vacuum -10hg @ idle

[yajreckless]

What can cause my vacuum to be so low? Idle fines, car drives/pulls great. Everything stock. -10hg at idle and will go probably -11hg-12hg at most after acceleration or releasing the gas paddle. Compression 150-150-50-150. Checked all hoses. Gauge reading -19hg on my brothers 99gst. What can cause this bad of a vacuum leak?

 

[crimsondragon]

I think you need a re-evaluation of what is considered running fine. If your motor is running on 3 cylinders, it's not going to be pulling as hard as 4. Check for vac leaks.

 

[sschambers]

"Normal" vac is anywhere from 17 - 21...
Re-check EVERY vacuum line.
*Hint* - check your EGR valve. I had an EGR gasket fail once, causing extreme low vacuum/crappy idle.

EDIT> just realized one of your cylinders is running 50psi? I'd be more worried about that than a vac leak...

 

[snowborder714]

You have a big problem if you have a cylinder that's reading 50 at a compression test.

 

[yajreckless]

typo, compression test was 150 across the board.
Smooth idle, smooth acceleration, drives perfect except for the low vacuum.

 

[yajreckless]

Would sucking up seafoam through one of the vacuum hose do the trick in finding the leaks?

 

[andy4g63]

My vacuum is like 10-11, with BC280's, OS valves along with some other things, but this is normal for my motor. Even though, after acceleration, when I let off, my vacuum goes to over 20+in/hg. So you have some kind of a problem. Most likely it is a boost/vacuum leak. Do a BLT and Let us know.

 

[e85_4g63]

You could do boost leak and leak down test's.

 

[sschambers]

So it idles perfectly at 750? Check your vac line that goes to your vac/boost gauge, maybe it's kinked?

I know every vac leak I've had, it made my idle screwy. But again, check the EGR gasket. Also check the PCV vac line, solenoid vac lines, all of them. And do a boost leak test while you're at it.

EDIT> are you 100% sure everything is stock? Is this a car that you recently got? If so, maybe there were cams that were put in. Having lumpy cams would cause low vac. Just a thought...

 

[crimsondragon]

I had a similar vac leak. It idled fine, WB showed 14.9 AFR on idle because fuel trims compensated, but vac was 10 ish. Checked the engine bay and found the WG reference line had come off and sucking in unmetered air.

 

[PIMking]

My vacuum is like 10-11, with BC280's, OS valves along with some other things, but this is normal for my motor. Even though, after acceleration, when I let off, my vacuum goes to over 20+in/hg. So you have some kind of a problem. Most likely it is a boost/vacuum leak. Do a BLT and Let us know.

Same with me I have comp 100-200 cams and I sit right around -10 in the vac but on decel im -20+ too.

 

[need2boostpsi]

My vacuum is like 10-11, with BC280's, OS valves along with some other things, but this is normal for my motor. Even though, after acceleration, when I let off, my vacuum goes to over 20+in/hg. So you have some kind of a problem. Most likely it is a boost/vacuum leak. Do a BLT and Let us know.

exactly the same only with HKS 272's. Timing and fuel ratios can alter the Vacuum at idle. I have 150 across the board because of my cams and thick headgasket on a stock block.