I recently noticed that boost was falling off at the higher RPM's and suspected a boost leak. All I wanted was 15psi throughout the powerband on my S16G turbo, but was seeing the boost fall from 15psi to about 12psi by redline. Hooked up my boost leak tester and heard air leaking out around the TB.

We all know that the TB shaft on pretty much every TB leaks, but this leak sounded substantial so I went ahead and changed out the TB gasket, hose clamps, and silicone coupler.

Before I had the idle set screw turned slightly in and the car idled perfectly at 800 RPM's, now I have it fully backed out (butterfly plate as closed as it can get) and it idles at 1K. Also, when I am in neutral and coasting to a stop, the RPM's jump up and sit at around 1500 RPM's, and only fall back down to 1K when I come to a full stop.

Took the car out tonight and did two 3rd gear pulls, and the boost issue seems to be worse now. On both pulls I had the initial boost shoot up to 14.6, and by redline it had fallen to 9.2!

I am running a Joe P Pro Z MBC. Any ideas guys?

i swear its ur MBC. can't think of anything else.

I hear ya Mikey, but Joe P's stuff does have a prety good reputation in the DSM community, and the Pro Z was designed around the 16G. Also, the MBC would not be responsible for this idle / coasting issue I'm having. To me that sounds like it must be either a vacuum leak or an IAC issue.

I will hook up the boost leak tester again this week when I get a chance and see how it sounds..

Originally posted by Ebster1085
I hear ya Mikey, but Joe P's stuff does have a prety good reputation in the DSM community, and the Pro Z was designed around the 16G. Also, the MBC would not be responsible for this idle / coasting issue I'm having. To me that sounds like it must be either a vacuum leak or an IAC issue. I will hook up the boost leak tester again this week when I get a chance and see how it sounds..

The idle coasting issue you need to learn to live with. The stock TB does it as well, only its not as bad because of the size of the TB.

You can try running a stiffer spring, but in the end it is the IAC that does it. The speedo sensor also feeds into the IAC, so if it sees movement, it sends the signal.

Thats what Dino told me when I was complaining of the same thing a year ago.

If it was just a matter of the VSS sending the IAC a signal to let it know the car is moving, then wouldnt I have had this issue all along? It only just started after I pulled off the TB and changed out the gasket, coupler, and hose clamps.

I KNOW TIM, DAMN, JEEZ, SHIT :afro

Fixed the problem. Pulled off the TB again tonight and removed the IAC. It was completely dirty from the 50K miles put on the car running an EGR valve and recirculating breather lines. I thoroughly cleaned it with RC electric motor cleaner and reinstalled it. Then, based on Tims recommendation, I set the TB spring (MP TB) to the tighter setting. Put it all back together, adjusted the idle stop screw, and now it idles perfectly at 800 RPM's and also idles correctly when coasting to a stop.

Thanks for the help guys!

Your EGR system would not contribute a whole lot to a dirty IAC as the gases are dumped in after it. PCV pulling oil in pre-throttle body is more likely.