3B boost problem (again...)

[newsh]

Without a wastegate a turbo would spin too fast and destroy itself. So your wastegate is set by its internal spring, to control the turbo. When the boost pressure reaches 0.5 Bar your wastegate opens and "wastes" some of the exhaust gases, directing them away from the turbo and back into the exhaust, so controlling the maximum boost that the turbo can make. If you clamp the pipe to the lower chamber or disconnect it and plug it this is the same as having no wastegate, the turbo will spin out of control and just keep on making boost untill something stops it. In the case of our cars the ecu will detect the excess pressure (overboost) via the vac/pressure line connected to it from the Inlet Manifold and will shut the engine down to avoid dame by cutting off the fuel supply.
So how do we get more boost than 0.5 Bar (or whatever is set by the spring)? This is where the active boost control comes in. The N75 or WasteGate Frequency Valve, can be controlled by the ECU to turn up the boost by restricting the pressure feed to the wastegate lower chamber. This means that the ECU cannot turn down the boost below the pressure set by the WG spring it can only turn it up.
So, if your car is making 0.5 Bar with the WG lower chamber connected to the IM but makes a lot more boost (I think you said 1.5Bar and fuel cutoff) if you disconnect the pipe to the WG lower chamber, then you basically have no active boost control. So if no codes are showing either the WGFV is not plumbed in right or the ECU is not sending it a signal. The ECU gets its instruction to send the signal to increase boost from the TPS (throttle position sensor) so check that the throttle cable is not still slack when your foot is on the floor and check the TPS is working correctly.

[max_s2]

Thanks for the huge message.
I know how that all works. Ok, tomorrow we'll swap TPS from my S2 and then I'll report back how it went out.

[newsh]

Thanks for the huge message.
I know how that all works. Ok, tomorrow we'll swap TPS from my S2 and then I'll report back how it went out.

You're welcome, I thought it would be useful just to get back to basics, Its amazing the conclusions some people jump to about these cars.. anyway another thought for you. Check the static timing is exactly on the marks, flywheel, camshaft and distributor. Make sure they are all lined up by rolling the car forward in a high gear till the flywheel 0 mark is lined up in the bellhousing window and then double check the dissy is exactly right. You can get some weird symptoms if its not correct.

[max_s2]

Timing and other stuff is OK. But we'll recheck it also. Tomorrow (owner today feels not well after a bit alko ) we'll try to change TB and a good K26 turbo. Current K24 has a little bit shaft play in all ways (in/out, up/down)

[andrix]

dount drink so much

[max_s2]

So, we changed WG, replaced K24 with a good K26 one, swaped TB with TPS - no differences. We disconnected N75->WG vacline, tricked WG spring with a weaker one and got 0.9bar aprox, car runs fine.. but without real boost control no faults etc.. what the hell is wrong with this S2, I'm out of ideas what else can be wrong

[max_s2]

Hello.
Still the same.. 0.5bar. Yesterday after hard acceleration, spido stopped working (not showing speed, rpms are ok). This has happend also before.. could this make only 0.5bar boost?

[BorisShakhov]

I have the same... spido, boost 0,6 , no faults... 100% same... and i don`t know what can I try...