Help me out w/ low vacuum at idle problem [Merged 8-9]

BlueShark · Jul 21, 2002

I've been trying to fix this low vacuum problem ever since the engine rebuilt. I used to get 17-18" Hg at idle but now I only get 14-15" Hg.

These are what I tried so far.
BISS o-ring replacement
Throttlebody shaft o-ring replacement (1" Hg improvement)
Replaced all the vacuum hoses
Tried tightening BOV spring
PCV replaced
Tried spraying CARB cleaner & soapy water all over the place.

Weird thing I noticed is that if I unhook the hose connected to PCV, idle shoot up close to 3000rpm and vacuum increases to 18-20" Hg. I thought if I pull that PCV hose engine was gonna stall or come close to stall?

Any idea? Any help would be greatly appreciated.
Help me out before I go insane.
Thanks in advance.

zippyshoe · Sep 12, 2006

kingjust said:
Why do I need to check for boost leaks when it drives better, not great, but better with boost. It's weird and I just don't have the energy right now to look for the problem.

The only way to tell for sure whether or not you have any boost leaks is to perform a boost leak test. Unless you have a huge boost leak, your turbo will still be able to build boost - it will just have to work harder to do so. Solving these problems is usually by process of elimination - if you boost leak test and fix any/all boost leaks and you still have the same problem, at least you eliminate this as a possible cause. Many consider boost leak testing a part of their regular maintenance.

andymoraitis · Sep 12, 2006

Boost leaks and vacuum leaks are two distinctly different things. While I advocate testing for boost leaks often, vacuum leaks are tackled in a different manner.

1. Check cam timing. With the motor at TDC, verify that the cam gears line up on the marks with a straight edge. If they're off, vacuum will be affected.

2. With the motor at idle, spary carb and choke cleaner on and around your vacuum lines and listen for a change in idle. If you hear one, you have a leak and the line needs to be changed. Even a small crack in a line is easily missed. Since you have idle issues, I'd also suspect the BISS o-ring which can leak at idle and cause a vacuum leak as well as a boost leak.

3. When you shut the motor off, does the boost gauge settle back to zero? If not, this is an indicator that the gauge may be screwed and is giving false readings.

Take a look at those things and let us know what you find.

Andy

dieseltech · Sep 12, 2006

Just a little FYI for those observing this thread. Boost leaks can be detected throught the entire intake system. Vacuum leaks will only occur after the throttle plate inside of the throttle body. :thumb:

Good info Andy, I just didn't feel like typing earlier when i saw this post

JLangevin · Sep 12, 2006

Just put your boost leak tester on your throttle body and pressurize to about 15psi. This is what I did and found that both EGR vacume lines had popped off...

Boost leaks are also effected by vacume leaks... reguardless, both ARE directly related.

kingjust · Sep 13, 2006

zippyshoe said:
The only way to tell for sure whether or not you have any boost leaks is to perform a boost leak test. Unless you have a huge boost leak, your turbo will still be able to build boost - it will just have to work harder to do so. Solving these problems is usually by process of elimination - if you boost leak test and fix any/all boost leaks and you still have the same problem, at least you eliminate this as a possible cause. Many consider boost leak testing a part of their regular maintenance.

Ok, I understand now, thanks!

kingjust · Sep 13, 2006

andymoraitis said:
Boost leaks and vacuum leaks are two distinctly different things. While I advocate testing for boost leaks often, vacuum leaks are tackled in a different manner.

1. Check cam timing. With the motor at TDC, verify that the cam gears line up on the marks with a straight edge. If they're off, vacuum will be affected.

2. With the motor at idle, spary carb and choke cleaner on and around your vacuum lines and listen for a change in idle. If you hear one, you have a leak and the line needs to be changed. Even a small crack in a line is easily missed. Since you have idle issues, I'd also suspect the BISS o-ring which can leak at idle and cause a vacuum leak as well as a boost leak.

3. When you shut the motor off, does the boost gauge settle back to zero? If not, this is an indicator that the gauge may be screwed and is giving false readings.

Take a look at those things and let us know what you find.

Andy

Number 1, I will do after I do the vacuum leak test. I have to go pick on up today and test all five ports. Number 2 was suggested also, will brake cleaner do? That's what was told to me by my friend of a friend who works as a mech. for mitsu. Number 3, yes the boost gauge settles, so it's good to go.

kingjust · Sep 13, 2006

JLangevin said:
Just put your boost leak tester on your throttle body and pressurize to about 15psi. This is what I did and found that both EGR vacume lines had popped off...

Boost leaks are also effected by vacume leaks... reguardless, both ARE directly related.

How about this scenario, intake mani gasket blown?

zippyshoe · Sep 13, 2006

kingjust said:
How about this scenario, intake mani gasket blown?

A blown intake manifold could certainly create a vacuum leak - a boost leak test should catch this problem, if this problem exists. I can vouch for item #1 in Andy's post - my intake camshaft sprocket was off by 1 tooth. In this state, the car idled rough as if it had performance cams. My vacuum at idle was about 18 inHg (in park/neutral). After having the problem corrected, the idle was smooth and my vacuum at idle was 22 inHg (in park/neutral).

kingjust · Sep 18, 2006

zippyshoe said:
A blown intake manifold could certainly create a vacuum leak - a boost leak test should catch this problem, if this problem exists. I can vouch for item #1 in Andy's post - my intake camshaft sprocket was off by 1 tooth. In this state, the car idled rough as if it had performance cams. My vacuum at idle was about 18 inHg (in park/neutral). After having the problem corrected, the idle was smooth and my vacuum at idle was 22 inHg (in park/neutral).

