Low Vac

[I-luv-em-all]

my car shows about 10,11,12 vacuum on my auto meter boost gauge, i think thats a bit low, if it is how do i change it and if i have a leak how do i check for it, i noticed that when i race my vac is at like 11 or whatever and sometimes after the race when i let off it seems to go a bit higher to like 15 or so......ya beats me what do u think?

 

[psychlow]

You're almost always going to get higher vacuum when decelerating at no throttle than at idle. The ground is putting a force on the engine, making the engine work, but all the pistons can do is suck air from inside the intake, and your throttle plate is closed.

There are quite a few things that cause low vacuum...

Offhand, I can think of -
- Vacuum leak (of course)
- Bad A/F ratio (MAF, fuel pump, ECU, injectors?)
- Bad valve or ignition timing (CAS misadjusted, t-belt slipped?)
- Incomplete ignition (coil pack, plugs/wires?)
- Plugged exhaust (in neutral, will have lower vacuum at a sustained 3000RPM than at idle)
- Worn piston rings
- Blown headgasket or burnt/bent valve(s) (vacuum needle bounces around at idle)

Start a process of elimination for as many items on this list as you can. First thing I would do is test for leaks. You can do so by following the boost leak testing procedures in the VFAQ.

If you suspect it's one of the latter three suggestions, do a compression check on the engine to verify that it's a sealing problem.

Oh, and FYI, you may want to use a parts store vacuum gauge tapped into the BOV vacuum hose to verify that the readings your in-car gauge is giving you are correct.

 

[Defiant]

You're tapped into a bad spot, or have a kinked sensing line. You should get about 28 on deceleration unless your motor's completely toasted. Idle should be around 15-18"Hg.

 

[I-luv-em-all]

really, well i think i have almost all of those problems u guys listed, which remindes me, anyone one want to buy a 91 plymouth laser rs turbo ..........time to get a 92 awd 5spd.

 

[psychlow]

Idle should be around 15-18"Hg.

Mine got 10inHg at idle with a bad A/F ratio (bad pump voltage).

(Verified with a shop gauge.)

 

[GinNBoost]

Idle should be around 15-18"Hg.

Really? I thought 20 Hg was what it should be at idle. Cool, that means I don't have a vaccum leak after all because my vac used to 21 Hg during the winter and now its at 17. Could the weather have caused that?

 

[psychlow]

The air is more cold (dense) in the winter, so that might make a difference on your gauge reading.

19-20inHg is good for those right at sea level, 15-18inHg is decent for those of us at higher altitutes.

 

[crankbender]

I must say not a bad list.

I should note that a vacume leak alone will have other symptoms and should be easily found .

Defiant is correct about your readings when decelerating. If you can't pull over 20 while coasting down in gear then something is very very wrong. Compression check the engine first then check the cam timing.

I donno about yall but i pull 22-24 in hg when the ac is off......stupid stupid auto meter gauges arn't acurate to more than 2-3 psi/in hg.

 

[ProjectTalon130]

You're all forgetting one more thing that i had happen...is it an electric boost guage??? My Auto Meter Cobalt Series is with a MAP sensor and I didn't have a constant power source at first for the Zeroing out and it was reading the exact same vac in Hg's, when i found a constant source off the ECU it ran 16 to 18 Hgs....maybe this will help..then again my boost was off too...unsure if this is the case??? just another thought .