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High Pitch Vacuum Noise??

9K views 17 replies 4 participants last post by  peva 
#1 ·
I recently replaced the water pump in our 2001 2.7L Intrepid. That job seemed to go very well, with all the help I got from this forum.

But, an issue that we are having now is the car is the daily driver of my son and he is telling me that he is hearing a high pitched sound that seems to go away when he hits the brakes.

I did hear this when I first finished the work but I thought it went away. Apparently his hearing is more acute than mine as he can still hear it.

I assume that this is some sort of vacuum leak but I can't find anything that is out of sorts from when I took it apart...

Any idea where to look or what it might be?? I am at a lost??

Thanks!!
 
#5 ·
Sorry, I got distracted and moved on to the suspension clunking......

Almost have that done and I am back to this whistle issue....

I drove the car tonight and I can hear it again. Not as loud as it was but it is definetly making noise and if you hit the gas or brakes it will stop momentarily.

Is the hose you are referring to the one that goes to the air plenum? I did remove that and it just created a huge vacuum.

It doesn't make the noise all the time (and not so much at idle) so I think it will be tough to diagnose by pulling that hose anyway.

Any other ways to determine for sure if it is really the brake booster? How difficult to replace the brake the booster?

Thanks!
 
#6 ·
Remember that I did say to plug the hose (or where it connects to the plenum depending on which end you pulled off). That way you will not have the large vacuum leak and you would be able to hear if the whistling is gone. Either way I would bet on it being the brake booster because of when you say the noise goes away. Really the only way to get a 100% guaranteed, this is the problem, answer is to have the vehicle idling in park while its making noise and then remove the vacuum source to the booster. If it stops the noise (which I'm betting it will) then that would tell you that it is the booster.

I've never had to replace the booster on an LH but brake boosters generally have two nuts on the front of them to hold the master cylinder in place, a vacuum line running to the plenum, and a couple of nuts or bolts on the back side to connect it to the firewall. I imagine there may be a thing or two to remove to gain access, and then maybe a little bit of a pain to get the brake linkage lined up (don't know for sure).
 
#7 · (Edited)
...I imagine there may be a thing or two to remove to gain access, and then maybe a little bit of a pain to get the brake linkage lined up (don't know for sure).
You'll have to remove the windshield wiper arms, the cowl at the bottom of the windshield, and possibly the windshield wiper assembly (I did when replacing the booster on both my Concordes, but I may have read that you maybe can leave the windshield wiper assembly in place if you pull their fuse in mid motion so the linkage arms are rotated out of the way - not a lot of work in either case). The master cylinder is on flex lines, which is nice because you don't have to open the brake fluid system to move it out of the way.

You'll need a 1/4" ratchet, extension, and 10mm socket on the cabin side to remove the 4 nuts that attach the booster to the firewall - very tight quarters, and you'll be standing on your head, but otherwise easy and quick work. There's a spring clip on the end of the brake pedal-to-booster input rod pin that you have to remove. It wraps around the end of the pin to latch itself in place - just pull the latch tab away from the end of the pin just a little bit and slide the clip out of the groove machined into the end of the pin. Pull the pin to disconnect the booster from the brake pedal. You'll want to re-use that critical part (the spring clip), so don't permanently deform it by pulling that end latch too far - just enough to allow you to slide it out of the groove.

This isn't the exact spring clip, but the same concept to help you visualize what I was saying. The straight part slips into a groove machined right at the end of the pin, and the curved part latches over the end of the pin. Again - your pin will look somewhat different, but you can figure it out from this:



Don't be shocked at the amount of rust on the master cylinder push rod on the front end of the old booster - it's normal. Put a light coating of silicone grease on the push rod of the new booster. Also, after the installation, smear a bead of RTV over the top surface where the master cylinder meets the booster - that will prevent any water splash from entering from the top, while allowing any that otherwise gets in to drip out the bottom and also evaporate.

Don't be surprised if you can take the old booster and shake it and hear water sloshing around inside - it's condensation that accumulates over the years.

Should be able to get a booster from Advance or similar for around $80 - rebuilt, but reliable (no company that wants to not get sued or otherwise stay in business cuts corners on such a critical part).
 
