low vacuum / idle / timing questions

[unior]

question my car has been having some vacuum issues recently. Before I
would get a solid 20 inches at idle no matter what. I was also always getting
25 inches or so under deceleration. Now I am getting random readings.
Sometimes I get the normal vacuum at idle but I have seen readings as bad as
16inches at idle and 20.5 under deceleration. I checked my compression and
got 163, 162, 155, 155 - and I think I got lower readings on the last 2 since
I couldn't find what fuse to unplug to stop the injectors from firing.

So, could this be my timing belt stretching close to the limits? Also I have
had slight idle problems. Idles always stable just idles a little high
somtimes (1000 rpm) but the low vacuum situations don't match up with the
idle situations.

Should I be replacing my timing belt before I go 16g? Will the bigger turbo be
putting more stress on teh belt?

also anyone know if the kevlar timing belt is worth it?

thanks in advance, Jr.

 

[Defiant]

Originally posted by unior
Before I
would get a solid 20 inches at idle no matter what. I was also always getting
25 inches or so under deceleration. Now I am getting random readings.

So, could this be my timing belt stretching close to the limits?

Sounds like an issue with the tubing feeding that vacuum gauge, or the gauge itself. Has your gas mileage died?

Timing belts don't appreciably stretch. Usually they shear off a tooth, then the tooth next to it, then all the teeth on the crank sprocket at the time.

 

[unior]

dah.. that means the gauge may not be reading boost properly either..


hmm... maybe ill check it versus a mechanics vacuum gauge from the shop or something...

btw - the boost gauge should have been tapped into the intake manifold after the TB right?

 

[Lordpaxin]

yea, the boost guage should be hooked into one of the lines running out from the manifold, down where the injectors are. thers 3 i think, i have mine tapped into the line feeding the butterfly actuator

 

[Doug99RS]

The high idle and the different vacuum readings actually do match up. You may be getting confused with what you're looking at because I've done the same thing.

Inches of Mercury (barometric pressure) is not the same as vacuum (absence of pressure). Barometric pressure at see level is roughly 29.5-30.0 In/HG so in your intake manifold you should see the number closer to 11 to 12 inches of mercury.

Inches of vacuum you should expect to see roughly 18-19 inches on a stock and properly sealed engine. As the RPM's come up, the there will be less vacuum present and more pressure. Non-turbo cars that are properly tuned may see 0 inches of vacuum and WOT for a brief amount of time but most will still see some vacuum.

High idle means more air is getting in to the cylinders, more air in the manifold means a vacuum reading closer to 0 inches and a barometric pressure closer to ambient pressure (29.x give or take at sea level).

I would recomend checking for vacuum leaks, cleaning the throttle body then setting your base timing. The last two would be to better tune the engine and not specifically addressing the idle concern. I would not recomend adjusting the Base Idle Set Screw until you have found your high idle cause, cleaned the throttle body and set the timing other wise you will just cover up the problem.
Doug

 

[unior]

I understand how higher idle means less vacuum however my higher idle situations and lowered vacuum situations don't always line up. Lately, the idle has been pretty decent and is normally at about 650rpm.

The vacuum however seems to not give to much regard to my idle speed right now. Another thing: my "high" idle hasn't gotten any higher than 1000 rpm. Not to bad - just enough to piss me off.

My goal tonight was to run a new vacuum line to my boost gauge or a friends boost gauge to see if they are reading the same. I'm just not trusting my gauge 100% right now.

Tonight after leaving work my idle was 650 and my vacuum was 18" and 20 under heavy decelleration. About 10 minutes into the 30 minute drive i started getting my normal 20" at idle and 25-26 under any decelleration. That should rule out base timing right?

As for the idle. Could my isc just be starting to stick since it operates fine sometimes but not all the time? I was thinking that if I turn the power steering and the idle still adjusts some to compensate even at 1000rpm then the isc is still working.

And lastly I'm also suspicious of my intake manifold gasket. I know the old 90s had major problems with them flaking away and such. Has anyone had problems with 2g intake manifolds leaking?

thanks, Jr.