Losing boost at WOT.

[tbonitz301]

Everytime i go wide open the car will go to 15psi which is what its set at, and then fall back to about 11. If i feather the gas the boost will go back to 15psi for a second. My wastegate is tee'd into the bov line. Im not sure if that could be the problem. Also im running a turbo xs BC.Any thoughts?

 

[MrBoxx]

Don't tee your BOV line for your MBC. Use a barbed fitting on the J-pipe.

Also, make sure you installed the MBC correctly, as the TurboXS MBC is setup pretty much backwards from almost every other MBC out there.

 

[mike_tsi]

Does it studder or anything of that nature when it goes down to 11psi? Because I have an xs bc and evo3 boosted to 15psi and it holds solid. Also its tee'd into the bov line.

 

[tbonitz301]

No it doesnt studder at all i can just feel the power drop off, and see the boost gauge. The mbc is hooked up correct. Ive hooked it up numerous timesAnymore ideas?

 

[MrBoxx]

Boost leak test is in order.

 

[dSmgs-T]

+1 on boost leak test!

 

[JayRolla]

Make sure you dont have the mbc tee'ed through the bov line. The bov line should be unmolested all the way to the intake mani. If thats how you have it then a boost leak test should be the next place too look. Also if its a cheap bleeder valve mbc they will spike and drop boost like that.

 

[dsmtalon2008]

i have the turbo xs boost controller and i was currently having this same problem, it turns out it was not my boost controller, it was my turbo bc the shaft was so wobbly that it would not pull consistent air threw, so the boost would just be all over the place,

 

[carlos]

Check for the boost leaks I had the same problem. And to my discovery a boost leak was found

 

[bob123131]

Not sure with your familiarity with bc's etc, but have you tired turning up the boost, ive seen others think there boost is dropping off, but in fact their bc is set at 11 and they are just creeping to 15? Whats you entire mod list? Also i would guess boost leak too, usually a little leak wont give you a studder, but you will get a surge of boost and 15 is easy to hit but once it hits 11 you start to leak and that first surge gives you a spike you but it just keeps leaking and brings you back down to 11.- The science behind that is no way a fact just what it seems like and my best guess with my experiences with different small boost leaks.

 

[JayRolla]

He hits 15psi then drops to 11psi which is a spike, not creep. He doesnt start to get more boost he loses boost.

 

[bob123131]

O true i wasnt thinking clearly i will edit my post thx.

 

[CanadianTSi]

Does it slowly drop down as the RPM's climb? If so welcome to the T-25...

 

[Simpleman1]

My Turbo XS MBC "T'd" with BOV was holding 16 PSI solid when I had 16G.

 

[Splitpi]

Stock T25? I think you reached its max flow at the upper RPM's and thus loosing boost as it no longer can keep up with the engine demand.

But boost leak, etc... its just a good practice. But I think your turbo cannot compete at that much CFM per revolution of the crank.

 

[bling5tatus]

I wish people would stop passing misinformation around and saying how their MBC worked "just fine" t'd into the BOV line.... this is just plain retarded.

YOU ARE MESSING WITH THE PRESSURE THAT IS USED TO HELP ACTUATE THE BOV, if you want to blow your turbo up over intermittent compressor surge problems, then be my guest. I imagine this also has an affect on the tbody seals as the bov won't properly release boost that will be backing up against the plate.

 

[joe]

I wish people would stop passing misinformation around and saying how their MBC worked "just fine" t'd into the BOV line.... this is just plain retarded.

YOU ARE MESSING WITH THE PRESSURE THAT IS USED TO HELP ACTUATE THE BOV, if you want to blow your turbo up over intermittent compressor surge problems, then be my guest. I imagine this also has an affect on the tbody seals as the bov won't properly release boost that will be backing up against the plate.

+1

 

[tbonitz301]

Ok guys i have abig 16g installed. No boost leaks at all 60sec+ to leak down 25psi. Im leaning towards a faulty mbc, but i think i will try the j-pipe pressure source first. The tee into the bov line was just temporary to get my car going when i installed the 16g.Please help

 

[quicksilver2102]

Ya know, I have the exact same problem. I was running a HRC Super20g and when I first got my car out of the shop it was a beast! It would smack 15psi by about 3800-3900 and hold there till redline, and now recently it has been hitting 14-15psi and then dropping off to 11psi. So I know that mine is not a spike to 15, since it used to hit that and stay there the whole way. Found some boost leaks but had them fixed and it's still doing the same thing, so I'm kinda stumped right now. Blew the turbo for some reason so now I'll be replacing it w/ the same one, so maybe it was the turbo but I guess I'll find out once I put the new one on.

 

[eclipsetech]

you want to tee the boost source for the mbc as close to the intake as possible not on the j pipe. this way you get a more accurate reading of what the engine is seeing.

 

[tbonitz301]

Uhhh, beings that the j pipe is connected to the compressor outlet i'd say thats a pretty good spot.

 

[MrBoxx]

you want to tee the boost source for the mbc as close to the intake as possible not on the j pipe. this way you get a more accurate reading of what the engine is seeing.

If you searched, or if you thought about what you said for more than a minute, you might see how your statement is moot.

Your boost gauge is what you use to calibrate an MBC (e.g.: my boost gauge says 13psi, so i turn my boost controller knob till the boost gauge shows 15psi which is my target). Since the boost gauge tap is on the intake manifold, REGARDLESS where the MBC gets its pressure source, you'll ALWAYS get an accurate reading of what the engine is seeing, assuming there are no boost leaks in the system.

The main purpose of not tapping the BOV line is to avoid putting a boost leak in that line, which would alter the opening and closing of the BOV. The same people who say that their car "runs fine" that way would probably also make the argument that a 255lph fuel pump doesn't overrun their FPR because they "don't notice anything different" or that their car runs just fine venting their BOV to atmosphere on the stock MAF.

 

[tbonitz301]

Is there anyway to test my mbc? Can i take it apart and inspect it?Thanks eric

 

[MrBoxx]

The spring could be weak, or broken, or it could be sticking. The only way to tell for sure is to take it apart and see if there's any debris in the body of the MBC. If you have a MightyVac and an air pressure gauge, that might help too.

 

[tbonitz301]

I'll check it out. Thanks again;