#8 ·
So, is it safe to drive the car with the vacuum line unplugged, just to see if the noise goes away?

It sounds like this is a part that should be replaced anyway, so maybe I should just do it??

Thanks for all the replies, and great tutorial.
 
#9 ·
So, is it safe to drive the car with the vacuum line unplugged, just to see if the noise goes away?...
It would not be safe to drive it at all without the vacuum hooked up. Without boost, you wouln't be able to make even a moderatley quick stop, much less handle an emergency.

Are you thinking you can't duplicate the noise unless the car is in motion? If so, test it on a flat private property area - don't drive fast, and allow *plenty* of stopping distance. Still not advisable - you do so at your own risk. Hit the brakes at 5 mph with no boost and you'll see what I mean. You won't be able to apply enough pedal pressure to slow it much at all.

I am fairly certain the booster is the problem. This is what they do when they go bad, and I've never experienced a high pitched whistle from anything else on these cars. The fact that the noise is modulated by brake pedal pressure reinforces that opinion.
 
#10 ·
Yes, I am not sure I can duplicate the noise without driving it. It does not always do it, but I don't want to take any unnecessary risks and since this is my sons' car I will just go ahead and replace it.

I'll report back my findings.

I just replaced one of the sway bar links and went for a test drive, I did not hear the whistle this time, last night I did....But the good news, the link seemed to take care of the front end clunking noise. The control arms and sway bar bushings probably needed replaced anyway and I did feel some improvement with the new parts - so no worries there.

Thanks again!
 
#11 ·
I bought the brake booster and while driving to get it the car started to make the noise. I pulled over and pulled the hose off and the noise did not stop?? It seems to be coming more from the cabin then under the hood.

Also, it continues to make the noise even after shutting the car off for a short period of time??

Still sound like the brake booster??

Thanks again!!
 
#12 · (Edited)
I bought the brake booster and while driving to get it the car started to make the noise. I pulled over and pulled the hose off and the noise did not stop?? It seems to be coming more from the cabin then under the hood.
Don't take the new booster back just yet.

The nipple on the booster that the vacuum hose plugs onto is a check valve (allowing air to flow out of the booster, but not back in, thru that port/check valve). Even after you disconnect the vacuum hose from the booster, the booster is still charged with vacuum (behind that check valve) for a while until the vacuum inside the booster leaks down, so you could still hear the noise if it was coming from the booster even with the engine vacuum source disconnected.

Is that possibly what you were hearing when you pulled over? If so, it would have eventually stopped (once the vacuum inside the booster had in fact leaked down).

If you can get the noise to start again, shut the engine off, and press the brake pedal two times to operate the booster. *IF* when you press the brake pedal, the noise stops (clearly quicker than it would if you didn't hit the brake pedal), then that would be proof that it is in fact the booster that is making the noise. (Pushing in on the brake pedal actuates the booster's internal mechanism, and immediately depletes the vacuum stored inside it behind that check valve).

Let us know if you find that to be the case. On the one hand, I'd hate for you to replace the booster if that's not the problem, but on the other hand, I'm thinking the vacuum stored in the booster would explain the noise (at least for a while) even after the vacuum hose is disconnected.

One more thought occurred to me from where you said "It seems to be coming more from the cabin then under the hood". Around the booster's input rod (the rod that is connected to the brake pedal), there is an air inlet. When you push on the brake pedal, the input rod opens a valve inside the booster to let air in (from the cabin side of the booster - at that area around the input rod) - sucked in by the vacuum on the other side (engine side) of that internal valving. When the booster starts making noises, it's because the valving inside it is starting to leak. The noise is high pitched squealing (air from inside the cabin around the input rod being pulled by the vacuum past the leak points in the valving), and because it is open to the cabin side thru that area around the input rod, the noise is projected into the cabin.
 
#13 ·
Peva,

I believe it will stop if I hit the brake pedal twice (maybe even once??). What you are saying about it being charged and the air needing to escape makes sense, as I mentioned it will make the noise even after I shut the car off but if I hit the brake pedal I bet it would quit immediately.

I'll experiment and see what I can find..but it does sound like I still need to replace it and it will fix the problem.

I got sidetracked again...by oldest son is home from college and his car had a leaking strut. Just replaced 4 struts on it last night. The fun never ends!!

The guy in the auto parts store commented to me "seems like you are here every day!". I didn't have the heart to tell him that he is one of four parts stores that I go to!

With three teenage boys driving, between parts and insurance, I am going to go broke!
 
#15 ·
For Advance Auto: 2 of 4 reviewers experienced the problem below. Has anyone else experienced this?

Brake booster seems to work great. The only problem is that the rod is too short for the brake pedal. I had to fabricate a spacer so the the brake light switch would work. I'm really disappointed that they had not corrected the connection rod length issue. I had to spend too much time trying to figure out a way to correct a problem that if the part was made to specification in the first place it would have cut my installation time.
Bottom Line No, I would not recommend this to a friend
 
#17 ·
Under Dash Whistle Cured

5-24-15 NAPA Brake Booster 54-73176, 2000 Concorde 159,766 miles FSM 5-27 to 5-29
1. Abbreviations: AR or R: air ratchet or ratchet, ls or ss: long socket or short socket, RS or LS: right side or left side, TW: torque wrench, FBSD: flat blade screwdriver, pstn: position
2. Windshield Wiper Arms: small FBSD to remove pivot covers. 15mm ls, 3/8 AR remove pivots nuts. Push down on spring loaded joint and pull up at pivot to remove. R & L under arms.
3. Wiper Valley covers: T-25 Torx socket, 3 ext, and 1/4 AR. Outside 2 screws are much longer. Remove RS first
4. Wiper Reinforcement: 9 bolts, including one for WW motor support. 13mm ls, 3/8 AR. (see FSM 5-28 location of 9 bolts). 4 longer bolts: 2 on each side at inside rear. Unclamp WW hose and remove valley
5. Windshield Wiper Linkage: rotate WW motor crank lever to 12 o’clock pstn (FSM 5-28, Figure 47). I should have tried turning the motor crank nut until crank arm was at 12 o’clock. Removed nut and washer for rear linkage bar (master link), 13mm ss, 12 ext, 3/8 BB. Remove from inner pivot with pry bar. Front linkage bar (slave link) pop off inner pivot with pry bar. (I should have done same with master link)
6. Battery Negative Ground: remove both nuts 15mm ls, 3/8 AR. Remove wires, otherwise brake lights will stay on after remove booster pedal push rod from pedal pin.
7. Brake Fluid Level Sensor on MC: Disconnect wire connector on RS
8. Master Cylinder (MC): remove 2 mtg nuts. LS nut: 13mm ls, 6 ext, 3 ext, 3/8R. RS nut: 13mm ls, 3/8 R. MC is on flex lines, which is nice because you don't have to open the brake fluid system to move it out of the way. Turn it, so it faces backward and place on top of LS VC.
9. Brake Vacuum Hose: remove from check valve at front of Brake Booster (BB).
10. Fuse Box Cover: remove at LS Dash, LF door opening in passenger compartment
11. Knee Bolster (KB) under Steering Column: 2 screws @ LS: 7mm ls, ¼ R. Pull back to disengage top clips. Remove trunk release connector. Parking Brake Release: pull rubber piece forward and rotate cable up so log slides R out of mount. Haynes 11-17
12. AC Heat Duct: remove, 7mm ls, 6 ext, ¼ R on outside screw
13. Pedal Push Rod: There's a spring clip on the end of the brake pedal-to-booster input rod pin. The pin is at rear of pedal push rod. Spring clip wraps around RS end of the pin to latch itself in place - Use small FBSD between tang of pin and center of retaining clip. Rotate FBSD ¼ turn and remove retaining clip (FSM 5-26, Figure 39). You'll want to re-use that critical part (retaining clip), so don't permanently deform it by pulling that end latch too far - just enough to allow you to slide it out of the groove. The straight part slips into a groove machined right at the end of the pin, and the curved part latches over the end of the pin.
