I took my JCW to the track yesterday. 1st session the car ran normal ~20 psi (Manic stage 1). 2nd session the boost dropped under 10psi for couple laps, then back to 15 psi for 2 laps, and finally dropped to 3~5psi for the rest of the sessions.

All clamps are tightened. Checked the wastegate actuator and found something suspicious. I am very new to Turbo car, hope someone can help to explain if this is normal:
- When engine is off, the actuator rod push outward (towards the passenger side). When engine is on, the rod instantly pushes inward.
- I assume the rod push outward opens up the wastegate? so my thought is if 0 pressure (engine off), the gate is open. if negative pressure (idle=vacuum), gate is closed. so if turbo is running to provide positive pressure, the rod will be pushed outward and stays open, which cause the low boost?

Thanks in advance!

 

The pressure converter might have failed.

http://www.realoem.com/bmw/enUS/show...diagId=11_3982

 

Just did a search on pressure converter, my car doesn't have that typical kazoo sound after shut off, is it still possible for a faulty pressure converter?

 

 

A video from youtube said mini/bmw wastegate is driven by vacuum, so my original thought was wrong...

Did you have low boost when the pressure converter is dead?

I just took my car out for a test, under 4k rpm, it has 3psi. when rpm increases, boost goes up linearly a bit but max is 5psi.

 

Yes, wastegate is operated by vacuum, so when the car is off the gate is open. This is actually a safe operation. This way if you lose any of your vacuum hoses, the gate will open and now worries about overboost.

That being said, if any component in the vacuum system is on its way out, you will have low boost. Check all vacuum hoses, vacuum pump, boost converter, etc... Also check the wastegate actuator to make sure it’s going fully closed under vacuum.

 

I was trying to understand how pressure converter works to control the wastegate actuator. It makes sense to me w/o boost, the engine creates vacuum which sucks the actuator to close the gate. But when creating boost, the vacuum is gone and the positive pressure in the system. I am curious how pressure converter converts the boost to vacuum & how it controls the wastegate actuator to open the gate when over-boost in system? Appreciate if anyone can help me understand.

 

Vacuum is provided by a vacuum pump, not the engine.

 

OK found the issue today. The outer nut to hold the wastegate pivot arm is GONE... The gate is stuck open, it approves that the boost will go up slowly w/ rpm goes up. Being said, with an open wastegate, the turbo can still make 5 psi....

next will check if all the parts are still in working condition, then tighten it back and adjust the wastegate. I guess at least to put it in service mode or take the entire front end off for easy access.