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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 15201
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have a 2006 express van with the 6.0. I am getting an

Customer Question

I have a 2006 express van with the 6.0. I am getting an intermittent LOUD hiss coming from under the hood. It is usually only when the vehicle is under a load going uphill, which suggests it is a vacuum leak. I suspect it has something to do with the heater controls, as it seems aggravated when switching from a/c to vent etc. The weird thing is the fact that its intermittent, and actually sounds like a "big-rig" purging air from it's tank!
JA: Where exactly do you think the noise is coming from? And how long has this been going on?
Customer: Cant tell. I can sometimes get it to do it while stationary, by revving the motor. My wife said she thought it sounded like it was coming from the RH side. I checked all of the vacuum connections and all are secure. Maybe a crack in a line?
JA: Are you fixing your Express yourself? What have you tried so far?
Customer: Yes. All I have done is check vacuum line connections.so far. It's a hard one to diagnose, as it comes and goes.
JA: Anything else you want the mechanic to know before I connect you?
Customer: Nothing else I can think of.....thank you.
Submitted: 1 year ago.
Category: Chevy
Expert:  david craig replied 1 year ago.

Hi Im David, thanks for visiting the site. please allow me some time to look up info on your vehicle, thanks

Expert:  david craig replied 1 year ago.

the only thing i can tell you is take a can of carb cleaner with the engine running and spray every vacuum hose, hose connection intake base, around the throttle body, maf , and see if the rpms change at any time.

Customer: replied 1 year ago.
I'm familiar with that technique, but am not very confident it will help me.
Expert:  david craig replied 1 year ago.

well unless its a turbocharged engine that is the only thing it can be. a vacuum leak. i would suggest that if you are familiar with that technique to do it and find the leak.

Expert:  david craig replied 1 year ago.

when you are under load your vehicle obviously needs more air so it will take all it can get and leave your hvac a bit starving until the load is no longer required. thus making it seem like its an ac issue

Customer: replied 1 year ago.
The reason I don't believe it will work is that it is not a constant (continuous) leak. It only happens when under a load. I know it's a vacuum issue. Going to be a little hard to spray carb cleaner while driving the vehicle.....
Expert:  david craig replied 1 year ago.

if its a vacuum leak it will show up regardless of if you are driving or not. you can have someone give the vehicle rpms while at idle.

Customer: replied 1 year ago.
I will try it. Vans are tough to get at the lines etc. Should be fun.
Expert:  david craig replied 1 year ago.

i hear ya. van are not fun to do anything on the engine......

Customer: replied 1 year ago.
Well I took the "dog house" off so that I could get at as much of the intake area as possible. I used up one can of carb cleaner to no avail. I squirted every vacuum line I could find from end to end. My thoughts are that I may have an intake manifold gasket leak? Can you tell me if the 6.0 chevys typically have manifold leaks? Any bulletins etc.?
Expert:  david craig replied 1 year ago.

i will find out if there are bulletins, and from experience, they dont go bad that often

Customer: replied 1 year ago.
I drove it with the dog house off and the hiss seems to be coming from the top of the motor somewhere. It's very intermittent .....it will just "spit" every once in a while. This one's going to be hard to track down I fear.
Expert:  david craig replied 1 year ago.

check the entire pcv hose/tube

Customer: replied 1 year ago.
I sprayed it. No change in idle.
Expert:  david craig replied 1 year ago.

pull the throttle body, put a new gasket on it.

Customer: replied 1 year ago.
Ok. That's easier than intake gasket. Won't have time for several days. How long can we stay in touch?
Expert:  david craig replied 1 year ago.

as long as i keep this open.

Customer: replied 1 year ago.
Ok. Thanks. I'll keep ya posted.
Expert:  david craig replied 1 year ago.

ok

Expert:  david craig replied 1 year ago.

any update?

