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Old 02-12-2007, 10:11 PM   #1
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Default Vacuum at 13

Well as the title states...my vacuum is at 13. Isnt it suppose to be near 20-21? I tried checking for vacuum leaks but didnt find much.. Can someone explain exactly which ones im suppose to look at just to make sure I did it correctly? and also can a blow off valve cause this? My car has a weird BOV that came with it that seems to be loose on the outer part where as the inner part is screwed tightly to my intercooler pipe.
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Old 02-12-2007, 10:30 PM   #2
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you might have low compression.get a tester or have it checked.
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Old 02-12-2007, 10:36 PM   #3
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How's the car run? If the car's fine, the gauge may just not be accurate.
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Old 02-12-2007, 10:55 PM   #4
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BOV won't affect it as it is before the throttle body (or should be). Chances are you have one of the following conditions:

1) Vacuum Leak (check the nipple cap on the rear of the intake manifold, it's the most common leak)
2) Timing belt is off a tooth or two (fairly common, if the car is running rough or seems to lack power in the low/top end this is probable)
3) Low Compression (unless you have over 150K miles this is unlikely)

Good luck.
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Old 02-12-2007, 11:04 PM   #5
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They bov won't affect it... but the line to the BOV could. Remember... the vacuum reading is between the throttle plate and the head... Not the turbo and the head.

I know my autometer vac/boost gauge is off by about 3inHG and 3psi. I'd double check the meter's lines and any others you might have fiddled with.
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Old 02-13-2007, 12:29 AM   #6
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a bad bov could have a leaky diaphragm and account for it.
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Old 02-13-2007, 12:33 AM   #7
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had the same problem, so I replaced all the vacuum lines on my miata. It doesn't have too many so it was easy. went from 18 or so to 21. Also ziptied all the vacuum lines too.
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Old 02-13-2007, 12:41 AM   #8
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BOV could be leaking at Idle......

are all your IC pipes connected securely? (assuming you are running one...)
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Old 02-13-2007, 12:45 AM   #9
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Quote:
Originally Posted by airbrush1 View Post
BOV could be leaking at Idle......

are all your IC pipes connected securely? (assuming you are running one...)
Large portion of BOVs are open at idle. This will not affect the vac reading... its a reading taken from between the head and the throttle body.

The only way the BOV is an issue... is if its diaphram(as mentioned) is toast/cut/cracked or the lines are off/cut/cracked etc.

Its not an idle/MAF issue afterall, it's a vac one
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Old 02-13-2007, 12:58 AM   #10
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wow you guys are awesome loll soo many things i can go check out now. Anyways sorry if I wasn't clear. I have a 1990 miata with 125k starion FMIC with a no name brand bov that came with the car. I just tried loosening the throttle cable a little because it seemed to be too tight and now my vacuum reads 15-16 and another thing my car has been doing is my idle keeps moving around.
I pulled the plug by the throttle body to test to see if the idle control was working and It was, because my idle went up to 1200.

And on a side note... I have already tried setting my idle with the idle screw and also connecting the diagnostic thingy...still the same thing
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Old 02-13-2007, 11:13 AM   #11
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my crank pully has separted (the timming mark was moving around). That caused me to set the timing off a bit and thus my hg/mm was off. Might want to inspect your timing with TDC markings and your dipstick
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Old 02-13-2007, 12:05 PM   #12
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I'm thinking of doing a compression test...anyone care to explain to me how to do it? or a link? I know that you're suppose to do both wet and dry and that you pull out your plug wires and put the other one in its place in order to do it. But thats all I know. And what are some numbers a typical miata should have around? Im assuming around 170-180? Thats what Ive heard. Im just hoping its not my short nose crank starting to die. There is no wobble or anything but It might be at its first stages
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Old 02-13-2007, 12:22 PM   #13
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WTF dipstick?
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Old 02-13-2007, 12:24 PM   #14
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Quote:
Originally Posted by magnamx-5 View Post
WTF dipstick?
I was thinking the same thing. Maybe he means using the dipstick to find TDC?
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Old 02-13-2007, 01:47 PM   #15
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Hmm im at about 17-18 in vac once its warmed up.

Never did a compression test, but the engine was rebuild about 10k miles ago, so i didnt think that was necessary.

Could it be giving me a bad reading because i have the boost gauge tee-d off of the same line that my wastegate is on?

Im gonna check for a leak, but i dont have a bad idle or anything like that.
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Old 02-13-2007, 02:18 PM   #16
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Fireindc,

There has been discussion about boost gauge accuracy and the effect of altitude on boost gauge reading. I wouldn't worry too much about your vaccuum numbers, I'm sure they fall with in the % error on most gauges.

Jay
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Old 02-13-2007, 02:31 PM   #17
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Ahh, great! Here at 7200 above sea level, im sure the altitude would surely effect my readings if this is true.

Thanks!
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Old 02-13-2007, 04:01 PM   #18
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i drove my car to colorado up in the high altitude.. vacuum didn't change much.

check for leaks!!
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Old 02-13-2007, 04:26 PM   #19
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According to the chart I'm looking at 7000ft above sea level should see a decrease of 22.8%. That would account for Fireindc showing 17-18 InHg.

http://www.anver.com/document/vacuum...20pressure.htm
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Old 02-13-2007, 07:39 PM   #20
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yeah it was to find TDC
and dont bother with a compression test. Get a leakdown test done because I have seen many cars show good comp numbers but fail on leakdown test with bad rings, etc
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