View Full Version : Weird boost fluxuation

05-06-2011, 07:46 PM
I finally got a video of this:


This happens at maybe 30% throttle, just accelerating a little bit like up a hill or something. It just bounces back and forth a few psi, throttle is steady the entire time. I can't figure out wtf would cause this. Also, sometimes (not always) when I'm at maybe 50-60% throttle (where the car typically hits 10psi) as soon as it hits 10psi its like the DV just decides to dump all the boost, it drops back to 0psi and then builds back up. Obviously this results in the car falling on it's nose for a second then as soon as the boost builds again it goes. I don't think it's the DV though since I'm on the throttle (DV is in stock orientation).

As far as I've been able to tell all vacuum lines are good, I've taken the manifold off and checked everything under there. Check valves have been replaced, N249 is bypassed, everything seems to be good but I don't pull enough vacuum at idle (18psi at idle with no AC, car in park) so I have to have a leak somewhere but I can't find it. I've heard the vacuum reservoir mentioned, but I don't know where it is.


05-07-2011, 04:10 AM
Good luck finding the problem, I have had the same problem for quite a while but not quite as bad of a fluctuation. My situation is the same as yours, only happens on part throttle between 7 and 9psi.

05-07-2011, 06:31 AM
So It doesnt happen with WOT? or is 10psi your max at any time.
How long has the f4hl been in? did the problem start then? Sounds like something that a wastgate set at to low a opening pressure could do, as exhaust gas builds it could overcome the wastgate and open it dumping your boost. That or a boost leak in one of your hoses- a split that opens up enough when under pressure only- but doesnt explain low vacuum at Idle- if your gauge is correct.

05-07-2011, 02:54 PM
Max boost is ~22 psi and no it never does it at WOT. It only happens at part throttle, and not all the time. The inconsistency of it is what drives me nuts. The only hose I haven't replaced that I probably should is the line from the manifold to the DV as it's the original (though it appears to be in good shape). I don't know where my idle vac leak is at, though there has to be one as my idle fuel trim is -3%. It's also recently gotten worse because my idle fuel trim used to be -1%.

This crap is driving me crazy. The car feels great about half of the time, the other half it's inconsistent and sometimes difficult to drive. I really should just go pay for a smoke test and see where that gets me.

05-10-2011, 06:19 AM
Bump. Any other ideas?

05-10-2011, 07:05 AM
Boost/pressure leak. Do a pressure test. How old is your n75? What revision is it?

05-10-2011, 07:31 AM
Boost/pressure leak. Do a pressure test. How old is your n75? What revision is it?

N75 is brand new, Revision E. I'm going to try to do a smoke test this week. I just spent 20 minutes with the car idling and clamping off every vac hose I can get to with no change, stil idles at 19inHg.

05-10-2011, 08:08 AM
Could be tears in the silicone as well, may not be leaking at the ends.

05-27-2011, 10:07 AM
I think I also found the source of my last vac leak.


Basically if you trace from 13 up to where that T is, the rubber T portion is what has the hole in it. Is it safe to cap that bottom line off the suction pump? That line connects into the crankcase and the pancake valve on the TIP, is that safe to cap off or should I leave it open? The way it is right now is I disconnect the T portion from the suction pump and left that open, then put a cap on the suction pump.

Because of these issues I think I will end up just ripping the PCV system out and do a catch can and have my tuner take out the SAI code in my ECU. I can't stand this stupid crap always developing leaks and I really don't want to spend the money to replace it all....