riddle for the boost control experts

[offroader1006]

Please don't ask for logs, my hd took a dump on my laptop.

I just installed a new turbo (F21) set the Wastegate actuator to 8 psi crack pressure myself with a digital Wastegate calibrator, installed with it a new tuned ecu and fmic/tip/exhaust mani etc.

I used my existing n75 and boost gauge that I've been running on my apr tune.

After install, I am running 30+ psi of boost at maybe 1/4 throttle. Extremely early. Serious stumbling at low throttle and 5psi.

Verify correct n75 routing.

Unplug n75, boosting 8psi. Wastegate setting and operation verified.

Swap n75 and bench test the other, same boost levels(way past 25 on my vdo gauge), both valves work great on the bench.

Install ball and seat mbc, in parallel to n75, plumbed correctly. Adjusted until the adjuster is falling out. Boost drops to 25psi

Seems to me I need to try another mbc and have a possible problem with the map(not likely) or tune.

Anything I may have missed?

[offroader1006]

Found the spring from my used, unknown brand MBC was stretched, causing constant ball pressure.

Removed the spring and have a max boost of 20 psi.

One problem solved.

I plan to monitor the N75 operation while driving using a scope.

If anyone has done this and has data I can use to compare, that would be very helpful.

[elRey]

Unplug n75, boosting 8psi. Wastegate setting and operation verified.

Unplugged electrically or unplugged from plumbing (removed)?

[offroader1006]

Unplugged electrically or unplugged from plumbing (removed)?

electrically so it's running turbo pressure straight to wastegate

[Steve Chicago]

I plan to monitor the N75 operation while driving using a scope.

VCDS can show you and log your N75 pulses.

Which revision N75 are you running? Various versions vent at different rates for the same pulse width. Your new setup may need a faster or slower venting rate. In other words, for the same amount of pules, you need a different amount of waste gate action. More action would get you a faster responding boost control and less overshoot. It sounds like your system might be overshooting the mark. Any idea what this tune is supposed to be hitting for boost numbers? VCDS can also show you requested boost verses actual boost.