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Bad Vacuum problem...

transparentdsm

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Jul 27, 2011
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Cherry Hill, NJ
Ok so i did a boost leak test and all was well, i need it to stop raining so i can do a compression test now.. im sitting at - 10 in hg at an 800 rpm idle after warmed up, when cold my idle starts out at around 1800 rpms and is at - 20 in hg. when i drive the car and i put it in neutral it will drop to - 22 to - 24 in hg. i did not build the motor, i bought it from someone who claimed it has a evo 8 valve train in it. could the valve train cause it to be that high? my other thought was could it be a bad wastegate actuator?
 

James

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Apr 7, 2012
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Port richey Florida
Mine is 10-15 at idle 20-25 at deaccel. /ubbthreads/images/graemlins/dunno.gif
 

James

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Port richey Florida
Just went outside to double check. I'm 15-20 at idle.
 

transparentdsm

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did a compression test and im 145,145,138,135 and the motor hasn't been turned over in like a month. also did a timing check and everything is dead on. i don't know what to gather from this... could it be the turbo??
 

JNR

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ca
How does it behave? I mean, is it steady, or does it move around, etc.? Can tell a lot on your vacuum readings.

Sounds a little low though, as I presume you're close to sea level...How's your timing? IIRC, retarded timing will produce lower vac readings at idle. I would suggest checking your lines too to make sure there are no fine cracks, they seat all the way and so on...doesn't sound like a major leak, but little things like that *could* affect the reading too.
 

ktmrider

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Tempe, AZ
Turbo cannot affect idle readings.

Where do you have the gauge hooked up to/at? TB, direct manifold, FPR hose?!?

What kind of hose to gauge? Rubber vac hose will expand when hot/warm like you are seeing. Hard plastic lines are a pain but don't expand like rubber.
 

transparentdsm

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it stays steady at -10 at 800 rpm's, every line, gasket and seal is brand new. i just put the car back together and the only two things that weren't brand new were the motor, it was a stock rebuilt w/ evo 8 valvetrain and 8,000 miles and the trans was out of 1375 with around the same mileage. everything else was brand new or freshly rebuilt. timing and everything is dead on. i checked like 3 times.
 

transparentdsm

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Quoting ktmrider:
Turbo cannot affect idle readings.

Where do you have the gauge hooked up to/at? TB, direct manifold, FPR hose?!?

What kind of hose to gauge? Rubber vac hose will expand when hot/warm like you are seeing. Hard plastic lines are a pain but don't expand like rubber.




what if the actuator is leaving the wastegate flap open, won't affect the vacuum reading?

i have it connected directly to the manifold and im using the vacuum home that prosport prvides with there boost gauge, i have had this gauge and hose in a different car and it worked fine and gave proper readings.
 

turbowop

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Apr 29, 2001
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Yakima, WA
Turbo/actuator/wastegate has zero affect on vacuum readings, whether the flapper is wide open or not.

Maybe try a different boost gauge? I can't stand Prosport junk. And I own two of their gauges. Oil pressure one stays pegged at 100psi, even with the car not running. /ubbthreads/images/graemlins/banghead.gif New sender didn't even fix it, so I'm guessing the gauge itself went haywire. Pieces of sh*t.
 
Last edited:

5OF2k

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colorado springs, colorado
That doesnt seem all that low to me, honestly. And if it does have Evo valvetrain in it IE cams/etc, youre going to see slightly lower than average idle vac readings do to the intrease lift and duration of the evo cams. Is your idle lopey at all? If it is, it's completely normal to see lower vacuum at idle.

My .02

-Jake
 

JNR

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you're up ~6000 ft though and he's closer to sea level I would imagine.

Missed that you already checked timing; sorry for the double then lol...I was thinking you were using a diagnostic (i.e. quality) gauge for some reason; yeah, don't rely on boost gauges for a vacuum reading, unless they are nice units and even then it's good to double check with a genuine vacuum instrument (calibrated) if you're in doubt.

check out scenario 10
 
Last edited:

transparentdsm

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Quoting turbowop:
Turbo/actuator/wastegate has zero affect on vacuum readings, whether the flapper is wide open or not.

Maybe try a different boost gauge? I can't stand Prosport junk. And I own two of their gauges. Oil pressure one stays pegged at 100psi, even with the car not running. /ubbthreads/images/graemlins/banghead.gif New sender didn't even fix it, so I'm guessing the gauge itself went haywire. Pieces of sh*t.




ugh i hate reading that....
 

5OF2k

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colorado springs, colorado
Quoting JNR:
you're up ~6000 ft though and he's closer to sea level I would imagine.

Missed that you already checked timing; sorry for the double then lol...I was thinking you were using a diagnostic (i.e. quality) gauge for some reason; yeah, don't rely on boost gauges for a vacuum reading, unless they are nice units and even then it's good to double check with a genuine vacuum instrument (calibrated) if you're in doubt.

check out scenario 10



You do have a point in regards to my altitude, but my main point was the fact that the vac reading is going to be lower than typical due to the cam profiles. Thats all. The fact that its reading normal at higher RPM leads me to believe his motor is healthy as it is. Also, the compression numbers are within specs.
 

transparentdsm

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Jul 27, 2011
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Cherry Hill, NJ
yeah with 272 i would expect to see -13 to -14 in hg. i know what is in my car and what my specs should be, -18 in hg is what it should be with the evo cams and the 1000 feet elevation. my concern is that it isn't a vacuum leak because my boost leak tester isn't showing any air coming out and my gauge is telling me -10 in hg. im really puzzled..
 

transparentdsm

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Jul 27, 2011
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Cherry Hill, NJ
yeah. the motor has been driven around, about 120 miles on it since i put it in the car. always had the -10, but i thought it was leaking throttle body seals. i replaced them, becuase when i did a BLT i heard air hissing from there. i just parked the car to fix the problem and with the crap weather haven't had time in the last month to even bother with it, plus the sunroof is leaking so i cant drive it anyway.
 
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