Problem with MBC

RobTarD · Post by **RobTarD** » 25 Nov 2012, 13:08

I have an 850R with a RICA tune that would boost to just about 1 bar. Today I bypassed the stock BCS with an MBC hoping to get a few more pounds of boost out of it but I couldn't, even with the knob cranked all the way down.

I took it on the highway and did a few pulls and was boosting to around 12 psi with the transmission in "Sport" mode but if I switched it to "Econ" I could boost to the full 1 bar, but no more. I also saw several pounds of fade.

I can hit 1 bar in either mode when accelerating from a stop but it doesn't hold.

Is my MBC kaput or is there something else awry? All I'm looking for is maybe 18 psi maximum.

Post by **matthew1** » 25 Nov 2012, 13:30

Mine fades as revs rise in any gear. I don't know if that's expected or means there is something wrong. I suspect it's expected. I have an R ECU in my '97 T5. It's been hitting 17psi lately, when the air is cold and dense. All summer it was 16psi.

RobTarD · Post by **RobTarD** » 25 Nov 2012, 14:28

I've never been able to boost past 1 bar. Could the CBV be giving out at high boost? I have an IPD replacement that I haven't installed yet so I should probably do that first.

Post by **matthew1** » 25 Nov 2012, 14:35

Absolutely, install that CBV. I too have one waiting to replace my original. My 850 has ~120k miles in case you're wondering.

https://www.box.com/shared/zdjf64r3k3 that's a CBV DIY.

RobTarD · Post by **RobTarD** » 25 Nov 2012, 15:45

Alright, I just went out and did a few pulls with the wastegate disconnected and still couldn't boost past 15 psi. Would this be a problem with the wastegate itself? I fiddled with the actuator arm and it seems to be working properly.

ARD-Lucky · Post by **ARD-Lucky** » 26 Nov 2012, 20:24

If the wastegate line is pulled from the wastegate actuator and the car still won't boost past 1 bar then the CBV could be at fault, it's not terribly likely but a quick vacuum test on that line will tell you. The CBV should hold vacuum when tested with a vacuum pump. Some slight loss is acceptable but should be minimal.

Other issues that might cause that are:
Clogged cat or exhaust
Restricted intake
vacuum/boost leaks
Wastegate arm not fully seated in the turbine housing (tap from above with hammer to seat)

Just a few thoughts, let me know what you find!
-Lucky

RobTarD · Post by **RobTarD** » 27 Nov 2012, 22:49

I'm kinda hoping it's a clogged cat so I'll have a real excuse to upgrade the exhaust....

I have an IPD intake, Snabb fresh air pipe, and silicone hoses throughout so I doubt it's something on the intake side. I've had the CBV sitting around for a while but haven't had the time to install it.

I won't be able to get to it until next month so for now I'll have to settle for "only" 15 psi.