Strange boost issues...

[JLangevin]

Hello there. First off, I have been working on V8s and other domestic muscle for years... this is my first turbo car, and first DSM.

1995 GST
Stock T-25
Hard upper pipe
TurboX MBC
Greddy Type S BOV
Stock sidemount
Recently rebuilt top end, all tensioners replaced, etc..

The boost is all over the place.. no matter where I adjust the MBC to, the car spikes 20 and fuel cuts, then back down to 15psi... if I hammer the gas at higher speeds, the boost hits 16psi, then backs down to 12psi and drops to 10 a few seconds later...

The turbo spins freely and has no shaft play. No matter how I adjust the MBC, it responds the same way. Please do not send me to the boost creep porting post as I have already ported the flapper and wall in the wastegate area. This is something that has started happening recently. I have not personally checked for boost leaks, but the mechanic I had rebuild the head said he performed the test before and after the rebuild... I trust him, although I should still do it first hand.

Does anyone have a direction they can point me? Im very new to turbo systems. I know how they work, but have not had the experience to tune and trouble shoot... thanks!

 

[JLangevin]

I will get the EVO RX (they call it the BC-42 now) with ceramic ball... its $120 from some stores, but a reputable Ebay seller has them for $89.99. I will give that a shot.

 

[MiSFiTz]

Im in the process of seeking the parts for a 14B upgrade, will I see the same spike issues? Or should I just jump straight to 16G... I dont care to have the fastest car possible... Id like 250 to the wheels and pull some decent 13sec numbers. Since the car is only FWD, I can see how easy it will be to overpower the traction. Im used to V8 RWD, so when I see my car boosting to 16psi and the tires start to chirp, I forsee traction probs.

If your having these problems w/ the T-2slow, your gonna see even more w/ the 14b, i have the 14b and control the boost w/ my foot by watching the gauge until i get some money and do things the complete right way, by this i mean, (this also goes for the b16g and evo3)

Get a GOOD manual boost controler *NOT HOME MADE*
Get a good o/2 housing tubular or extremly ported
Get a good exhaust system 2.5" back or better from the o/2 housing
Of course the turbo b16g, evo3, 14b whatever your style
Get a good aftermarket wastegate setup! vent or not *this will fix 90% boost problems*
Port your exhaust manifold or get a different one your choice either will work..

With all that and of course your fuel/injectors/sfac if u choose that u will be set nicley and no boost spike problems...

My 14b on my fmic w/ no boost leaks, the vacuum line straight from the lower j-pipe to the wastegate i still cannot control boost at all.. i have 3" exhaust no porting no nothing and its just not working for me... do the above and be done

 

[oldman]

MiSFiTz, you have boost creep, the original poster has boost spike, two completly different problems which requires two different solutions. This is how you solve boost creep.

 

[ddavisaf]

The bleeder valve is tricky to master.
I had the TurboXS bleeder valve. I found that a tiny tiny *tiny* adjustment would actually help with the spikes. It's hard to explain but I'll try.
You'll set your MBC to where you want it (15psi from reading the post). Usually it will spike like crazy upon the first setting. After this I always barely increased the boost again and it almost always cured the 4-5psi spike. I would still spike 1-2 psi, but I'll take 1-2 over 4-5 any day.

 

[cb3016]

Well, if I want 15psi, the car will spike to 20 then drop to 15... its a spike, not a creep... so I figured that it was the hose to the MBC... I may have mis typed the current hose specs.... the current hose on the MBC from the turbo and back to the actuator is roughly 3mm.... Its the same size as the hoses comming off the throttle body.. very small.. they are also about 15" on each side.. so thats 30" of 3mm hose. Im guessing thats entirely too small for the critical job of the boost controller.

with the mods that you have your car can't do 15PSI and not be killing the car, bypass the MBC and see if that corrects your problem. You said you have a greddy BOV make sure the screw on top isn't too hard or too soft. Also make sure that your return dump to your intake is sealed correctly.

 

[ddavisaf]

You can run 15 psi safely. Just no more than that.

 

[JLangevin]

I know what I can and cant run with the car, it can handle 15psi just fine. Ive ordered a Hallman EVO RX mbc. We will see what happens.