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Strange vacuum noise from Firewall

Avalacher

Avalacher

Full Member
Joined
Dec 29, 2021
Messages
34
Hi everyone, I have this odd vacuum noise that is coming from the drivers side firewall. It kind of changes pitch when I turn the steering wheel a bit, but doesn't change much when I apply the brakes repeatedly, but the RPMs fluctuate quite a bit though with the brake application. This happens when in drive. When in park the RPMs are steady and idle is fairly stable, maybe a mild fluctuation, but in drive it will be stable then do some odd fluctuations. Brake force application is fine, but seems a tad slow to respond but the pedal returns normally and effort seems normal.

Could it be a brake booster diaphragm on the way out and need a new booster? I don't think it's power steering related because I can't hear it from outside. I can try and make a video if that helps. Truck runs perfectly otherwise and has 466,000 kms.
 
I would say it sounds like a brake booster as well. You could spray some carb cleaner around the engine compartment area while the engine is running to find an exterior vacuum leak if it's not the booster. Spray where the vacuum like connects to the booster too.

After you cut your truck off, do you still have a normal feeling brake pedal for about 1 or 2 presses? Or is it hard almost immediately?
 
His booster is powered by Power steering pump as it is hydraulic.
 
ygmn said:
His booster is powered by Power steering pump as it is hydraulic.
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Sorry, but it's to new to be a hydro boost, the 2500's got the hydro booster's 02-06 and the HD 1500 (2500) 2500, 3500 and bigger trucks.
 
OP is driving a 2007+ (avatar pic and section he started this in) so it is a 1500.

Hydro-boost brakes started in 1500 avys since mid 2003 model Year.
Link to thread in Problems: Brakes with pics from a 2006 1500 cladded with hydro-boost brakes:

Leak, brake booster or brake master?

I noticed my power steering fluid was empty ( the noise it made!) the other day so I serviced it up and drove away. However, I have had a leak on the left side of the engine for about 3 or 4 days, before I serviced the steering reservoir. Its worse now. And the steering fluid has gone down in...
www.chevyavalanchefanclub.com www.chevyavalanchefanclub.com
 
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screen shot from Rock auto for 2003 1500 avy
Notice it shows both vacum and hydroboost.Screenshot 2023-04-18 060305.png
 
now would gm go back to vacuum boosters with model change in 2007+?

do not remember reading about any 2007+ boosters so lets see what OP says.
 
ronthehandymanga said:
Ok, OP will have to confirm.
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My Avalanche is definitely vacuum boost. It gets vacuum from the rear of the manifold. It isn't the hydro boost system. My truck has parts code JD9 which also validates that. I think I am going to risk buying the booster to see what happens. Is there enough room to remove the booster without having to disconnect the brake lines from the master cylinder. If I can avoid bleeding the brake system, that would be ideal of course. Hahaha. Thoughts?
 
You should be fine with doing that, if there is any brackets below the master just disconnect the bracket.
 
I bought the booster. Hopefully it will arrive soon. Not as expensive as I thought it might be. Wow these trucks can be fairly affordable to fix. Compared to my Prius and the Model 3 that came before it, not bad at all. The model 3 was one of the most expensive cars to fix I have ever owned. Nice while under warranty but out of warranty, Holy crap. Need a new headlight? $2300... New door handle? $850... Upper Control arm? $700... And that is parts. Avalanche is just far far cheaper to own I find overall. Anyway, enough off topic, I will report back after the repair. I hope that fixes the weird hissing noise. Maybe I will record a video if I can and post it.
 
I noticed that about a year ago you had to replace your rack and pinion steering because of a leak, did you bleed it real good.
 
So I replaced the booster and the idle has improved a bit but still hovering at 520-550 rpm... A little low I think. No check engine lights at all. Is this normal? But the braking is so much better... Like wow better. The hissing noise is still there though. I tried bleeding the rack again by going left to right 4 times all the way. No change. Still a strange sound that seems to be related to the power steering somehow. The idle is stable in neutral at about 650 rpm and doesn't move a muscle. But in drive, it can fluctuate from time to time a bit. Also noticed the 1-2nd shift can be a little bit rough while 2, 3 4 shifts are butter smooth. Any ideas?
 
yes 500+ is idle to keep emissions low and mpg high.
 
You did not bleed the steering correctly it take 15 to 20

See attached follow the instruction exactly you have to get the front tires off the ground.2007steeringbleed.JPG
 
MS03 2500 said:
You did not bleed the steering correctly it take 15 to 20

See attached follow the instruction exactly you have to get the front tires off the ground.View attachment 235986
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Oh wow, I watched videos online ... Ughhh.... Thanks so much that is unbelievable. I will do it first thing in the morning and report back. Wow. This is so helpful
 
So I did the bleed procedure... oMG the amount of bubbles in the system, it gurgled like crazy. Now the entire truck just quieted down so much. Wow. Engine seems smoother too. No words to describe my thanks to you for your help! Wow. YouTube videos suck. What a fail. But you are awesome. All of you. Thanks so much everyone. Love all of you!
 
No problem after I read your previous posts and saw you changed the steering rack I kinda figured you had not bleed it.

But this is what the club is about members helping other members.


Oh yeah check the fluid level quickly.
 
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MS03 2500 said:
No problem after I read your previous posts and saw you changed the steering rack I kinda figured you had not bleed it.

But this is what the club is about members helping other members.


Oh yeah check the fluid level quickly.
Click to expand...
Oh yeah I did that. After spewing it guts of air bubbles, it was about half full. Topped it up. Wow the entire truck is just so smooth. For something with 466,000 kms, I am azed at how solid it still feels. Really impressed. Had a Tundra with similar mileage and it felt all rickety but to be fair the frame was rusted all to hell on it. But thanks again! Really appreciate it.
 
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