Okay so I was test fitting all my gauges and running all my wiring and such.. anyhow, the boost gauge shows about 22-24 vac at idle which is good.. but when the A/C goes on and both radiator fans are on, the gauge reads 20vac but it kinda vibrates there.. it's a slight vibration on the needle.. visible, but not enough to get off of 20vac. Anyone else experience this?

I've never seen this on any other car (non-Scion) that I've worked on.. I just want to make sure there isn't any issue before I do the install this next weekend.

 

which boost gauge do you have.. i have a digital gauge that constantly changes from 18-21 on idle but the vibration on idle i usually notice when idle afr goes a lil lean usuall 15.1 and vibration gets stronger when it gets leaner than 15.5.. enriching idle might help the vibration but a constantly changing manifold pressure could cause that too, ac and fans causes change in manifold pressure too.. i bet it gets crazier if you have cams

 

I have an Autometer Cobalt mechanical boost gauge. The idle vac reading is rock solid at 22 vac.. but it only "vibrates" when both fans are on. I don't know AFR cuz my w/b isn't installed - and I'm not even boosted yet.

The vibration isn't significant enough to change the reading.. it's just at 20vac but shakes there... until the fan turns off then it's okay again... when fan comes back on, it does it again.

 

maybe the vacuum reading is constantly changing so it kinda vibrates.. but i do notice that vac reading is a lil more unstable when fans or ac is on.. i dont know but it prolly how our stock tune is.. just get you kit installed and you will worry on something else

 

its normal for it to vibrate at idle with the a/c on

 

Cool.. confirmation is good.

I hope more people chime in with the same. This is a phenomenon I have yet to witness on other car's I've installed mechanical boost gauges on.

 

yeah most dont notice coz they dont install gauges when theyre stock.. i wish i did so i will know how stock numbers are and maybe tune it like stock at vacuum.. if you can put your WB on stock setup. drive it around that would be better

 

I know! Trust me I wish I could... I don't have jack/jack-stands and not even allowed to do it in this complex.

I already have an extra bung welded for the w/b on the stock S-pipe.

 

This happens to me to..

 

lol i could relate, i spend hundreds even thousands having a shop do stuff that i can do just coz im not allowed to do it in my covered parking haha.. do it man, if you know the stock numbers you can really have the car tuned like a factory boosted car

 

it's a normal thing, vacuum systems aren't always at one stable position, it's constantly changing. dont worry about it

if it goes lower than 20 then there might be a leak though.

 

Actually I've experienced this already. I installed the TT vac block and had to "plug" one of the fittings I wasn't using. Well to plug it I used electrical tape.. and that came off shortly. The effects were a lower number on the vac side.. like around 12 or so... and a CEL code P0171 (system too lean). Coincidentally, this was the same code I got when my O2 went bad. Anyhow that's fixed now.

 

my idle vacuum is 17.5-22 but it varies on different conditions, i didnt find any vac leaks so far but its always around 18-19s most of the time.. you guys think i have a leak?

 

The TC pulls a lot of vacuum and it can throw off the guages. I had a similar problem which was corrected by installing a restrictor in the nylon line going to the boost guage. You don't want to restrict it too much, just a bit to slow down the flutter.

 

Good info to know. Does anyone have a list of 'expected' readings for Idle, cruise, etc...?

 

Idle should be ~22/23 vacuum and cruise depends on the gear and throttle position. With very litle input it should be close to idle figures.

 

wouldnt vacuum reading affected by other factors too like air flow, temp, idle rpm and such.. im check again my vac setup for leaks lol

 

Sure, vacuum is your engine's ability to pump air. If you have a leak, that could effect it. So could a bad valve or ring.

 

my vacuum sits at about 17-19 at idle and under decel it is around 22

 

exactly like mine