Been noticing low vacuum readings across the board recently. When I first went turbo, I would pull 22in.hg at idle, 25 on decel. Right now I'm doing 16-17 at idle and about 19 on decel. No power loss, still chatters 2nd, barks 3rd and 4th. I figured I was losing a cylinder, so I did a compression test. 200-190-185-185, so no major losses there (and not bad for a 150k motor with 20k of 13+psi in it). However, the 4th plug was significantly sootier than the others, and they were fresh plugs about a week ago. As soon as I get ahold of a leakdown tester I'm going to go ahead and do that, but perhaps I've got valve seals leaking?

And just for kicks, the specs of my planned build. Nothing fancy, going to try to keep it under $2k for the shortblock. Goal is to pick up a little power and a little response with more compression, and have a motor that will do 250whp stone reliably.

-Machine work (2mm over, deck) - $400ish? Haven't looked into it in my area.
-FM Weiscos, 9.5:1 compression, +2mm (if I can get them in that) - $600
-ETD rods - $450
-Boundary billet OPGs - $150
-ARP hardware - $250
-$500ish for head gasket, gaskets, seals, motor mounts, etc.

 

Leakdown test will discriminate, but from what you say it could be rings as well as. Does the motor use oil? Leaking rings makes it use oil, but the oil seals up the rings to a point such that it doesn't show up on a compression test, but it will on a leakdown.

 

Compression numbers doesn't explain the amount of loss of vacuum. A motor would have to be in very bad shape to show that much loss of vacuum.

What are your AFR's at idle? Timing? Does the vacuum gauge jive with the MAP sensor?

 

Air leak? I'm guessing bad gaskets, or loose bolts on the intake or head.

 

I idle at 28kpa, so I dont know why you are complaining at 22.

 

22 ~hg. not 22kPa.

I idle at 33-35kPa...I have a funny feeling I did my timing belt wrong when i replaced the pulley, I made the teeth jump so I had to redo it, always slightly lower vacuum than I was used to. I remember idling more around 29-28kPa.

 

I have a log from last year showing idle MAP at 27-29kpa. Current is between 32-34kpa. :( If I get bored I'll open the valve cover and see if I jumped a tooth again. The motor burns about a quart of oil at every track day, but around town it burns virtually nothing. Mabye half a quart in 3000 miles.

 

What timing value do you idle at sav? I've idled at 2* and had 40 kPa, and now I idle at 17* degrees and have 23-24 kPa. I'd check timing before assuming the motor is bad. Also what AFR do you idle at? I get the strongest idle at ~11.5-12:1. Idling 12:1 right now and it holds around 24kPa.

 

Sorry.
Third time I have made the same mistake.

 

Don't feel bad. I can't get thorough and through right. Stupid, but I fail at typing the right one or picking up that I used the wrong one.

 

Are your readings based off of MS or a vac/boost gauge?

 

I just pulled some logs and checked mine. Fully warmed up I run 29-32, right in the middle of your observed range. I wouldn't be surprised if this varied somewhat with climatic conditions.

Your oil consumption sounds pretty familiar too. When I spend a day tearing up the canyons or even just running down the freeway at extended high RPM, I'll go through about 1/2 quart in a couple of hours. OTOH, daily driving is about as you observed, maybe just a touch more (I might burn 1/4 per thousand.)

I try not to think about it. The motor will run as long as it feels like, and when it's finished, it'll let me know.

 

Protip: Make sure the vacuum line feeding signal to both the MS and the boost gauge doesn't have a tear in it. Replaced the plastic tee with a brass tee fitting and my idle/decel vacuum is 100% again. Idle 21-22, decel 24-25in.hg. Idle 27-28kpa.

Who knows how much boost my motor's been seeing recently

 

Little question--- where can you find brass tees?

 

Got it at my FLAPS. 1/8" FIP brass tee, 1/8"MIP-3/16" barb fittings for the vacuum line, 1/8"MIP-1/8" compression for the boost gauge.

 

apparently I drive like a *****.

Back when I had the vr6, I could throw it in 4th from 117mph and bark the tires, lol.