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My car only is pulling about 15psi to redline. It used to easily boost from 18-20psi. I only have a pulled wastegate hose, catback, and filter. Any ideas as to why this is happening? Anything I should check?

thanks
-Steve
 

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It's probably going to be your vsv switch located towards the back of the firewall on top of the motor asuming the car is twin turbo.
 
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What do you mean "VSV Switch" ??? . I only have a pulled wastegate hose, catback, and filter as well. I ask because I am experiencing the same problem as Darkgrey3K.
 

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this probelme is common enoough you could do a search on it on the forum here is a reply I gave for trouble shooting the turbo system:

from the upcoming revision to MKIV FAQ Has not gone though the review process yet
2. how does the stts(sequential twin-turbo setup) work? sequential turbo diagram first off, the is no #2 wastegate. there is only one wastegate and it comes off #1 turbo because that turbo is always on line, therefore you always have a wastegate. there are 4 sets vsv's, actuators, and control valves for the sequential turbo system. each vsv is simply a solenoid that is either 100% open or closed, allowing manifold pressure to pressurize the different actuators that open/close the four different valves.wastegate: when the manifold reaches 11psi of boost, the ecu sends a signal to the wastegate vsv, this allows manifold pressure to build in the wastegate actuator which opens the wastegate.exhaust gas bypass valve: somewhere around 3500 rpm, the ecu sends a signal to the exhaust gas bypass valve vsv, which allows manifold pressure to build in the exhaust gas bypass valve actuator which opens the bypass valve. this is a small opening inside the #2 turbine housing which allows some exhaust gas to go through the turbine of the #2 turbo which makes it start spinning, and dumps the exhaust gas out the exhaust piping coming off of #1 turbo. since it is a small amount of exhaust gas, it pre-spools the turbo and does not get it up to full operating speeds. this will smooth out the transition from 1 to 2 turbos. this valve is similar to a wastegate in design, but is located after the turbine wheel instead of in front of the turbine wheel like a wastegate would be. this is not a wastegate!exhaust gas control valve: this valve is located in the exhaust piping downstream of the #2 turbo. when this valve is closed, all exhaust gas must go through the #1 turbine wheel to get out through the rest of the exhaust system. at around 4000 rpm, the ecu sends a signal to the exhaust gas control valve vsv, which allows manifold pressure to build in the exhaust gas control valve actuator which opens the control valve. this allows exhaust gas to go through #2 turbo and out the exhaust system which brings the #2 turbo up to full operating speed.intake air control valve: this valve is located in the intake tract coming off of #2 turbo. it is closed below 4000 rpm so that boost pressure coming off of #1 turbo cannot backup through the #2 turbo and back out the air cleaner/suction of #1 turbo. there is also a 1 way reed valve within the same housing of the intake air control valve. as the #2 turbo starts to prespin at 3500 rpm, it will build some boost. if it builds enough boost, it will open the 1 way reed valve to allow this boost into the intake tract to join with the discharge boost pressure coming off of #1 turbo. at somewhere over 4000 rpm, the ecu sends a signal to the intake air control valve vsv, which allows manifold pressure to build in the intake air control valve actuator which opens the control valve. this allows the full boost pressure coming off #2 turbo to join in with that coming from #1 turbo and you are now fully on line. usually, the exhaust gas control valve will open first, which gets the #2 turbo spinning at full rate so that it is building good boost before the intake air control valve opens, allowing this boost to join in with that coming off #1 turbo. if the intake air control valve opens before the exhaust gas control valve, the boost pressure coming off #1 turbo will go backwards through #2 turbo, spinning it backwards if there isn't sufficient exhaust energy to keep it spinning forward. when the exhaust gas control valve opens, and the #2 turbo has to reverse the direction of the spin. this is a tremendous strain on the turbo shaft and bearings. if the sequential operation is not a well orchestrated symphony of motion, it is easy to see how the #2 can be prone to failure. For an alternate explanation go to new car features, ncf 91- 95 are the appropriate pages. back to start
3. what does a vsv do?vsv is short for vacuum switching valve. it is just an electronic solenoid, which either opens or closes 100% when energized. this will allow the vsv to either pass boost pressure through it from the actuators (like the wastegate actuator) or block it off.back to start
4. How to trouble shoot the sequential system.
As you can see from the discussion in number 2 above the operation of the sequential system is complicated and requires all it s components to be functioning properly. Failure of a component will cause the disrupt the proper sequential operation of the turbos and can lead to failure of one or both of the turbochargers.
If any of these valves aren't working properly, you'll have low boost problems:

1. BOV (boost will get bled back to intake)
2. EBV (loss of exhaust energy)
3. EGCV (loss of exhaust energy)
4. WG (loss of exhaust energy)

Now if the EGCV, or EBV aren't working properly, you should have strange transition problems, as well as low boost on the low end.

If the problem is the WG, you should suffer low boost consistently, whether it's only #1 or 1+2.

One way to sort his out is to put the car into True TwinConversion(TTC) as a diagnostic tool. TTC instructions If your boost problem goes away with TTC it has to be one of the VSV or actuators that is malfunctioning. ( as an aside many list members feel that TTC is safer for your turbos because by eliminating the sequential operation eliminates the chance that the turbo will boost against a closed valve and twist the shaft).
The actuators can be checked in the car. All you need to do is hook up a pressure source (I use a 60cc veterinary syringe) and pressurize the actuator, then watch to see if it leaks down. It's pretty simple. Don't go over 20 psi with this as you'll blow an actuator diaphragm or overextend an arm.
The VSV's are the same. Hook up pressure to see if they leak. Then apply a ground to the wire going to the ECM to actuate the VSV and see if it opens. Of course they are not all normally closed, so you may need to check flow just the opposite,

If it's on the compressor side, it could be your BOV passing at low boost. These are easy to check and just need a source of vacuum. There is the suck and blow test… suck on the smaller tubing while a friend blows down the larger tubing if he can blow through with out resistance it is normal. If he cannot blow through while you are NOT sucking that is normal. operation.

Another possible source of the problem is a vacuum leak. The newest of the surpas are 3 years old as this is written (Dec. 01) The oldest is 8 years old. The various rubber control lines get brittle and leak. The leaks may not be obvious to a superficial look. I suggest replacing them with silicon vacuum hose which has the additional benefit of looking great. Short of that if you suspect that is the problem pull the individual hoses to inspect them .

go tothis thread as there was some other good pointer for tracking this down



low boost thread
 
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