Replaced TBV , loss in boost?

Phil Rose pjrose at
Wed Dec 3 10:38:16 EST 2003

At 7:27 AM -0700 12/3/03, Andy wrote:
>I recently replaced my turbo bypass valve due to car stalling 
>symptoms. Immediately after the replacement, that was done w/o 
>incident, my boost pressure guage on the dash computer is 
>registering .1 to .2 less. I know the guage is unreliable as an 
>exact boost, but it should be relatively accurate(?).
>  I regularly would see 1.7 and now will rarely see 1.5. Any thoughts?

Yes it should be fairly "repeatable", but is not "accurate". It 
should give more consistent readings than what you describe, though.

>I know the small vacuum(?) line can be a problem.

Not "can be", but there almost surely _will_ be a problem unless that 
vac hose (it's also a pressure hose) is not replaced within 100K 

In a previous response I wrote:

>At 10:36 PM -0400 6/15/03, Phil Rose wrote:
>>  When replacing the bypass valve, make certain that the
>>vac hose is in good condition. That hose gets cooked and becomes brittle
>>from turbo/exhaust heat; it usually splits and falls apart beneath the
>>heatshield. Remove the intake heatshield and have a good look.

Did you do that (check the hose condition)? I should also have 
suggested doing a vac/pressure test of that hose, since looking good 
doesn't guarantee that it will hold pressure or vacuum. If it's an 
original hose, it's very likely to need replacement.

>>  If it needs
>>replacing, consider using a length of copper tubing instead of rubber hose
>>for the section running beneath the heatshield--or simply use a longer vac
>>hose and reroute it away from the engine--e.g., along the inside of the
>>fender behind the airbox in order to avoid turbo/exhaust heat altogether.

>  Could I have exacerbated a problem w/ manipulation of this line 
>during r&r? Some posters suggest replacing this w/ a longer line 
>that is routed outside the heatshield on the right side of engine. 
>Good idea if I replace it?

Since it's manifold/turbo heat that cooks the hose, it would seem to 
be common-sensical to avoid that heat in the future--and one easy way 
is by using a few extra feet of hose.

>  Where should I get some new line? Is it standard vacuum line?

You can use regular vac line, as long as it has an appropriate i.d. I 
seem to recall using vac hose with 5/32" i.d. Take a section of the 
old hose with you when you buy the replacement. Afterwards, I 
discovered that the oem-type fabric-covered hose is available at some 
auto supply outlets for a buck (or two)/foot-- but avoid buying it 
from the Audi dealer, who, IIRC , will charge you about $30 for only 
a few feet of it.

You might save yourself some hassle by not replacing the first foot 
or so of old hose--i.e., retain the rearmost section that is clamped 
to the (relatively inaccessible) manifold port (nipple). Just cut the 
old, brittle hose back to a point where it's in OK condition and 
"splice" it to the new hose. There are small diameter, plastic or 
brass connectors available for this (Pep Boys, or pet-shop aquarium 
supply, etc). Don't neglect to clamp the hose at the nipple of the 


Phil Rose
Rochester, NY USA
mailto:pjrose at

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