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!

 

[ddavisaf]

Sounds like you're having an issue with the wastegate. Make sure the vaccum line going to the actuator isn't torn or ripped. And yeah, run a boost leak test

 

[JDNSBR]

Sounds like you're having an issue with the wastegate. Make sure the vaccum line going to the actuator isn't torn or ripped. And yeah, run a boost leak test

If you don't....Oldman will find you.

 

[ddavisaf]

It's so true too....

 

[Strm Trpr]

You should still have your Boost Control Solenoid mounted to the chassis and plugged in, or you'd have a CEL.
Disconnect your MBC and connect the lines as if they were hooked up stock to the BCS.
If this helps with your irradic boost issues, then your MBC is the one to blame.

Is the MBC Tee'd into your BOV line?
If so, fix that immediately.

Since you're on the T25, you have a perfectly located boost source from the nipple on the compressor housing.

Hook that hose up to the input of the MBC, then run the output hose to your wastegate actuator.
Done deal.
Turn your MBC knob all the way CCW and go do a pull.
Then adjust your MBC accordingly to the desired boost level.

Also, after you're done with that, here's a link that shows you how to build a <a href = http://www.vfaq.com/mods/ICtester.html>Boost Leak Tester</a> if you haven't done so already.

I see you have a hard UICP, do you know if you have a hard LICP?
If not, then you'll have to do a boost leak test on the compressor inlet.
Do the test with the motor hot so the turbo seal will seal better for you.
If you're confident the LICP isn't leaking, then do the test on the Inlet of the IC.

If you do these 2 things, you should be good.
Oh yeah, don't forget to get a spray bottle with a lot of dish soap so the bubbles will be more apparent while searching for boost leaks.
Start at the turbo and spray every coupler, all fittings and gaskets of BOV, Thottle Body shaft seals and gaskets like to leak on older cars, fouled up PCV valves like to stay open and allow boost to flood into your Vavle Cover, Injector seals like to leak too if they're old and finally intake manifold gasket.

Good luck and keep us posted.
Sorry for such a long post, but I had a few things to say....

 

[ddavisaf]

I've been outdone! Good post

 

[JLangevin]

Thanks a LOT for your help! I solved the problem... The first thing I checked was the diagram from TurboXS and found out that the MBC was backwards... the tech hooked the inlet to the actuator, and the outlet to the turbo face. So, I sawpped the hoses at the MBC and turned it completely clockwise and the car wouldnt boost past 10psi.... then I started turning it out till I got it to 15psi and Lo and Behold, the car stopped spiking... now the boost still drops, but thats because I believe the BOV is set too soft... now that I have a working MBC I can sweet spot the BOV.

Also, the MBC is mounted just above the radiator directly above the turbo... the line used was some very small possible 1/4" Vacume hose... Im guessing this may be too small? I read on the TurboXS site that 6mm is the hose that should be used, and kept as short as possible.. do I will swap out the hose tomorrow and let her rip.

I already am seeking the parts for a 14B setup, so Im eager to compare to the stock T25... I heard its a definite seat of the pants feel.

 

[Strm Trpr]

Cool, I'm glad you got it dialed in.
6mm hose is a 1/4" hose.
6.35mm = .25", just an FYI, so I don't think changing the hose will help you at all.

Now, at what rpm is your boost droping off?
T25's run out of boost above 5500rpm, if you're going to redline with a T25, you're not doing anything but stressing it out.

I shift at 5500-6000 and when I shift, I'm still at 14psi.... that's just what you see in my profile, I don't even have my MBC hooked up yet.

I'm doing a fuel pump rewire and installing my MBC to 16psi this weekend....

 

[JLangevin]

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.

 

[oldman]

Size of the hose doesn't matter because it's pressure not volume that counts, the proper sizing depends solely on the size of fittings involved, one should always use the smallest hose possible without over stretching them on the fittings.

Spiking>>>settle>>>falling off is typical of a T25 but 20psi does seem excessive, I suspect it might have some to do with the TurboXS Standard MBC, a bleeder type valve, well know for boost spike. In any case, boost leak test first, they told you I would come after you.

Good post Strm Trpr. Unlike some lazy wiseman I know.

 

[JLangevin]

Forgot to mention, It was boost tested, no leaks... also the car idles at about 18mg of vacume. I hear thats normal?

 

[JLangevin]

Im running out of things to check... I cant get the car moving like I want since in order to get it at 15psi WOT, It will spike to 20 and bog/sputter. The fuel pump is new and was replaced along with the fuel filter when the head was repaired.

Im getting tired of not being able to actually give this car any throttle for fear of the reprocussions of excess boost.

Things I have checked already...
-Fuel pressure (38psi)
-New Fuel Filter
-MBC adjusted quite a bit
-BOv adjusted a bit
-Boost leaks
-????

Are the bleeder valves really that crappy?

 

[Strm Trpr]

Yeah, I remember seeing that you said it was boost leak checked, but these things can develop boost leaks over night.
It doesn't hurt to follow that link I posted to build one and have one handy.

