Weak brake boost when engine is cold and pedal pulsating at slow speeds?

[avanti284]

Thanks to all for helping me out...

I am trying to fix two brake issues in my 02 GTI 1.8T. I am not sure if they are related or not but they seem to be getting worse and I've already stumped a mechanic...

1 At slow speeds and in sharp turns the brake pedal sometimes pulsates just before coming to a complete stop. My car does not have any fault codes active or pending and there doesn't seem to be any driving technique that prevents it from happening. I heard that a pulsing brake pedal and no fault codes indicates dirty wheel speed sensors, but after cleaning them, no luck.

2 When the engine is "cold" (been off for several hours), the brake boost is weak (little to no vacuum) for the first 30 to 45 seconds that the engine is running. After that, it seems to work normally (other than the pedal pulsing).

The mechanic's diagnosis was a vacuum leak in the diaphragm of the brake booster. I replaced it myself (saving $600) but it did not fix either problem.

Both issues feel like they're deteriorating so I need to fix them soon. Anybody have any thoughts on how?

[Anony00GT]

You've gotta watch wheel speeds while driving. This requires VCDS and a portable computer, even a good shop probably won't have this unless they do a lot of VW/Audi work.

Each wheel has its own speed signal. One of the four is not like the other three, and it very well may not set a fault code. Once you find the oddball wheel, visually inspect the sensor, wiring, and tone ring. If all passes the visual check, replace that sensor.

You're never going to be able to fix this without VCDS though. Generic OBDII is of no use here.

Re-evaluate the hard pedal after the ABS issue is fixed.

[avanti284]

Thanks, I'll definitely get my wheel sensors checked out the right way. But if a wheel speed sensor is the cause, then it must mean that the weak brake boost is being caused by something else...

Does anything about this picture of my brake booster assembly seem unusual to you?
http://www.flickr.com/photos/7782568...in/photostream

I'm talking about the hose on the bottom of that tee fitting in the brake booster. If it were a vacuum hose, then I would have been able to get a technical drawing of the brake booster assembly showing it but I found none. All of the drawings I found used a simple check valve and one vacuum line to the intake manifold. Has anybody ever seen this tee fitting style assembly before and can you clarify what that "bottom" hose is for (it goes about 2 feet back behind the firewall as if directly under the center console area)?

[Anony00GT]

As for that pic...

Follow the lower hose and see where it goes, depending on what's on the other end of it, that may or may not be your assist problem. That doesn't look OE. I just took a glance at an '03 GTI I've got in the shop right now, and it only has an elbow into the booster, not a T-fitting like you've got.

Is your CEL on? Post up a complete list of mods if you're not stock.

Where are you located?

[avanti284]

As for that pic...

Follow the lower hose and see where it goes, depending on what's on the other end of it, that may or may not be your assist problem. That doesn't look OE. I just took a glance at an '03 GTI I've got in the shop right now, and it only has an elbow into the booster, not a T-fitting like you've got.

Is your CEL on? Post up a complete list of mods if you're not stock.

Where are you located?

Interesting, I didn't think that the tee fitting assembly looked OEM either (no part #/doesn't say VW on it and it's not a check valve, just a plain tee). I've done no mods to it but I've found other strange "repairs" done to it before. As for that "bottom" hose, it's really hard to tell what it goes to, if to anything because I have to look with a mirror but it seems to end right underneath the center console.

I think I'll try switching out that tee fitting with a typical check valve that I got when I ordered a replacement brake booster and see if it fixes it.

[Anony00GT]

I believe the check valve should be between the booster and intake manifold in that car (on the vacuum line), not on the booster.

You'll probably have to get under the car to follow that hose. I wouldn't disconnect it until you know what's on the other end of it. There's something there, if you had a vacuum leak that big you'd know it.

[avanti284]

Yep, there are two check valves on the vacuum line coming from the intake manifold and both are functioning properly. I just tried looking under my car for the end of that hose and it appears that it's covered by a shroud that can't be removed unless you have a pit to work in...So instead I'm going to test for vacuum from this hose with the engine running.

