Reasons for bad vacuum reading on boost gauge?

[Deadly BlaZe]

I have a friend who's getting a reading of 11-13 inHg vacuum on his gauge at idle. What are the reasons for why this could be so off? I've read normal vacuum is 18-20 inHg. The car is a 1gb manual with 2g mas and 16g turbo with injectors and a rich tune on the s-afc. I've searched, but couldn't find anything iron clad. The car also idles like crap when it starts up. Is it possible it's just cause of the tune/idle, or could it be internal damage? Besides the idle, the car runs pretty good and pulls pretty nice.

 

[xxraptor660xx]

Seems like a boost leak to me. Test it and let us know..

 

[subie22]

yea mind idles at like inhg 16 or so.. i must have boost leak somewhere, i was also told that the rings could be bad... Doesnt burn oil, just leaks it typical dsm... But i love her..

 

[oldman]

Things that can affect your vacuum reading, boost/vacuum leaks, valve timing, ignition timing, RPM and AFR.

 

[absolute_DSM]

Wheres the gauge tapped in to?

 

[Black_Bullet]

Or monster cams .


Im tapped to the fpr and I read -20 vac at 900 rpm idle with a slight boost leak somewhere at the tb...
From what ive seen vacuum leaks seem to be the biggest culprit to cause poor idles.

 

[Defiant]

A boost leak isn't likely to affect your vacuum reading. The vacuum is only going on between the inside of the throttle plate and the back sides of the valves, not a likely place to find a boost leak.
A kinked sensing line, or having the line hooked up at the wrong location will give you a bum reading. Jumped cam timing will as well, but that usually results in a lousy idle, as do hot cams.

 

[oldman]

A boost leak isn't likely to affect your vacuum reading. The vacuum is only going on between the inside of the throttle plate and the back sides of the valves, not a likely place to find a boost leak.

Don't do much BLT do you? Some of the most likely leaks are actually located in between the throttle plate and intake valves.

 

[Black_Bullet]

Like inj seals, intake mani and tb gaskets... Ive fixed all those leaks though.
Im getting a leak somewhere around the back of the tb near the throttle cable pulley.?

 

[a95gsx]

A couple of areas to look at after the TB are the injector seals and any/all vac tubing. The stuff looks fine 'til you touch it and then the cracks become apparent.

Since the car runs ok on top cam timing isn't a likely culprit. As we don't have mechanical advance I doubt timing is gonna be an issue either.

 

[oldman]

Im getting a leak somewhere around the back of the tb near the throttle cable pulley.?

TB shaft seals.

As we don't have mechanical advance I doubt timing is gonna be an issue either.

What do you mean by this?

 

[bvass]

I had a vacuum leak on mine causing the guage to read low vacuum and boost was going up to 7-8 PSI on the aftermarket gauge while my stock gauge was still showing it all the way up.

I have tapped my gauge to the FPR line and the vacuum leak was in the hose that goes from the intake manifold to the FPR. As mentioned before a kink somewhere can have the same effect.

 

[a95gsx]

What do you mean by this?

Back in the day timing advance was done mechanically in the distributor. It was not uncommon for the springs and flyweight mechanism to seize or jam locking the timing at a certain position causing issues. We don't do it this way (and have even ditched the whole distributor idea) hence my declaration that we don't have 'mechanical' advance as timing is all done electronically. But you already know this......

just edit real quick to add if base timing was off the car probably wouldn't be running too good up top.

 

[oldman]

Back in the day timing advance was done mechanically in the distributor. It was not uncommon for the springs and flyweight mechanism to seize or jam locking the timing at a certain position causing issues. We don't do it this way (and have even ditched the whole distributor idea) hence my declaration that we don't have 'mechanical' advance as timing is all done electronically. But you already know this......

A 1G CAS is adjustable, that is why I listed ignition timing as one of the possible causes in my original post.

http://www.plymouthlaser.com/timin.htm

 

[a95gsx]

Which is why I mentioned that it wasn't likely the culprit since the car is said to pull pretty nice up top. If base timing was off far enough to kill vac at idle it'd pretty much wreck the top end too.

 

[oldman]

Which is why I mentioned that it wasn't likely the culprit since the car is said to pull pretty nice up top. If base timing was off far enough to kill vac at idle it'd pretty much wreck the top end too.

Since the car runs ok on top cam timing isn't a likely culprit. As we don't have mechanical advance I doubt timing is gonna be an issue either.

Back in the day timing advance was done mechanically in the distributor. It was not uncommon for the springs and flyweight mechanism to seize or jam locking the timing at a certain position causing issues. We don't do it this way (and have even ditched the whole distributor idea) hence my declaration that we don't have 'mechanical' advance as timing is all done electronically. But you already know this......