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Old 12-19-2008, 11:03 PM   #1
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Default No exhaust gas?

OK guys, here goes. I finally got my car running, and now there seems to be sudden problems. Before I tell all the problems, lemme tell the things that have tested to be fine. Compression, timing, turbo, bypass valve, the 305 injectors, the afpr, the walbro 190lph. now the whole DIY kit i pieced is fine, but we're showing a vaccuum leak to -10psi. Its supposed to be around -18 to 20 right? We've tested pretty much everything, but for some reason the car seems to be oddly misfiring and running rough. We drove the car two days before and it was tuned and running fine. We blew off an intercooler pipe a couple of times, but the last time it ran oddly rough after. We've tested for a vaccuum leak and we cant find one. there has to be a leak somewhere since were getting -10psi right? Also when we start the car and rev it while running in this condition, as it revs, there is less exhaust gas coming out. Its like as the rev goes up, theres less exhaust gas coming out. Do some of you more experienced DIYers and boost runners knows what could possibly be goin on?
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Old 12-20-2008, 10:35 AM   #2
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First, clarification on your units of measure is required. There's no such thing as -18 to -20 PSI, at least not on the surface of the earth. When speaking in relative PSI, negative numbers are relative to ambient atmospheric pressure, so on a day where baro is exactly 1 bar (14.5 PSI absolute), then a perfect vacuum would only be -14.5 PSI.

Now, if you're talking about inches of mercury (which is how most boost / vac gauges are scaled) then yes, 18 to 20 "hg would be about right, corresponding to ~33-40 kPa (give or take the accuracy of the gauge.)

At only 10"hg actually in the manifold, I'd expect the engine to be idling somewhere near redline. This strongly suggests that a vacuum leak, if present, is in your gauge or the tubing leading up to it. Simple test- disconnect the hose from the manifold, and using a hand-held vacuum pump, pull a constant vacuum on the gauge and hold it. If it leaks up to 0, or can't be drawn all the way to at least 25"hg, then you found the problem.
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Old 12-20-2008, 12:24 PM   #3
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Sorry for the incorrect units, but anyways the gauge works properly and was already tested. Any other ideas because it really shouldnt take 3 months to get a finished car to work. Its really making me wanna just go back N/A. Even if there is a big leak, why is there less exhaust gas at higher rpm than when it tries to idle?
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