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ossaman55 · Posted: Wed Mar 14, 2007 11:42 am Post subject: varying boost

I just drove the 931 for the first time since december and noticed that the boost readings drop from 7 psi to aroung 5psi after the car warms up. What could cause this and has anybody else experienced this. The turbo is fresh and has less than 500 miles on it.
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Darren
1980 931
1984 Maserati Biturbo
1978 Triumph Spitfire
1986 Camaro Z28

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Gotta be something going on with the WG. Maybe sticky when cold?
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Lizard · Posted: Wed Mar 14, 2007 2:47 pm Post subject:

should be 7 PSI all the time, where is the reading being taken from? and what is your A/F guage reading
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3 928s,

ossaman55 · Posted: Wed Mar 14, 2007 8:23 pm Post subject:

The reading is taken from a hole that i drilled in the intake manifold. I jb welded my guage in. I dont have a A/F guage but i noticed that my vacuum has been low, around 13 psi.
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Darren
1980 931
1984 Maserati Biturbo
1978 Triumph Spitfire
1986 Camaro Z28

Smoothie · Posted: Thu Mar 15, 2007 9:08 am Post subject:

Your main culprits for low vacuum (other than a very worn engine) are ignition timing, valve timing and plain olde vacuum leaks. Don't overlook that fat vacuum line from intake manifold runner 1 to the turbos' bypass valve.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

ossaman55 · Posted: Thu Mar 15, 2007 10:33 am Post subject:

i found my vacuum leak, it was caused by my boost gauge installation( i had to use a small amount of fuel line to connect a piece of copper pipe to the small nylon line and forgot to put a clamp on )but i dont think that it has anything to the boost being low after a few minutes of driving.
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Darren
1980 931
1984 Maserati Biturbo
1978 Triumph Spitfire
1986 Camaro Z28

Smoothie · Posted: Thu Mar 15, 2007 2:56 pm Post subject:

Any possibility that the turbos' bypass valve was assembled backwards? Paul had this happen, but I believe it limited his boost to 4psi and at all times, not just after warm-up.

Double check your vacuum line routing - especially the vacuum advance and retard. -Make sure the advance line is on the distributors' advance diaphragm and the retard line is on the retard diaphragm. There are also some coolant temperature controlled vacuum switches - make sure the advance-retard lines either are or aren't routed through them as appropriate according to the vacuum diagram for your car.

It might also be just a general tune-up issue - plugs, wires, their connections, weak coil, etc.
Put a CIS tester on it and check that the control pressure goes up as it should during warm-up.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox