Vacuum reading inconsistent
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This is a discussion on Vacuum reading inconsistent within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; I have been battling a rough idle / misfire that seems to move between cylinders at will for a couple ...

  1. #1
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    Vacuum reading inconsistent

    I have been battling a rough idle / misfire that seems to move between cylinders at will for a couple months. I have run injector cleaner, replaced plugs twice, moved coils, moved injectors, replaced fuel filter, checked coil leads, checked injector leads all to no avail. I can drive 100;s of miles then CEL comes back on. when CEL comes on it has been for each cylinder... never all at the same time. Ususally only 1 cylinder misfires at any one time, but if I don't clear the code, eventually all 4 will be added to the list.

    Car has no mods other than Cobb APv2 running stage 1 93 map, 138,000 miles.

    I just recently installed a boost gauge and have noticed peculiar vacuum readings. I have no problem hitting and momentarily holding 13.5 psi or so in 1st or 2nd (until gear change), so i assume I would be hitting target boost in 3rd, 4th, etc.

    What I have noticed is irregular vacuum readings. I can get in my car, start it up and let it warm up and go somewhere. While driving if I push in the clutch and hold it in, it will hold steady at 18 - 19 inHG (pretty much normal vacuum level). Then hours later I can get back in my car on the way home and under the same circumstances (car warmed up, moving holding in clutch) It will dip down to around 18 or 19, then rise up and hold steady at 12 inHG. Blipping the throttle does nothing to raise the resting vacuum level. No AC on, no wheel turning making the PS pump work lowering vacuum... just intermittent low vacuum. also worth mentioning idle does not improve when vacuum is at full compared to low.

    Sorry for the long description, just trying to be accurate. Is there some system that is only on sometimes, that could have a vacuum leak. Could this be causing the rough idle and misfire? any help or ideas would be appreciated.

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  3. #2
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    Did a compression test - 150 152 140 148

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    Registered User mudferret's Avatar
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    There's an excellent sticky about misfires here: IF YOU HAVE A MISFIRE CODE ,PLEASE READ THIS!!!

    I'd suggest a leak down test as well; compression doesn't tell the whole story.
    ...Rob.
    MMVI WRX - VF43 Airboy Tuned @ 3500'

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    Thanks for the link. I have read that and been following it with the low cost items first (fuel injector cleaner, plugs, injectors, moving coils, moving injectors, fuel filter, etc.). I was hoping that the vacuum symptom might be indicative of a specific problem. The next step is dropping 100 on a leakdown test, then if that is inconclusive, another 500 to have the valve clearances checked. I was just hoping these symptoms might point to something so to avoid spending 600 + for additional diagnosis with no chance of the money spent actually fixing the problem.

  6. #5
    Moderator Donkey's Avatar
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    The vacuum readings would indicate a vacuum leak. Check all of your vacuum hoses for cracks or being loose.
    Resident Adviser @ WTF Tuning, LLC
    2002 FORESTER S
    1998 FORESTER S




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