Yes, 18 in/Hg is perfectly normal....

 

[oldman]

Forgot to mention, It was boost tested, no leaks...

Highly unlikely though I could be wrong, how much pressure was your intake tract able to hold (reading on your boost gauge) during the pressure test? How long did it hold that pressure? Did you spray soapy water every where?

 

[JLangevin]

We only pumped it up to 20psi seeing as I will never put more boost than that, even when I start the performance build. We tested at both compressor and upper pipe. The guage did not move nor could we hear ANY boost leaks in the car. The entire head is new, along with all seals, gaskets, fittings, hoses, turbo gaskets, etc.

 

[oldman]

We only pumped it up to 20psi seeing as I will never put more boost than that, even when I start the performance build. We tested at both compressor and upper pipe. The guage did not move nor could we hear ANY boost leaks in the car. The entire head is new, along with all seals, gaskets, fittings, hoses, turbo gaskets, etc.

Do you mean the boost gauge was showing 20psi and it didn't move after you stopped pumping? Or you set the compressor to 20psi and the boost gauge never moved?

 

[JLangevin]

We set the tract pressure to 20psi according to the gauge. In any case, if I had a leak, I would have expected it to be in the hose area and there were no leaks. Ive read the tech articles as well as searching for HOURS... Im begining to get frustrated with the car which is a pitty, its a beautiful car.

 

[oldman]

We set the tract pressure to 20psi according to the gauge. In any case, if I had a leak, I would have expected it to be in the hose area and there were no leaks. Ive read the tech articles as well as searching for HOURS... Im begining to get frustrated with the car which is a pitty, its a beautiful car.

You still have not really answered my questions clearly but anyway, I will leave the boost leak issue to you.

Let's recap the problem a little. You're mostly stock, BOV recirculated, T25 and TurboXS Standard MBC, problem is when set to 15, spikes to 20, settles at 15 and drops off at higher rpms, is that right?

 

[JLangevin]

Yes, thats exactly the problem. I am currently reading up on building my own ball spring type MBC.

The car will not settle at 15 without spiking to 20 first... it wasnt always like that, but I havent had a lot of time to spend with the car, used to belong to a friend. I appreciate the help oldman, I need the experience from you guys to solve this... I certainly havent had a lack in research, its just everyones problems are slightly different.

 

[oldman]

Let's do a little test while you make your mbc. Bypass the mbc by connecting the compressor cover fitting directly to the actuator fitting, this should bring your boost level down to about 10-12 psi. Spiking before settling at boost setting, like I mentioned before, is very typical of a T25, 2-3psi being the norm. If after bypassing the mbc spiking is reduced to 2-3psi, a properly made ball/spring type mbc will help greatly reduce the T25 spike. If you're still spiking 4-5psi, the problem is most likely wastegate related. Believe it or not, porting of the tubine inlet to ease the entrance to the wategate path, same areas of porting as for boost creep, will also help reduce if not cure boost spike.

Of course all this is under the assumption that the system is leak free.

 

[JLangevin]

... I know you love the boost leak comments, but I assure you, it IS boost leak free... myself and another very expirenced technician tested it... I dont understand why it is so hard to see someone have a boost leak free car... I paid $2200 to have the head rebuild along with a lot of other things replaced... all hoses, belts, tensioners, seals, gaskets, valves, guides, pvc, etc.... The only thing not replaced in the rebuild was the bottom end. This car is seriously boost leak free....

I forgot to mention also that I did bypass the MBC and hook the turbo outlet nipple directly to the actuator (local shop reccomendation) and the car did level at about 10psi after spiking to 12... so Im pretty confident now that the problem is the MBC... You say "properly built MBC"... do you have an aftermarket valve that you would reccomend?

 

[JLangevin]

... Ill be very pleased when the spiking is cured since on the way past 15psi, the car actually pulls like a raped ape! Sure, its a T25, but Im very impressed with the potential of the 4G63!

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.

 

[oldman]

I have a Hallman, a little pricy but very well made. That said, any ball and spring type from any reputable vendor should work fine. I get a little worried whenever I hear home made mbc, a badly or wrongly designed mbc can cause alot of problems and possible serious damages. IMO MBC is not something you want to cheap out on.

 

[JLangevin]

What about the TurboXS Ball spring Type? Have you personally got an opinion on it? I dont know how reputable TurboXS is... so far with the bleeder valve, Im very unhappy... thats the only link that keeps getting pointed at.

PS. Thanks for all your help! I would give you reputation upgrade, but dont see a button.


EDIT!! I see several Hallman types... BC-10, BC-41, BC-42. The BC-42 says it has a Ceramic ball... what is reccomended?

 

[oldman]

What about the TurboXS Ball spring Type? Have you personally got an opinion on it? EDIT!!

Sorry, I don't have an opinions on it but that's probably a good thing.

I see several Hallman types... BC-10, BC-41, BC-42. The BC-42 says it has a Ceramic ball... what is reccomended?

Damn, they must have changed the model numbers. I have the EVO RX with the ceramic ball and it works fine for me.