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.

you might have low compression.get a tester or have it checked.

How's the car run? If the car's fine, the gauge may just not be accurate.

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.

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.

a bad bov could have a leaky diaphragm and account for it.

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.

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 :)

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

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

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

WTF dipstick?

I was thinking the same thing. Maybe he means using the dipstick to find TDC?

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.

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

Ahh, great! Here at 7200 above sea level, im sure the altitude would surely effect my readings if this is true.

Thanks!

i drove my car to colorado up in the high altitude.. vacuum didn't change much.

check for leaks!!

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

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

fwiw my 99 has always read ~18" of vacuum at idle here at sea level. Before and after the turbo (as if that even matters) and its well within compression specs.

ok so my vacuum is showing 18-19 now so thats good however my idle is still all rough whenever i rev it. In neutral if i rev it till about 3k and then let go of the throttle...the rpm goes down to about 200 and then goes back to idle. what the hell could this be? Seems like my idle air control is working so im clueless

That is DEFFINATELY a vac leak imo.

Check your bov to make sure its not opening near idle,

I had my bov way to loose, and when i revved it up to about 2k it would drop to 200 up to 1100 down to 200 up to 1100 pretty much indefinably.

Gl.

According to this, would my boost reading also be off? My gauge is showing 5 psi, does this mean its not actually giving 5 psi? Hmmmmm.

Someone enlighten me, id love to turn up the boost :bigtu:

no. Boost is boost, it doesn't change no matter where you are, however it may make your turbo have to work a little harder in order to reach the same pressure.

Im @ ~22in HG Vac, used to be ~10 ;p

Was fixed when I realized my bov was a piece of crap and plugged the vac line going to it...woah, +12in hg of vac! awesome!

lol, plug the vacuum line going to the bov, if it doesnt help, you bov ain't leakin. Easy test :)