Jumpster74

iTrader: (1)

Installed my boost and AFR gauges in prep for the turbo, but am wondering if there is a vacuum leak or similar after looking at my readings.

I've looked up and down on the forums and I only see that most people get mid-20s psi of vacuum at idle. Nothing about decel. I'm currently getting ~16psi at idle, and ~22psi during decel. I'm not knowledgeable enough yet to understand if decel should pull more vacuum, but that seems to make sense. Could the restrictor t-fitting for my boost gauge off the driver's side of the cruise control line be causing lower readings? What type of values are yall getting?

18psi

iTrader: (76)

It's not psi, it's in/hg but yes you should be at least -20 to -21 at idle unless leak or low compression

hks_kansei

Yeah around 20in from memory.

And yes, decel (IE coasting with throttle closed) should be more vacuum than idle

borka

iTrader: (1)

My idle with ac off is 21

Jumpster74

iTrader: (1)

Whoops, mental slip. Didn't mean PSI.

Compression checks out and all my lines running from the IM are clamped. Guess I need to make a vacuum tester, but that restrictor wouldn't lower readings, right? Fluid mechanics isn't exactly my specialty haha.

borka

iTrader: (1)

Its air vacuum past the throttle body

SpartanSV

What is your MAP reading at idle?

Godless Commie

iTrader: (2)

What is your idle RPM, timing and AFR?

Any of these three will affect your vacuum if they are not just right.
Motorcycle guys adjust idle mixture for best vacuum, for example.

Jumpster74

iTrader: (1)

Sorry, headed to work at the moment so can’t verify, but I believe idle was just the standard 800 rpm. If not, then pretty close. AFRs we’re happily bouncing around 14.7.

MS is going in next week so don’t have MAP or timing sadly.

For now I suppose I’ll just test for post TB vac leaks.

hks_kansei

Normally on gauges the restrictor is to slightly dampen the readings so the needle isn't fluttering with the small rapid changes in vac as each cylinder draws in air.

So, no, it shouldn't lower the vac reading.

Jumpster74

iTrader: (1)

Ok thanks. That’s essentially the conclusion I had come to after thinking more about it. Verified the idle is 800 and AFR is 14.7

Also figured out the vac leak is from my TB butterfly valve. With the IAC hose removed from the stock intake tube and plugged with a finger, there’s hissing coming from the butterfly valve and if I remove the TB intake pipe, you can hear it leaking past and the car dies. Also If I spray some brake cleaner pre TB in the joint, rpms change, so a definite TB leak.

Kinda confusing me because with everything correctly in place, at idle I’m getting 800rpm / 16in/hg and with the IAC hose removed and plugged, I’m getting 400rpm / 17in/hg. Not sure exactly how to decipher that but I knew the IAC hose pulled vacuum so wanted to rule that out.

Jumpster74

iTrader: (1)

Im kicking myself. Just noticed the hole on the upper side of the 1.8 TB just before the butterfly... assuming this pulls vacuum for the IAC and explains my “TB vac leak”.

If so then I’m back to square one and I’ll have to wait till I install my megasquirt to really troubleshoot more. Can’t seem to find any other vac leaks.

Jumpster74

iTrader: (1)

We’ll what do you know, thought glowshift would be an ok brand to buy from but looks like I’ll be going auto meter from now on...

Used my compressor, some spare hoses, and a known working tire pressure gauge to check the accuracy of my glowshift boost gauge. Turns out the spring in the gauge is way off... my homemade tester was reading 15psi while the boost gauge was only reading 9

problem solved, kids please just do your build right the first time.

hks_kansei

Yeah, ebay sort of stuff can be hit and miss. It's always worth checking how well it's calibrated before use.

I've got an ebay gauge arriving today, first thing will be to hook it to the compressor and check how accurate it is compared to my proper vac gauge.
If it's pretty close, i'll put it in the car, if it's way off, i'll send it back.



One of the issues i've noticed is that a lot of the decent stuff, like autometer, VDO, etc are either hard to find for sale anywhere, or just look cheap and nasty (seriously, why the hell does autometer still mainly offer flood lit gauges, it's not 1974 anymore)

sometorque

iTrader: (1)

At idle with AC off it hovers between 21-22 in/hg. It jumps a little with the AC on IIRC (usually 17-18 in/hg but i forget exactly). more than that and you have an obstruction in the intake side. Less than that and perhaps vac leak or low compression.

Fireindc

iTrader: (13)

Just a data point, high altitude will also cause lower vac readings at idle. At 7k ft elevation I usually see 18-19 instead of 21-22 in/hg.