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Old 01-08-2010, 05:16 PM   #1
tobylazur
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Default weird problem with boost gauge, not sure how to diagnose...

I've been noticing my boost gauge tends to "vibrate" (i.e.-the needle swings over about a 2-3psi range really fast) a lot over the past two months. It idles pretty close to where it's suppose to, it just vibrates while it's there. Once you give the engine some revs though, the needle holds stable.

Here's a short 35 second video (the needle is actually swinging faster than the video can catch I guess. It seemed to be swing twice for each time that was caught on the video):

http://s200.photobucket.com/albums/aa57/tobylazur/?action=view&current=MVI_2253.flv

I found this from in the manual, and it looks like my gauge is similar to the #6 condition.



How do I diagnose a faulty ignition system or throttle chamber idle adjustment though. Any help is greatly appreciated because I'm completely stumped.
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Old 01-08-2010, 09:10 PM   #2
danmezz
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I had the same problem when installing an aftermarket mechanical boost gauge... it vibrated like you're saying and drove me nuts until I could figure it out. What fixed it for me was installing a "restrictor pill" on the boost gauge vacuum line to reduce the vacuum strength. After that, the needle stopped vibrating and everything worked fine.

You can get a restrictor pill at most auto parts stores for a dollar or two and it's really easy to install... that should take care of it
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Old 01-08-2010, 10:07 PM   #3
tobylazur
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Originally Posted by danmezz View Post
I had the same problem when installing an aftermarket mechanical boost gauge... it vibrated like you're saying and drove me nuts until I could figure it out. What fixed it for me was installing a "restrictor pill" on the boost gauge vacuum line to reduce the vacuum strength. After that, the needle stopped vibrating and everything worked fine.

You can get a restrictor pill at most auto parts stores for a dollar or two and it's really easy to install... that should take care of it
any idea what size you used?
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Old 01-09-2010, 07:34 AM   #4
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not sure if they have different sizes of restriction or not... I just grabbed the only kind they had on the rack. It did have a very small hole though (like a needle).

Anyways, it's just a plastic "connector" thing that goes in line with the vacuum hose. Just cut the hose and put it in between. As long as it's made for the same size vacuum line it should be fine
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Old 01-09-2010, 02:07 PM   #5
tobylazur
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I'm already using the plastic line that came in the kit, so it's pretty small to begin with.
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Old 01-09-2010, 05:43 PM   #6
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The line that came in the kit may be small, but not small enough. If you want to try it or not, it's up to you. It's most likely the solution to your problem though... search the internet and you'll find others saying the same thing.
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Old 01-10-2010, 01:11 AM   #7
lil'redwagon
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make sure your plastic line isn't kinked. or make sure it wasn't kinked before. it happens really easily around where it goes through the firewall. a kink can cause a pinhole leak that makes it shake. or, obviously, it can restrict the flow. shaking from a kink usually happens while it's reading boost, but it would be worth checking. otherwise the pill sounds like it would work. it would basically slow down the reading.
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Old 01-11-2010, 02:04 PM   #8
GatoEnFuego
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Autozone sells a coupler that has a small brass restrictor built into one side. It's like $2 and fixed my flappy boost gauge.
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Old 01-11-2010, 09:36 PM   #9
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Quote:
Originally Posted by lil'redwagon View Post
make sure your plastic line isn't kinked. or make sure it wasn't kinked before. it happens really easily around where it goes through the firewall. a kink can cause a pinhole leak that makes it shake. or, obviously, it can restrict the flow. shaking from a kink usually happens while it's reading boost, but it would be worth checking. otherwise the pill sounds like it would work. it would basically slow down the reading.
traced it back, no kinks in the piping

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Originally Posted by GatoEnFuego View Post
Autozone sells a coupler that has a small brass restrictor built into one side. It's like $2 and fixed my flappy boost gauge.
I just got back from Autozone. (had to pick up a spring compressor to fix my STI strut) Are you using one of the brass compression couplers? They had one, but it didn't really seem to be any smaller than the inside diameter of the current plastic piping. How small was the port for the restrictor?
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Old 01-13-2010, 11:39 AM   #10
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bump....
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Old 01-13-2010, 11:57 AM   #11
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Wow... Hooking up a vacuum gauge to manifold vacuum is actually a way to check for problems. If your needle is rapidly vibrating, it is an indication of a bad intake valve.

However, seeing how your gauge was reading 20"HG, I think you should be okay. I'd test the gauge itself by unmounting it from the column just to make sure the vibrating needle isn't vibration being transmitted through the column. It could also be vibration coming through the vacuum sense hose. If it also vibrates in boost, I think it is indicating the same, the hose is vibrating and making the diaphragm in the gauge vibrate.

Test it by hooking up another gauge to it. I freaked out when my vac/boost gauge read 15"HG at idle. Then I hooked up a fuel pressure/intake vacuum/intake boost multi gauge, and it read 21"HG vacuum at idle. Ultra crap aftermarket junk gauge.
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Old 01-13-2010, 12:03 PM   #12
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looks good to me, while the motor is idling, the IACV will do its best to maintain set idle speed, which may require it to let a little more or less air into the manifold. it would show up on your gauge as a loss or increase in vacuum.
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Old 01-13-2010, 04:09 PM   #13
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Quote:
Originally Posted by WeldingHank View Post
looks good to me, while the motor is idling, the IACV will do its best to maintain set idle speed, which may require it to let a little more or less air into the manifold. it would show up on your gauge as a loss or increase in vacuum.
The IACSV cannot make the vacuum needle vibrate. It can cause the manifold vacuum to fluctuate though.
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Old 01-13-2010, 10:10 PM   #14
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Originally Posted by Chi_San View Post
The IACSV cannot make the vacuum needle vibrate. It can cause the manifold vacuum to fluctuate though.
i didn't watch the video very closely at work, but now that i am at home it does seem to vibrate more than just iacsv can make happen. i don't think that video does it justice.
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Old 01-13-2010, 10:31 PM   #15
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the motor is perfectly fine.... ive had this happen with every mechanical boost gauge ive had on the car. You need that little restrictor in there. the needle vibration is the gauge picking up the pulses of air getting sucked into the combustion chamber, put the restrictor in there and watch this problem go away.

By putting the restrictor in there you slow and even out those pulses and get rid of the needle vibration
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