944S: what should my vacuum be?

gtroth · 08-08-2005, 06:48 PM

I'm preparing to pull my intake manifold to go after my sticky ISV valve (TPS and vacuum lines too, probably clean the injectors...). So I measured the vacuum before all this as a point of reference.

I Tee'd my mityvac vacuum gauge in at the fuel pressure damper (a mere inches of line to the IM) and got:

13 inHg at idle
18.5 inHg while reving

What should we expect (no luck searching)?

carsontc · 08-08-2005, 07:16 PM

I'm doing exactly the same thing next weekend. Tonight I Tee'd my mityvac vacuum gauge in at the fuel pressure damper and got 15 inHg at idle and know I have a vacuum leak by the way its running. Just a few minutes ago I spotted a loose hose under the intake manifold.

I'd like to know what's normal...I'm betting it more like 18 inHg or more.

tgcattle · 08-08-2005, 08:20 PM

I've ask this question before. I never got an answer. I have 18 on mine and my new fuel pressure regulator still showing 48psi. So i'm thinking that aint enough. I think but I aint sure the more vacuum on the regulator the less the fuel pressure should be. I'll continue to look for a big vacuum leak

tgcattle · 08-08-2005, 09:03 PM

well i been searching. the best i can tell is it should be from17 -20 at idle. I wonder if a pinched return line would make frp pressure high? prolly so i'm thinking

gtroth · 08-09-2005, 06:11 PM

Thanks, guys. Yeah, 13 inHg at idle sure sounds low, doesn't it? The thing is the car is running very, very well, except that the idle flops (drops perilously low) sometimes when the AC is on or sometimes when cold in the winter. It has stalled two or three times in the winter.

gtroth · 08-09-2005, 06:23 PM

Hmmm... intermittently, maybe. TPS clicks convincingly when I pull the throttle cable a touch, and the switch contacts measured correct at the DME connector (at least while I was measuring it).

gtroth · 08-10-2005, 03:39 PM

Oh, ok - TPS could switch normally even though the throttle hasn't really fully closed. Doesn't seem to me like the the idle would flop if the throttle's open a little...low vacuum and idle flop may be unrelated, I suppose.

gtroth · 09-22-2005, 03:16 PM

Ok - all back together now (all new lines under the IM, new ISV, capped the port on the venturi that's no longer used with the new hose, throttle clean and reseal, TPS, IM gasket, injectors WitchHuntered).

Runs great, idles perfectly, and vacuum is:

16 inHg at idle
19 inHg while reving

krahm · 09-23-2005, 06:29 AM

Hey gtroth, thanks for reviving this post. I was following it earlier because I was having vacuum problems, too. Unfortunately, my car is still running like crap and the 02 sensor is suggesting that the mixture is seriously lean (even with the AFM cranked to the richest setting). Vacuum is a bit lower than your readings at idle, with some fluctuation. I'm going to pull things back apart when I have time, so I'm curious about what you did with the venturi connector. What "new hose" did you use? What did you eliminate or bypass, etc.? I'm not seeing any venturi delete kits anywhere for the "S". Of course, I'll also go over the intake boots and connections again, too. Thanks.

gtroth · 09-23-2005, 02:12 PM

I just kept the venturi and capped off the port that the new hose did not use:

venturi thread

Big tip for the 'S' - remove the fuel rail, air-box, AFM, dip-stick, brake booster hose at the brake booster, other small hoses and obvious things, and then pull the intake manifold, throttle, and j-tube out as one piece.

krahm · 09-26-2005, 08:42 AM

Great! The pics really helped. Thanks for taking the trouble.