have just bought an 1993 S4 but it has no boost at all the gauge on the dash sits at 0.5 at idle then goes to 1 when driven hard.
have checked
air flow meter
all the hoses
waste gate inc spring
and have just removed plug on exhaust pipe and fitted a gauge on it to measure back pressure in exhaust and getting about 20 psi at 4000rpm
don't know alot about these and don't have the bit of kit to read the fault codes
anyideas or tests i can do?

 

You either have a big *** boost leak or I have no clue. Wastegates perhaps?

 

Check the line from the gauge to where its taped on the manif? Might just be its kinked or just a bad gauge.

When you're driving and you feel the turbo pull at all?

 

20 psi of backpressure in exhaustis way too much, You should explain exactly where you tapped in and how. With that much back pressure the vehicle should barely run or not run at all.

 

can't turbo kick in at all
am testing the pressure in the exhaust at the small bit of pipe on the down pipe just after the turbo which has a blanking cap over it i guessed this was the test point? and just put a pressure gauge onto this point

 

Usually it is easy to just pull the oxygen sensor and hook tester there but that spot could very well be a test point and since it should be in a good spot then you should really not have that much pressure. general rule of thumb is no more than 2.5 psi at 2500 rpm and generally no more than 6 psi at full load. your cat could be plugged in some way. Unhook from catalytic convertor back and try again. If you have that much back pressure your turbo won't want to spin since it is the expanding of gasses from the high pressure to low pressure that spins it.

 

all fixed now thanks all!
took cats off and banged the guts out and now i get boost![sm=burnout.gif]