14. Booster Mounts: 1/4" ratchet, 6” extension, and 13mm socket on the passenger compartnment side to remove the 4 nuts that attach the booster to the firewall.
15. Old Brake Booster: remove. Pedal push rod cover: I removed and several pieces of green felt fell out. No water sloshing inside.
16. New Brake Booster: Dimensions and pedal push rod seem exact same as original Bosch 04779072AB. Move white foam FW gasket from old booster to NAPA booster. Mount: reach in behind booster and guide pedal push rod thru hole in FW. The guide right upper stud into hole in FW. Rock a little until all 4 booster studs go thru FW. 13mm long socket, 6 ext, ¼ AR & then ¼ to 3/8 adapter on Torque Wrench, @ 250”lbs. On the upper right nut, I had to use a swivel because of the steering column joint.
17. Sil-Glyde: lubricate outer surface (bearing surface) of pedal pin where it rotates in the pedal push rod.
18. Retaining Clip: slide into groove of pin until clicks in place, center tang beside pin
19. Brake Vacuum Hose: push onto Check Valve
20. Underhood Wiring Harness: place above MC studs on BB
21. Booster: Coating of Sil-Glyde on the front push rod of the new booster.
22. Master Cylinder: Turn and position on 2 studs at front of BB. Use fingers to guide booster push rod into indent at rear of MC piston. Wire Harness Bracket goes on studs after push MC into position. Spin on nuts at first by hand. LS Nut: 13mm ls, 6 ext, 3 ext, ¼ R. RS nut: 13mm ls, 1 ext, ¼ R. Install 2 mtg nuts @ 250”lbs. RS nut, 13mm ls, ¼ to 3/8 adapter TW. LS Nut: 13mm ls, 6 ext, 2 ext, ¼ to 3/8 adapter, TW
23. Brake Fluid Level Connector: reinstall on RS MC
24. Battery Negative: reinstall both nuts, 15mm ls, 3/8 AR
25. Brake Test: I jacked up the front of the car. I had an assistant put the car in neutral and press the brake while I was spinning the wheel.
26. Brake Light Switch: brake lights remained on with pedal fully released. Removed brake light switch by turning switch 30 degrees CC and pulling out of bracket. Pulled plunger out to its ratcheted end of travel, about ½”. Reinstalled switch with brake pedal depressed. Gently pushed back on pedal until it reached fully released pstn. Plunger on brake light switch ratchets to its correct off position by this process.
27. AC Heat Duct: reinstall, 7mm ls, 6 ext, ¼ R on outside screw
28. Knee Bolster: engage Parking Brake. Cable end log vertical in place. Push rubber cable surround into plastic bracket. Install Trunk Release Connector. Push KB upper clips into place. Install 2 LS screws, 7mm ls, ¼ R
29. Fuse Box Cover: install
30. Test: Drive car forward and backward. Hit brakes hard. Stopping correctly and brake lights operating correctly.
31. WW Linkage: Master Link at Crank Arm. 13mm ss, 12 ext, 3/8 TW @ 180-200”lbs. There was no spec for torque, so I guessed. I held crank arm in place against WW motor mount with pry bar. I should have used pry bar to pop master link off ball. Cloth over channel locks to pop slave link onto ball.
32. Wiper Reinforcement: 9 bolts, including one for WW motor support. 13mm ss, 3 ext, 3/8 AR. (see FSM 5-28 location of 9 bolts). Torque: 8 mtg bolts, 13mm ss, 3 ext, 3/8 TW @ 240"lbs
33. WW Valley Covers: T-25 Torx socket, 3 ext, and 1/4 R
34. WW Arms: turn WW on and then off to reposition linkage at park. Install WW arms on pivots: 15mm ls and 3/8 TW at 235"lbs
35. Test WW, Brake Lights, and Brake Fluid level Warning Light
36. Test Drive to assure proper brake system operation. No whistle from under dash.
 
#18 ·
Good procedure write-up.

I still recommend leaving the brake light switch in place to calibrate. It takes longer to R&R the switch than to simply pull the plunger out with pedal pressed and then let pedal return to its rest position, and sometimes it can be difficult to press in on the switch with enough force while simultaneously twisting it back into position - binds if slightly tilted. YMMV.
 
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