overboosting, new mbc

talonite · 08-14-2012

ok so just today my car overboosted like CRRAAZZZYY it was set for about 14psi on a cheapy ebay MBC, but yet it built boost to around 24 to 28psi. so i figured that was the problem. bought a new grimmspeed MBC from MAP thinking it was gonna be a clear solution. but it is still doing it. im running an evoIII turbo with the stock internal gate on it. any ideas on where to start? I mean i triple checked and everything is hooked up correctly, and I'm just kinda stumped at the moment.

**goodhart** · 08-15-2012

You're sure you have it turned all the way down and not all the way up right? LOL. Have you tried replacing the vacuum lines you have connected to it?

**scheides** · 08-15-2012

Make sure it's not backwards.

Then try running south the line directly from the boost source to the wga, maybe it's boost creeping really bad?

**asshanson** · 08-15-2012

My first guess is a leak in the hose from the intake manifold to the MBC. Could also be a broken wastegate actuator, like a hole in the diaphragm.

**Shane@DBPerformance** · 08-15-2012

Don't run any boost controller, just a new vacuum line and see what it does. You might have a wastegate actuator problem though.

19TsiAwd90 · 08-15-2012

had the same issue scared the shit out of me when i was getting on the freeway and i saw my boost gauge spike to 29psi =) it was fun for a second lol ended up being a hole in the wastegate line i would check that first, i have a grimmspeed mbc and it works flawlessly..

talonite · 08-15-2012

ok, so went through checked all the lines, the one from the mbc to the wg actuator was rubbed a little bit wasnt leaking, but i replaced it anyways. still doing it. used the smoke machine at work to do a boostleak test, doesnt seem relevant but i have a couple small leaks from my throttle body and b.o.v. i don't think they would relate to an overboost issue but i thought it was worth a mention. now would there be a way to manually test the wastegate actuator, or is it gonna come down to dissasembling it?

**goodhart** · 08-15-2012

Take out the MBC and put the vacuum line from the nipple on the j-pipe/compressor housing straight to the wastegate actuator (what scheides already mentioned) so it runs on wastegate boost. If it still goes to 29psi, either you have a magical 29psi actuator or a bad one.

**Shane@DBPerformance** · 08-15-2012

You can extremely carefully apply pressure from an air compressor to the port on the actuator and see if it starts opening when it should. A temporary boost gauge on the line going to the actuator would help a lot. Just don't try to put 50+psi to it, you would want to slowly ramp up the pressure.

talonite · 08-15-2012

if i go straight from the j-pipe to the actuator wouldnt it just build max amount of boost? or does the actuator itself have a limit? sorry about the newb questions here.

**Shane@DBPerformance** · 08-15-2012

The actuator has the limit. No MBC at all would run the lowest possible boost. A boost controller blocks boost from getting to the actuator, which fools it into going higher.

talonite · 08-15-2012

ok well thanks for all of the help guys, used a leakdown tester to put pressure to the actuator and its taking about 29psi to move, and only holding about 10psi at the diaphram itself. so out with the actuator and probably just going to an external gate. again thank you.