
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Old 10-24-2008, 02:09 PM   #10 (permalink)
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would my 93 civic dx 5spd have DFCO?

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