Re: Ok, I don't get it.... - Saab General Bulletin Board - Saabnet.com
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Re: Ok, I don't get it....
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Posted by JimBlake [Email] (#141) [Profile/Gallery] (more from JimBlake) on Fri, 17 Mar 2006 20:22:40 Share Post by Email
In Reply to: Ok, I don't get it...., Aaron Gilbert, Fri, 17 Mar 2006 18:25:28
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When you take your foot off the gas to upshift, a carburated (or older injected) engine will run VERY rich because the air is suddenly shut off. The fuel isn't able to shut down enough. That's really bad for emmissions & the EPA got into nagging the car companies about that.

So now (since mid to late '80s) there's enough computing power in the car to run all these cool features. When the computer sees that the throttle is completely closed, yet the RPM is substantially above idle; it'll shut off the fuel completely. Never mind that the air isn't completely shut off...

This happens for a moment every time you upshift. But it also happens when you leave it in gear during long downhill coasting. And also when you simply take your foot off the gas 400 feet before a red light...

The fuel comes back on at some rpm, usually around 1500. That's just high enough so that it doesn't stumble & sputter as it comes back on.

I'll leave it in gear whether I'm planning to stop or not. If I need engine braking for mountain downhills or just for fun, I'll downshift. Otherwise I'll leave it wherever it is. If I'm actually stopping, I'll clutch out at around 1000 rpm or so.

Normally, I hold the clutch out with the tranny in gear unless it's gonna be a long time. I know that's supposed to wear out the T-O bearing, but after 8 or 9 cars & well over 700k miles, that hasn't been a problem.

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