over boost.

[dennis]

Just being lazy I guess, but here goes. Is there a quick cheap way to trick the boost pressure sensor into thinking that boost pressure is lower? I keep having to re-start the engine to get out of limp mode? I have VAG software via Launch.

Thanks

Dennis

[410]

Sounds like the vanes are stuck in a closed or almost closed position.  Going into limp mode in this case is preventing the turbo from exploding so trying to fool the computer that it's seeing less boost than actual will lead to a catastrophic failure.  

As the actuator linkage wears on the turbo, you end up with less spring pressure that pushes the vanes open.  You can usually free the vanes up by forcing the lever through it's throw manually.  Once you get that lever moving again, lengthen the pushrod on the actuator to take up the wear on the pivot on the pushrod.  This will restore the spring tension forcing the vanes open preventing the over boost issue.

If you want an actual spec for setting the length of the pushrod, the lever should just start moving at 3-5 inches of vacuum applied to the actuator.  I set them closer to 5 to add a little extra spring tension. 

[Toby]

The spring tension is all in the actuator. Once it is off the stop the spring pressure does nothing to take up the slack in worn linkage. The lever will still flop around, if its loose. I would think that adjusting to the high side just lowers the boost a bit.

I would crawl under it with a vacuum pump and check for smooth operation of the actuator. As they age they get "notchy". I think this happens when the diaphragm get hard. They kind of jump forward and stop and jump forward as you apply vacuum instead of moving smoothly. This will cause an overboost that will put you in Limp Home Mode.

[410]

Setting it to the high side does not lower boost at all.  If you set the actuator to start moving at 5" of vacuum, you cannot pull the lever from it's full stop (vanes closed) at idle when full vacuum goes to the actuator.  The problem is the actuator can put upwards of 300 lb. of force to close the vanes using vacuum but the spring in the actuator is only capable of a few lb. of force to open the vanes.  A little preload on the spring is only helpful with no drawbacks. 

[Toby]

I think you need to redo your math. The diaphragm is ~2" in diameter. Pi x R2(squared) is the area if the diaphragm = 6.28 x 14.7 psi (atmospheric pressure) = 92.3 pounds of pull, not 300+ lbs. That is only if you could pull a perfect vacuum. The pump on an ALH is only good for about 25" Hg.

I suspect that you are correct about the extra preload helping with a notchy actuator. I have to replace the one on my wifes car today when I do the cam belt. I will try that and see if it helps with the overboost issue.

[410]

I think you need to redo your math. The diaphragm is ~2" in diameter. Pi x R2(squared) is the area if the diaphragm = 6.28 x 14.7 psi (atmospheric pressure) = 92.3 pounds of pull, not 300+ lbs. That is only if you could pull a perfect vacuum. The pump on an ALH is only good for about 25" Hg.

I suspect that you are correct about the extra preload helping with a notchy actuator. I have to replace the one on my wifes car today when I do the cam belt. I will try that and see if it helps with the overboost issue.

Looks like I grossly overestimated the force pulling the vanes closed but you got the point I was trying to get across.  Your math isn't correct either I'm afraid.  The area of a 2" diaphragm would be 3.14" squared cutting your answer in half. 

In the end there's way more force available to close the vanes then open them.  Good luck to the OP on your over boost issue.  Let us know what you come up with.