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Kyle C
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Engine Vacuum Test
I did a vacuum test on my engine using the vacuum port on the very front of the intake distributor (elbow for AC idle increase) and got a very erratic reading which bounces between ~19-15 inches mercury. Did I test at too large a vacuum port, or does it not matter? If not, does this indicate bad valves?
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1974 911 1978 928 5spd hillclimb special 1984 928S 5spd, 1986.5 928S auto 1983 944, 1984 944 track car 1995 Saab 900 S, 1997 Saab 900 Turbo, 1999 Saab 9-3 S |
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08-02-2010, 03:14 PM
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Registered
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Try it at different spots looking for the same erratic readings. If consistently erratic, you may have a leak.
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08-02-2010, 03:59 PM
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Madman
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depending on the location it could be too close to the intake ports or a burned intake valve (also depending on the speed of the vibrations) I recommend a compression test.
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1979 Porsche 928 3sp auto, black (gone)   Yamaha FZ6R, Raven Black. (gone) 1997 VW Jetta GLX VR6 swap in. black (gone) 1988 Jeep Cherokee 4.0 lifted, mudders, roll cage, black (wife's) |
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08-02-2010, 10:16 PM
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Kyle C
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I will check some other ports to see what I find.
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1974 911 1978 928 5spd hillclimb special 1984 928S 5spd, 1986.5 928S auto 1983 944, 1984 944 track car 1995 Saab 900 S, 1997 Saab 900 Turbo, 1999 Saab 9-3 S |
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08-03-2010, 03:23 AM
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Madman
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a compression tester is about $10 from harbor freight, cant go wrong there.
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1979 Porsche 928 3sp auto, black (gone) Yamaha FZ6R, Raven Black. (gone) 1997 VW Jetta GLX VR6 swap in. black (gone) 1988 Jeep Cherokee 4.0 lifted, mudders, roll cage, black (wife's) |
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08-03-2010, 11:26 AM
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Kyle C
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I will take vacuum readings at the front fuel pressure damper (with t connector) and vacuum advance (with t connector + revving up a bit) and compare. I should be able to do a compression test at school within the next week.
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1974 911 1978 928 5spd hillclimb special 1984 928S 5spd, 1986.5 928S auto 1983 944, 1984 944 track car 1995 Saab 900 S, 1997 Saab 900 Turbo, 1999 Saab 9-3 S |
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08-03-2010, 02:47 PM
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Kyle C
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Further vacuum testing gave the following results:
- Intake distributor front port (large diameter vacuum hose): rapid fluctuation between 19" and 17", sometimes down to 16" - Fuel pressure damper (front center of engine): steady 18" (small fluctuation, less that .25" either direction) These readings were taken at idle. Interesting thing is that after revving the engine to 1500 RPM about 5 times, the reading at the intake distributor then held 18" at idle with a rapid fluctuation of no more than .5" either direction. How would small throttle blips then returning to idle change engine vacuum???
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1974 911 1978 928 5spd hillclimb special 1984 928S 5spd, 1986.5 928S auto 1983 944, 1984 944 track car 1995 Saab 900 S, 1997 Saab 900 Turbo, 1999 Saab 9-3 S |
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08-04-2010, 04:38 PM
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Registered
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This is a very interesting approach to troubleshooting. What's inspiring this anyway? I can't remember. IPhone surfing is troublesome sometimes.
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08-04-2010, 04:50 PM
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Kyle C
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I am doing these tests now because 3 months ago before I replaced injector seals, intake gaskets, etc, I had several drivability issues (cold and hot start). I did a vacuum test then, and it was very erratic, I don't remember exactly but I think the readings may have been worse. So i retested after I did some work on the engine to see if anything changed (besides the cold/hot start issue which is now gone - injector seals were toast). Either way, my end goal is to evaluate the condition of the parts of the engine I did not work on (bottom end and heads) as I have virtually no service history on the vehicle.
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1974 911 1978 928 5spd hillclimb special 1984 928S 5spd, 1986.5 928S auto 1983 944, 1984 944 track car 1995 Saab 900 S, 1997 Saab 900 Turbo, 1999 Saab 9-3 S Last edited by Opelotus; 08-04-2010 at 05:17 PM.. |
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08-04-2010, 05:15 PM
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Risky Bidness