[avanti284]

Last night I tested suction presence from the bottom hose with the engine running and pedal depressed...there was never any. Afterwards I could suck air quite easily through the bottom hose while only feeling a slight bit of resistance...that means that suction was being lost from there!

So then I swapped out the bottom hose assembly for the typical vacuum hose and check valve assembly and drove. Not only was the boost fully present at startup (5 seconds or less) but it also improved significantly during normal braking. So as far as the bottom hose assembly goes, it appears that it's just another strange thing about my car, and I definitely cannot see how it is supposed to help my brakes.

Although I fixed problem one, I'll still be checking out my wheel sensors as I did feel the pedal pulsing still. Thanks again for your help.

[Anony00GT]

Somebody probably just wanted to tap a vacuum source, not help your brakes.

The question is...why? If it was my car I'd find out what's on the other end of that hose.

[avanti284]

Somebody probably just wanted to tap a vacuum source, not help your brakes.

The question is...why? If it was my car I'd find out what's on the other end of that hose.

Been in the process of moving and I finally got a chance to bring my car in for the wheel speed sensor problem that I suspected. I went to a VW dealer and explained how the pedal was pulsing at slow speeds (didn't mention the wheel speed sensor) and they found the passenger front sensor not functioning properly (great news until...). I got it replaced, but then they told me that it did not fix the problem and that they had no explanation of why

They took fault for their mistake and took care of the unnecessary $180 replacement. Sure enough I drove and that pedal still pulsates when it shouldn't be. I scheduled a second diagnosis today and they determined that the problem is extensive because there aren't any fault codes whatsoever and that they would need an entire day or more to dig into it. That's where I'm at right now, waiting for a time to bring it in for this diagnosis.

I also thought to ask the mechanic about that "bottom hose" connection, to see if he'd ever seen it before. He said that turbo engines (like I have) always have a vacuum pump because when the turbo provides boost during acceleration, most of the vacuum in the intake manifold goes away for a few seconds, requiring the vacuum pump which is installed underneath the car and connected to the brake booster by a second hose (that's what I have). I proceeded to tell him that it's got to be malfunctioning because of my weak boost on startup (it is actually still weak on startup). So when I bring my car back they are also going to find out what's wrong with the boost pump system.

[avanti284]

After all this time I finally managed to fix my car completely and so now I'll explain what I did. I basically had two separate problems, being the occasionally weak brake boost and the pulsating brake pedal.

I managed to find the reputable VW service department that I needed. They began with the weak boost problem since they thought it was a simpler issue. By performing a simple test when I brought my car in, they determined that there was no vacuum leak from the intake manifold to the brake booster, which meant that they could proceed to check the vacuum system from the brake booster going to the boost pump underneath the car. There was a vacuum leak on this side of the system being caused by cracked hoses, which they replaced and then concluded that the brake boost system was functioning properly.

When testing for the source of the pedal pulsation, they concluded that the ABS was activating based on false readings from the right front wheel speed sensor. Since I had told them that it had just been replaced, the technician noticed that the spacing between the reluctor ring and sensor was too large. He said the extra spacing was likely caused by a mismatched part in the wheel bearing assembly, and quoted $400 to disassemble and locate the mismatched part.

The work done so far had come to $450, but I immediately thought that a simpler fix could be found for the extra spacing between the reluctor ring and wheel speed sensor.

Visually I noticed that the right side sensor to reluctor ring spacing was greater than on the other side so I began looking for a way to move the sensor closer in. After removing the sensor, I determined that I would adjust its location relative to the reluctor ring by pressing an existant brass bushing through part of the sensor's plastic housing. Using a feeler gauge, I checked the left front sensor (the sensor functioning correctly) spacing to be 0.052", whereas the right side was more like 0.080". I used my small shop press to push the bushing out about 0.030" (thus moving the sensor in by that amount), reinstalled it, and then measured the spacing to be exactly 0.052". A subsequent 300 mile trip with enough braking action along the way assured me that this had fixed it. True definition of a $0 fix.