Re: Redlines when I floor it. - Saab 900 Bulletin Board - Saabnet.com
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Re: Redlines when I floor it.
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 19 Dec 2008 06:22:11 Share Post by Email
In Reply to: Redlines when I floor it., np [Profile/Gallery] , Thu, 18 Dec 2008 14:56:26
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I'd inspect the lines carefully - I'll bet you've got a cracked line. When the line is at vacuum, the crack collapses in, opening it up. Under boost, the crack bulges out, somewhat sealing it. More common the other way around, but I've seen some funny hose issues (OK, not hilarious) where a crack turns into a one-way valve.

But if the car idles, that is empirical evidence that the manifold is at a lower pressure than the outside air (some level of vacuum). An engine is nothing but an air pump, sucking air in. At idle, the throttle plate is almost entirely closed, so the engine is sucking against a closed plate, dropping the pressure in the intake manifold. And if it weren't so, air wouldn't flow in. When the turbo kicks in (which it doesn't at idle), it forces air in, raising the pressure.

So if the engine is running at idle, the manifold WILL be at low pressure. If the gauge doesn't indicate this, it is an indication problem. It could be a gauge that has a mechanical issue and sticks, or it could be the hose. Probabilities are on an 18 year old hose in a hot engine compartment. And cheaper and easier than a new boost gauge. I always go cheap and easy first. Yes - I meant to say that, and I'm proud.

posted by 99.159.103...


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