Vac on decel

[andrewjscott]

my motor has been recently rebuilt...all is well 5k into but but my vac readings are a little strange. I've got 19-20 inHG (depending on the day) at idle only 20-21 on decel...I shuld be getting 24-25, correct?. I have no drivability issues what so ever. What could this be?

 

[Talesin]

Depends on your altitude and the density of the air on a given day. I idle at 18-20 at home, decel anywhere from 21-24. Add some altitude and I idle at 15in/hg, decel at around 17.

I wouldn't worry about it personally, but if you want to be absolutely certain, pull each vac line and test it. Then set the crank so only one set of valves is open (ie: no overlap), cap all the vac nipples, put a pressure tester on the turbo intake (essentially capping it off) and apply pressure to the intake manifold. It'll probably leak back a little through the TB to the intake piping, but if there's a more severe leak you should be able to locate it.
Most likely location would be the intake manifold gasket to the head.
Remember: a vacuum leak is after the throttle body. There's very few suspects.

Now I'm trying to remember which vac lines are supposed to leak... I vaguely remember something about the EGR system having a vac line involved that does so, but I cannot be certain. Anyone else have information on that? Also, most MBCs will leak a bit of pressure... usually only an issue when boost leak testing (which would be a good idea at the same time), but something to keep in mind if it starts to hiss.

 

[andrewjscott]

I have done a boost leak test and i plugged my MBC to test...all good.

I'm right at sea level at all all times. I don't have any vacuum lines except for the FPR....they're all capped off. Before the rebuild I had 15-16 at idle, and 22-23 on decel. The idle vac went up, but the decel vac went down....

 

[olmytsi]

19-20 is accurate, thats what i used to get at idle on stock cams.

 

[andrewjscott]

19-20 is accurate, thats what i used to get at idle on stock cams.

I know my idle vac reading is fine....Its the decel vac reading that i'm curious how it went down by 5 inHG after a rebuild.

I'm still running the nylon hard line for my boost gauge, I guess a kind could cause a lower than accurate reading, especially at the extremes of the spectrum.

 

[I_eat_DSM's]

Have you done a leakdown test or at least a compression test lately? I'd recommend it because that will eliminate some variables.

 

[andrewjscott]

compresson showed 165's accross the board.

 

[I_eat_DSM's]

It's beginning to sound like you have a leak that doesn't appear until somewhere around 22inches of merc which is why your readings are so close together. As long as you don't have any diveability issues I wouldn't worry about it. The problem will become greater soon anyways if it's going to be an issue and then it will be much easier to diagnose.

 

[Defiant]

Or, high-vacuum could be sucking a line closed.

 

[oldman]

The gauge has also never seen more than -21inHG so it's also a suspect, test it with a vacuum pump.

 

[andrewjscott]

The gauge has also never seen more than -21inHG so it's also a suspect, test it with a vacuum pump.

The gauge has seen 26inHG before the rebuild. So I'm sure the gauge is fine

I'm gonna replace the hard line with a rubber one. I'll also check that the single vacuum line is not being sucked closed.

 

[andrewjscott]

There was a small kink in the hard line, replaced it with a soft line. I am back to 25 on decel.