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Has the car ever boosted properly since you have owned it? Your situ sounds very similar to my 8V-T when I first got it.
There are a couple of tests to try. First is to make sure that the APC is working. Since you say that boost decreases as you accelerate, I suspect it is working OK but we should make sure.
First test is easiest. Disconnect the APC solenoid, its up by the radiator, small plastic can with three vacuum hoses leading to it, and a small 2-wire connector at top. Just pull the connector off and go for a drive. If APC is working, there should be much less boost than normal, probably barely into the yellow. If the car behaves the same, APC system is offline and you will have to troubleshoot that before proceeding, see the FAQ section of this website for a guide to APC or "base boost" troubleshooting.
Assuming that the APC is working, the next test is to hook up a calibrated boost gauge (leave the APC solenoid disconnected). The boost gauge can be purchased at an autoparts store for the equivalent of $20 US, (sorry I don't know what that is in pounds). Tee the gauge into the intake manifold somewhere or tee it into any of the vacuum hoses coming from the intake manifold and lead a long vacuum hose into the cabin so you can see it while driving.
On this next test drive try to get as much boost as possible (remember the APC solenoid should still be disconnected). Punching it in third gear up a hill is the best test. This tests the "base boost" setting of the turbo.
The base boost should be at least 5 psi (or the equivalent in bar), I suspect you will find it is much lower. If your car has an intercooler, you can set it a bit higher 5.5 - 6 psi. In my 8V-T, base boost was just 3 psi when I first tested it...
The next step will be to adjust the wastegate rod at the turbo - let the car cool first! You will be shortening the rod to increase the base boost pressure. Search this site for details on this step or research it online - it is not difficult at all.
You may have to do several more test drives and adjustment of the rod to get the base boost up to where is should be. Once you are there (~5 psi). The system should be set up right. Reconnect the APC solenoid and if the rest of the engine is healthy, you should now see much more boost - close to, if not reaching, the red zone on the gauge. When I did this to my '84, the difference in power was amazing, transformed the car from a slug to a real performance sedan.
Good luck and greetings from Seattle, USA.
- Mark
1985 SPG
1984 900T
1978 99T
posted by 12.144.1...
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