Customer: replied 1 year ago.
I've been out of town for the weekend at a family wedding. I did try spraying some hard to reach areas that I may have missed the first time but no luck. I will be back to work Mon, but hope to have a chance to work on it during the week at some point. Thanks for checking in. Appreciate it.....
Expert:  david craig replied 1 year ago.

you bet

Customer: replied 1 year ago.
Is it possible that the vacuum purge valve could be bad? Would it still run without a CEL? I have not had time to replace the throttle body gasket, but I did spray all around the base of it.....no change in idle. And yesterday I noticed that it does it when letting off of the throttle, and at a steady (no load) as well as under a load. Really scratching my head here....
Expert:  david craig replied 1 year ago.

i dont think so. check the vacuum tank. check the hoses attached to it. and check the tank itself for small cracks

Customer: replied 1 year ago.
By "tank" do you mean the round, baseball size unit under the battery? It has one line going to it. I have sprayed that line on both ends, but have not inspected the tank itself yet. It will require taking it out of there. (which I'm willing to do, obviously.)
Expert:  david craig replied 1 year ago.

yep the round ball. dont worry about taking it out. have someone sitting in the van giving it gas while you listen for the origin of the hissing.

Expert:  david craig replied 1 year ago.

go to harbor freight and buy a stethoscope.

Customer: replied 1 year ago.
I'll keep you posted. Thanks! Gotta get to work now...
Expert:  david craig replied 1 year ago.

ok

Customer: replied 1 year ago.
last evening, I took the pcv tube, and both connecting hoses off and replaced them. Then I removed the vacuum canister, and checked it over for any obvious cracks/leaks. It's hard to tell by looking at it, but I didn't see anything, and when I sprayed it, no change in idle speed. Put it all back together and drove it.....still doing it! The only other "simple" thing to try, will be to remove/replace the canister with one from a donor vehicle (junk yard) and replace all the vacuum hoses on the other (RH) side of the plenum, that go to a tee and branch out to the canister, and the HVAC. I'm really getting discouraged, but want to do everything I can before the whole intake plenum comes off......As for using a stethoscope, the nearest harbor freight is quite a distance from me, but I can't get it to hiss unless I'm driving it, so that may be difficult anyway? Did you ever get a chance to check for bulletins on the manifold gasket, or any related problems? Thanks!
Expert:  david craig replied 1 year ago.

ill check this morninig

Customer: replied 1 year ago.
Thank you.
Expert:  david craig replied 1 year ago.

nothing as far as tsb's. the next thing you can do is smoke the engine. if there are any leaks they will definitively be seen.

Customer: replied 1 year ago.
Ok. I have a friend that has a smoke machine. He's really busy but I'll see if I can get in contact with him.
Expert:  david craig replied 1 year ago.

i think, thats the only way to determine if it a leak. if the smoke test ends up being ok, there have accounts of accessory components i.e. alternator, ps pump, etc... causing similar noise under a load.

Customer: replied 1 year ago.
Good to know, but I'm pretty convinced it's vacuum mostly because switching the heater controls aggravates it. But definitely food for thought.
Expert:  david craig replied 1 year ago.

The vacuum line passes through the firewall underneath the blower motor or hvac housing down there, I have had it come disconnected there, it is very tight and I had to use a mirror to find it?

Expert:  david craig replied 1 year ago.

the line for the vac ball is what im talking about

Expert:  david craig replied 1 year ago.

Over by the accumulator there is a 3 way T for the vaccum, one is the hard line that goes to the inside, one goes to the vacuum ball and the other line which is soft goes across the front of the motor over onto the drivers side of the intake manifold just even with the throttle body, the pcv valve line comes out of it and then the vacuum source line is just underneath it. If you remove the air cleaner you can get to it.

Customer: replied 1 year ago.
Sorry.....been at the u-pull-it yard. I got a vac ball, and that tee you spoke of. I had already found that, and wanted to make sure it wasn't the culprit. Will replace all of those hoses (except the small hard line) and see if that helps. Thanks for the suggestions.
Expert:  david craig replied 1 year ago.

you bet