Here is a fabulous update for you guys. Well, I went out and got me a compression tester and a vacuum tester. The first thing I did was the compression. And low and behold, cylinder 1 had no compression. I was like ####, so I had to take it to mitsu to figure out the problem. Fast forwarding, they told me that I have a blown HG (of course with the oil in the coolant), but it's either a burnt valve that is not seating right, or a bent valve. Either way, extended warranty is covering it. WooHoo. I also got some arp's for the head so when they put it back together they can install that at no charge. Thanks for all the help guys, you guys "ROCK"!!! LOL

91awd · Apr 3, 2007

So my car has about 140lbs compression on all four cylinders and my vacuum reads about 9lbs at idle. Is that normal for that low of compression? I was also wondering what is the lowest compression you should go before you rebuild? It still pulls pretty good needs a little help in the tuning department dsm link coming soon.

thanks,
brian

RedTurboEclipse · Apr 3, 2007

Check for boost/vac leeaks?

turbosax2 · Apr 3, 2007

http://www.geocities.com/dsmgrrrl/FAQs/compression.htm
Service limit = 121 psi

Talesin · Apr 4, 2007

What altitude are you at? 'Normal' idle at sea level produces 18-23in/Hg. I get about 20in/Hg at 700ft above sea level. Going to 6000ft drops it to 14in/Hg.

Compression doesn't seem to be a problem, unless you have one or two cylinders dramatically off from each other. I'd suspect a vacuum leak, myself. Vacuum hoses, maybe the throttle body isn't fully sealed, or the intake manifold gasket may not be seated. Heck, might even have a bad intake valve or something. A vacuum leak happens after the throttle body though... check all your vacuum hoses and your PCV valve first.

91awd · Apr 4, 2007

So I did a boost leak test on my car and found a big crack in my intercoler. Will this cause a vacuum leak? This has happend before I guess I should mount it so it's not so ridgid. I was told with it bolted right to the body it puts too much stress on the intercooler when the body flexes like going up a really steep drive way.

jonboyb · Apr 4, 2007

No, a leak upstream of the throttle plate will not affect your vacumn reading.

While the cracked IC is a problem, it's not the cause of your low vacumn. As Talesin and RedTurboEclipse said, need to look for leaks after the throttlebody.

91awd · Apr 4, 2007

Ok I'll keep looking I just can't seem to find anything else wrong.

TURFNNSURF · Apr 4, 2007

I have a 1g with 140psi compression on all four cylinders. I have a fully race ported head with 1mm oversized valves and FP2 cams and still draw 14-16 inHg at idle. This is at 250ft above sea level. Check the vacuum lines behind the intake manifold they can get brittle from the heat under the hood after 16yrs.

91awd · Apr 5, 2007

Well I'm going to pick up in intercooler from the shop. I'll retest for vacuum leaks after that I get it back on and stops idle surging so bad from the boost leak.

oldman · Apr 6, 2007

91awd said:
Well I'm going to pick up in intercooler from the shop. I'll retest for vacuum leaks after that I get it back on and stops idle surging so bad from the boost leak.

1. Definitely finish up the leak test after re-installing your intercooler.

2. What are you using to tune your 2g mas/550s?

Dream On · Apr 10, 2007

jonboyb said:
No, a leak upstream of the throttle plate will not affect your vacumn reading.

While the cracked IC is a problem, it's not the cause of your low vacumn. As Talesin and RedTurboEclipse said, need to look for leaks after the throttlebody.

Uh...Yes, a cracked intercooler will definitely affect your vacuum readings. Probably a large part of the problem. Still search for more leaks though.

oldman · Apr 10, 2007

Dream On said:
Uh...Yes, a cracked intercooler will definitely affect your vacuum readings. Probably a large part of the problem. Still search for more leaks though.

Adam, vacuum is created due to the closing (complete or partial) of the throttle plate, as in the engine out flowing the TB plate's air flow capicity, therefore leaks before the plate will not affect vacuum readings. With that said, vacuum leaks and compression numbers aren't the only things that can affect vacuum readings, A/F ratio, idle RPM and ignition timing can also affect vacuum readings.

laserspeeddemon · Apr 10, 2007

Make sure the line to the boost gauge is not cracked or improperly installed.

Burnett03 · Apr 10, 2007

Dream On said:
Uh...Yes, a cracked intercooler will definitely affect your vacuum readings. Probably a large part of the problem. Still search for more leaks though.

No it wont.. Vacuum is created on the other side of the throttle plate (engine side)

surgeon_general · May 29, 2007

This is not on my car this on actually on my fathers dsm haha. its a basically stock 92 awd laser. He has very low vacuum around 12 in-hg. the car runs really good otherwise the idle is a little weird and sometimes it haunts a little but other than that it runs fine. We tried everything the compression numbers are 160 acrossthe board. The car hauls ass seriously for being stock plus like an exhaust, filter and a boost controller. there is no loss of power. the car smokes a little at idle i think that is just turbo seals though.

Could bad valve seals cause a bad idle? I dont think the timing is off anywhere.
it was boost leaked recently and nothing really showed up. its all the stock piping and smic. the vacuum lines weve been checking replacing and eliminating what can be taken out to try to remedy the problem. I need some help though. thanks guys.

JohnCIV_Talon · May 29, 2007

Boost leak test!

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Help me out w/ low vacuum at idle problem [Merged 8-9]

BlueShark

zippyshoe

andymoraitis

dieseltech

JLangevin

kingjust

kingjust

kingjust

zippyshoe

kingjust

91awd

RedTurboEclipse

turbosax2

Talesin

91awd

jonboyb

91awd

TURFNNSURF

91awd

oldman

Dream On

oldman

laserspeeddemon

Burnett03

surgeon_general

JohnCIV_Talon

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