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Vacuum leak???


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#1 KoD

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Posted 26 January 2006 - 12:43 PM

Hi folks, I hope this is the right place to post this.
I have a problem that seems to mystify everyone. 2 GMC dealers, and nothing.

Anyway, the issue is this. I have a Sierra 2500HD 4x4 with ~ 11,000 miles.
The only mod so far is a Diamond 8' plow added in December. (Not sure if this is what has started the problem.)
There seems to be a noise coming from under the hood that sounds like a vacuum leak. It only happens on steady acceleration and will go away if I "gun" it a little, or take my foot off the accelerator.
It's very noticble only while driving, so finding it at a stop is very hard.

I may have narrowed it down, but don't really know how to describe it. The intake on this truck appears to be a one-piece molded part, but it has what looks to be a PCV valve molded into it. (It isn't removable). The line off this piece goes to the drivers side, just behind the exgaust manifold, to so type of canister. Vacuum I pressume. When I take this line off, the truck idles rough and almost stalls. If I put my finger over it, it comes back to life and smooths out.

I guess the big question would be, has anyone else experienced this? Dealers are telling me it's "normal", but I never heard it until after the plow was put on. (Yeah, they must have hit something under there running wires.)

I am considering disconnecting this hose at one end and blocking it for a test drive, but am curious if this will harm the vehicle in any way?

I'm at my wits end with this and can't seem to find the problem. :cheers:

I can add that this noise, at least for now, seems to just be an annoyance, and hasn't seemed to effect the operation of the truck. It still runs good, shifts fine and nothing noticable other than the vacuum "sucking" sound.

Any thoughts?

Thank you in advance, and my apologies for explaining all this like a noob. :fume:

#2 Durymcgee

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Posted 26 January 2006 - 01:04 PM

Might be stupid, but have you considered that the extra aerodynamics of the plow have made the noises?

#3 dewfpo

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Posted 26 January 2006 - 01:34 PM

Can you duplicate the noise with the truck stopped by reving the engine in neutral or under load (in gear, foot on brake and accellerator)?

You have your highest vacuum when you take your foot off the gas, so the noise should be the loudest IF it were vacuum related, but you say the noise goes away when letting up on the accellerator.

If you had a leak (loud enough that you could hear from inside the truck), you would most likely set a code.

The engine is designed to run with air moving thru the PCV so disconnecting it and plugging it will cause a rich condition and the engine may idle rough. If you discconect it and plug the line you will kick out a code and it's probably not the source of the problem anyway.

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#4 KoD

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Posted 26 January 2006 - 01:46 PM

Might be stupid, but have you considered that the extra aerodynamics of the plow have made the noises?

<{POST_SNAPBACK}>


I thought of that initially too, howerver it makes the sound with the plow off the truck as well.
I am almost certain it's the PCV valve now, but for some reason I can't get it out. I don't think it's molded into the manifold.

#5 KoD

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Posted 26 January 2006 - 02:11 PM

Can you duplicate the noise with the truck stopped by reving the engine in neutral or under load  (in gear, foot on brake and accellerator)?

You have your highest vacuum when you take your foot off the gas, so the noise should be the loudest IF it were vacuum related, but you say the noise goes away when letting up on the accellerator.

If you had a leak (loud enough that you could hear from inside the truck), you would most likely set a code.

The engine is designed to run with air moving thru the PCV so disconnecting it and plugging it will cause a rich condition and the engine may idle rough.  If you discconect it and plug the line you will kick out a code and it's probably not the source of the problem anyway.

DEWFPO

<{POST_SNAPBACK}>


The only thing I can duplicate when stopped is if I give the accelerator a quick shot, I can hear a burst of air. Almost like the old carb days if you took the ari cleaner off and gassed it. You'd hear that quick rush. I just was out and had it up to about 2000 RPMS in both park and neutral, and didn't get the same effect. Hard to tell though like that because the truck obviosly has some power and is loud. :cheers:

Odd thing is, much like the unusual noise the power streering pump makes because of the vacuum assist with the brakes, I can only seem to hear this from inside the cab.

Maybe I am just being overly anal about this, since it's just a noise, but where I didn't here it when I bought the truck and then 6 months later I do, I have to think something is not like it was.

#6 asilverblazer

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Posted 26 January 2006 - 02:24 PM

I bet it is normal, my engine and lots of others I've heard all sound like they leak air, which was new to me compared to the old 5.7 I had.
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#7 KoD

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Posted 26 January 2006 - 02:36 PM

I bet it is normal, my engine and lots of others I've heard all sound like they leak air, which was new to me compared to the old 5.7 I had.

<{POST_SNAPBACK}>


I don't doubt that, and it's possible. I would be more receptive to that if it did it all along though. Maybe it just needed some "break-in" time. :cheers:

#8 Haulin' 8.1

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Posted 26 January 2006 - 07:05 PM

From the conditions you hear it I'll say it's an airflow produced harmonic. Did you see this guys post a few spaces down? http://www.gm-trucks...showtopic=49075

I wonder if there's been a throttle body redesign or the intake ducting that has caused this.

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