Losing boost at higher RPM

[deletrius]

So I had my car dyno tuned by a reputable shop about 2 weeks ago and was saddened to find out it only put down right at 230whp and 240tq. I asked what could be the cause of this and was told that the car is bleeding boost off at higher RPMs. First thing that came to my mind was boost leak, so one was performed and resulted in nothing. Next thing that came to my mind was a weak wastegate so I shimmed it out using 2 washers per bolt. Still no improvement. The next thing I could think of would be an exhaust leak pre turbo. Turned out that the turbo to manifold stud would back itself out and be loose enough to remove by hand with just a couple turns, so I replaced it with a bolt. No improvement still.
What happens is the car will build boost like a 16g should, it will hit 21psi right at 3300rpm or so and pull nicely until about 4500rpm where it starts to drop off steadily and by the time I hit redline it is down to about 13psi! I searched but could not find any situations that were exactly similar to mine, any clue what could be the problem? If I had to guess again I would say this time that the turbo is shot or that the turbo/exhaust manifold gasket is blown. When I get some free time (maybe tonight) I'll drop the turbo enough to check the gasket, although I am kinda hoping it is the turbo, that way I have an excuse to upgrade to a t3/t04e 50 trim setup

 

[Black_Bullet]

Maybe theres something wrong with your wastegate flapper, it may be blowing open irregularly or something in that assembly is a dud.
16gs and similar small turbos usually dont hold boost very long especially when engine VE is dramatically increased. But what you describe sounds like another issue.
My 16g can hold over 20psi to 7k with full supporting mods.
I doubt an exhaust leak would be your issue , that would more effect spool up.
Its definitly related to the turbo... Check for shaft play and make sure your car doesnt smoke, or burn oil. If none of that happens then the compressor is good and you be likely to have a w.g. or boost controller or blow off valve problem being the culprit..

 

[deletrius]

Oh I forgot to add that I did check for shaft play, it has no up/down play but it does have a good bit of in/out play, which is bad. And when I have the car in gear (engine under load) and am decelerating (engine brakeing), when I come to a stop and put the car in neutral it shoots out some oil smoke for a couple seconds. Could it be bad turbo seals?

 

[DGajre777]

It could be a number of things:
1) Wastegate not staying shut
2) Actuator not being able to hold pressure
3) Boost controller leaking/gone bad
4) Small boost leak somewhere that doesn't really make you lose air until you hit 4500RPMs.

 

[fastclip]

yea i had a DS boost cont from dejon, and that is what was happening is the boost controller was leaking when the pressure went to wear it was suppose too and it would all the boost to bleed off. So i replaced it with a hallman RX, i really like it. I also changed my PCV valve and it seemed to fix the problem

 

[architechnik]

I would say your turbo is going bad, assuming your mbc and vac lines are all set and no boost leaks.
Exhaust side wouldn't cause much difference, as with higher rpms you're pushing more air to compensate - as far as minor waste gate fluctuations (they would have to be major).

My analysis:
While your turbo is spooling quick, under extended turbo rpms (much higher than the engine) it cannot push enough cfm with the increasing demand. The turbo may even be overworking itself under lower engine rpms although it is not enough to make a noticeable difference. Under greater demands that shaft play is showing its colors and cutting back.

Try reducing the boost to around 17psi and seeing how long it holds. If it drops off at the same rpms (holding steady up to) that would be a sign of the turbo. If it's a slow drop, then that probably eliminates the turbo and any cfm demands.

 

[deletrius]

I would agree that it is the turbo, after all it is the "china GT." I bought it before I heard about the bad rep they had a couple years ago, plus the shop I bought it from said it was a genuine MHI, I didn't realize it wasn't until I got the car home and actually read "EVO III GT" on it. Oh well, time for a garret t3/t04e 50 trim. Anyone have one for sale? Like I said it has been boost leak tested multiple times and no leaks were found.