Boost spike and boost drop at high RPM

[blkGSTspider]

I couldn't find the good answer to my problem.
MY car is 99 GST and it is stock T25 turbo.
It is very close to stock and has manual boost controller, intake and Greddy Type-S BOV. Car has about 125000 miles.

The boost spikes to 0.9 bar when the RPM reaches 3k and it immediately drops down to 0.73 bar. When the RPM reaches at 5k, boost starts to drop slowly and by the time the RPM reaches at 6k, boost dropped down to 6.6 bar and hold there.

After I install the manual boost controller, the small pin under the turbo inlet where the vacuum hose goes is left open with out vacuum hose nor anything.
Is this something to do with the boost drop?

What is the reason of this kind of boost drop and the solution to it?

Please advise!

 

[JusMX141]

Dropping boost is commonplace on a turbo as small as a T25, but there's a good chance you have a boost leak if it begins dropping as low as 3000 rpms.

 

[TsiJohn333]

When you say pin, do you mean the nipple coming off the compressor housing? Your manual boost controller hooks up between your wastegate acuator and your compressor housing. I bet all your boost spikes and your boost drops are caused from improperly hooking up your mbc. Leaving the nipple of your compressor housing open like that is causing a boost leak causng you to spike and not able to hold a steady boost. Try hooking it up like this.

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[blkGSTspider]

Thank you for the reply.
I change the MBC setup but it didn't really help.
How can the boost leak happens when I didn't modify my car for about 2 years and I didn't touch nor change the setup of any vacuum hoses.
Could it be something else than boost leak?

 

[pj91gsx]

Thank you for the reply.
I change the MBC setup but it didn't really help.
How can the boost leak happens when I didn't modify my car for about 2 years and I didn't touch nor change the setup of any vacuum hoses.
Could it be something else than boost leak?

Some of these cars are from up north, most common thing i have seen is stock intercoolers corroding where the hose connects. its a very hard place to see it leaking in a boost leak test. Also age. intercoolers corrode, throttle body seals, and fuel injector seals dry rot and leak. the biss idle screw oring drys up and leaks there, the throttle body gaskets can leak over time as well jpipe gasket for 1g 14b guys. search building a boost leak tester. i havent found a turbo car without a boost leak.

 

[TsiJohn333]

All it take is a small rip or tear in a vacuum line or intercooler hose for you to be spiking like that. I agree with pj91gsx. Start by doing a boost leak test because 9 time out of 10 thats usually it and the symptons. Also it could be that your turbo is on its last bit of life. You can check for shaft play.

 

[mitsu-luver]

well if you could just close that wastegate 45* your problem would be solved. The turbo loses momentum once the wastegate fully opens. reliving a shiz ton of pressure off the turbine thus loosing a small amount of boost. Idk maybe im wrong but i feel it only makes sense. check all your hoses and charge pipes and let us know if you find anything. if you dont i think my point is proved...

 

[blkGSTspider]

I turned the top knob on the BOV to hard but it didn't help.
If I floor the gas on 5th gear at about 75 MPH / 3000RPM, the boost stays at about 0.78-0.8 bar and it takes a while to reacht 4.5k RPM since I'm on 5th gear but there is no boost drop until 4.5K. My question is, if there is boost leak, sholdn't the boost has to drop at any RPM?
No matter what gear I'm at, the boost depends on the RPM.
3K - 4K RPM: boost stays on 0.77-0.80
4.5K - 6K RPM: Boost slow drops and by the time RPM reaches at 6L, boost drops to 0.57-0.60
6K - above RPM: boost stays at about 0.57 - 0.60
boost

 

[mitsu-luver]

The wastegate doesn't open fast enough.

Sent from my DROIDX

 

[Jarbo440]

I have a 96 GST with a similar problem. Mine is an automatic so it is hard to do a 3000 rpm up pull, because it downshifts at WOT. If I run it at just enough throttle to max boost it will spool up around 3k to 14-15 psi then by 4500 it will drop to 12 psi and stay there. I have noticed at WOT the AFR will drop to high side of lean (on a narrowband autometer gauge). I am not terribly concerned with the AFR because I bought the wrong gauge before I knew better.
The only mods this car has are the cat has been removed, boost and AFR gauge installed, and a cone type air filter.
Since I added the cone air filter I have noticed a whistling sound when the car is making boost (any amount). It doesn't sound like the turbo spooling, instead it sounds like a huge air leak very similar to the sound it makes when I let off the throttle and the BOV exhausts. I didn't notice this until I changed the air filter and removed the extension pipe on the BOV connection inside the stock snorkel. Could this be the BOV leaking, I have ordered a replacement, but just a cheap one from ebay. I plan to add a FMIC and blow thru MAFT setup so I don't what to spend a bunch of money on parts I won't be using with that. Hence the reason I stayed with the stock snorkel. Any help or ideas would be greatly appreciated. Thank you.

 

[DJ23GSX]

Are you using a ball and spring type boost controller? I had a similar issue with spiking and ended up putting a softer spring inside the boost controller and that cured it. The stiff spring wasn't opening the wastegate fast enough causing a boost spike. Also make sure your vacuum lines to and from your boost controller are as short as possible. The longer they are the better the chance of boost spikes. Def do a boost leak test as stated before its easy and can help eliminate one factor in dropping/spiking boost problems. A bad/weak wastegate actuator could blow open also causing boost to fall off in the upper rpm's as well. I've had to deal with that one recently but a few washers to shim the wastegate fixed that.


500th post woot woot!!

 

[kornfedgsx]

check for a boost leak check all couplers, if everything checks out good your wastegate actuator could be bad.

 

[2ndGenOrDie]

I would like to start off by saying, if you're running a stock t25 it will drop off at 5k. Before I got mine bored, the power band was from 3-5k. Dragging out your shift to 6k is out of the turbos power band and so naturally it will lose boost. Correct me if Im wrong because this is my experience with my car before I put a mbc on it. Definitely check the turbine for shaft play.