Very Low Vacuume, 7-8in hg

[Zelgadis69]

About 9 months ago i had my timing belt changed at a shop. Before the change my boost gauge read around 17in hg at idle. After the change it read 7-8in hg. It doesnt really fluctuate but this is a horrible vacuume leak.
The mechanic said i had a cracked xaust manifold and the o2 sensor may be getting some messed up readings. I knew about this and thought it may have just gotten worse and didnt really understand how that would mess it up that bad but i just figured he was right.
First i replaced my boost gauge with another autometer thinking it may just be broken, still showed 5-10. Compression test showed 155-160 on all cylinders. So i replaced the manifold and gasket. didn't fix the prob. then replaced the o2 housing and gasket still didnt fix the prob. I just gave up on it until about 2 months ago when i thought i should really start trying to fix the damn thing. All the stuff i'm going to list did nothing to raise my vacuum at idle.
Replaced o2 sensor, ISC motor, replaced all vacuume lines then just decided to remove and cap all of them appropriately, replaced intake manifold gasket, replaced throttle body gaskets, replaced TB seals, replaced TPS, replaced BISS o-ring, intake leak tested at 20psi(heard nothing), replaced injectors with 450cc and injector seals, replaced ignition coil with spark plugs (gapped correctly) and plug wires, ignition timing and valve timing are fine... i think thats about it.
My car runs perfectly besides having a very light rough idle. No smoke out the back such as a rich mixture. Vacuume shows 21in hg under deceleration and my turbo spools fast as hell to 14psi(where it's set). My car seems as fast as it should be (wooping honduhs and mustangs easily despite losing 2nd gear on my a/t, planning conversion next month or so).
I am just completely stumped and dont know what else to do. I plan to get a leakdown test done soon and replace my vavles but i think i would have noticed damaged valves. Any input would be helpful. Sorry about the long post, just kinda in the typing mood i guess.

 

[leet]

Check the timing, I bet it is off...

 

[92redman]

Yeah cheack the timing, if it is off then the exhaust pressure could be restricting the air entering your cylinders simulating a restricted exhaust. That "restriction" would cause you to have low vacuum. Other than that, it sounds like you have checked everything.

 

[ryanman]

I think the 2 guys above mean check the CAM timing, not the ignition timing.

 

[Robby Camero]

Yeah man, took me 4 times to get the t-belt on right! () Line up the crank mark, and oil mark. Then use a ruler to make sure the cam sprocket marks are deadly. I bet its off a tooh.

 

[Zelgadis69]

Valve timing is about all i could figure would cause such a vacuume that low but i have checked and am sure it is perfect.

 

[Robby Camero]

Are you sure you checked the marks with a straight edge??? It is VERY EASY to be off a tooth. It might look like its lined up but you cant tell for sure unless you use a straight edge accross the cams, with all your other marks lined up. The rough idle is a give away as well.

Like I said, it took me 4 trys to get it right. When the tensioner applies tension, it moves all the sprockets and could throw your belt off one tooth. It might not happen on the cams, thats why you should check it with all the other marks lined up then check the cams. If you did, sorry for bringing it up again.

Does if feel like your loosing power at all?

 

[Zelgadis69]

There no loss off power at all as far as i can feel and im getting 20mpg. There seems to be no bad side effects except a slightly rough idle. I checked the timing with a straight edge after matching up the other marks.

 

[4geze63]

I just installed a autometer boost gaguge and the vac doesnt go below 0 ever, whats could be the problem.

 

[6bolt2gen]

i am having a similar problem, i have checked the timing and it is dead on but i am also having a pretty loud noise when reved up and i do have significant power loss. it is hard to describe the noise, it kind of sounds like a supercharger at full spool.
any way any input would be great and if you fix yours let me know how and i will do the same
oh and by the way if the timing were off the compression would be low

 

[TwinOptiX]

what would be considered low? i have 150 compression across all cylinders on a motor with the same problem.

 

[bigjangin]

it sounds like your cam timing is off. Look at it again and make absolutely sure its correct.

 

[scuRRed whiTey]

or we can look at it this way...ur not having an power loss...nothing is making noises...

how bout u datalogg ur car...make sure its pulling the right type of timing and there is no phantom knock going on...itll read all errors that a sensor is putting out


ORRRR

we own DSM's they are proned to do the silliest shit ull ever think of
IT MIGHT JUST BE THAT ITS GONNA READ -8 vacuum...and ucant do anything about it...its just that way...

 

[Robby Camero]

Do another intake leak test and be SURE there is no leaks past the throttle body, like intake manifold gasket or something cause this can screw you up bigtime.

 

[6bolt2gen]

again though, if it were off on cam time it would have low compression and it should not vacuum 8in. but i agree this is the most tempormental,unreliable, yet fun car i've owned

 

[kyle h.]

i've got the exact same problem. every last detail, except mine idles at 10inHg. fun cars right!

i've tested everything and have a few other wierd problems..but what can ya do.

 

[1fast97gsx]

I have a similar problem only my vaccum reads 15 .. it used to read 17-18. I can't find a leak anywhere and my timing marks all line up. Car feels a little slower and gas mileage is horrible.

 

[1fast97gsx]

Could be burned valves

Yea that was my thoughts too ... car smokes white smoke at idle after startup. It stops after driving it for a few minutes ... but once I'm sitting at a light or something for an extended period of time it starts smoking again. ( It's not the turbo )

 

[Defiant]

I just installed a autometer boost gaguge and the vac doesnt go below 0 ever, whats could be the problem.

You're connected to a ported orifice instead of the main plenum.

 

[Defiant]

There seems to be no bad side effects except a slightly rough idle. I checked the timing with a straight edge after matching up the other marks.

You shouldn't have an idle that's rough at all. Something is WRONG, and the shop who did the work needs to FIX it.

 

[jotho]

Sure would like to know if anyone figured out the problem. I've got the exact same thing.
Did rebuild, now low vac, rough idle, runs fine, no loss of power. Very aggravating.

Zelgadis69 - Did you take your car back to the shop? I wonder if they screwed up the timimng belt and nicked a valve or 2.

Thanks,
Joe

 

[keltalon]

thread like these take up unusual amount of space! they post a problem but never post the results as to if the problem was fixed or not, big waist of time and space and should be deleted IMO

 

[HearMeSpoolin]

mine reads around -7 to -9 at idle... thought it was normal?

edit, now its reading -20.. just messed around with vac lines

 

[VaB90gsx]

I'm also having similar problems. I'm getting around 8 - 10 inhg and when I drive it around I can hear a whine similar to a supercharger. There is also a vibration at idle, but it goes away above about 1300 rpms. I don't wanna go around buying all these parts for nothing. I've check all the vacuum lines / ic pipes. The timing is money. There is no oil / coolant mixtures, and I have good compression. My intake manifold gasket has about 1k miles on it, along with just about every other gasket. When I'm looking under the hood I can hear the vibration is mainly coming from around the header area.

 

[beatshack]

i am having a similar problem, i have checked the timing and it is dead on but i am also having a pretty loud noise when reved up and i do have significant power loss. it is hard to describe the noise, it kind of sounds like a supercharger at full spool.
any way any input would be great and if you fix yours let me know how and i will do the same
oh and by the way if the timing were off the compression would be low

I had the same problem, it was a boost leak at the turbo outlet j pipe. Sometimes boost leaks whistle really loud. I'm assuming the sound doesn't really change pitch much, just kind of stays one pitch, like this: http://www.beatshack.com/exhaustvid.avi

That high pitched sound you hear was my boost leak, until I fixed it. No more supercharger sound