I have a 98 a4 1.8 with a gt28 on it. Has an Mbc with the n75 blocked off and out of the way. Also a new wastegate actuator rated for higher strength. Problem is if I try to set it at more then 15 lbs boost it runs bad. Feels like the wastegate can't hold the pressure back and is snapping open. If I try to adjust it any it just makes it worse. If I dial everything back to 14-15 lbs, it runs great. Why can't I get anymore boost out of it? What am I missing

 

What tune/injectors does the car have?

 

Not sure. Bought the car like it is and he told me it was tuned and does have bigger injectors. Didn't know enough about it then to know to ask what tune.

 

Has it always performed this way?

Reason I ask, is that generally speaking with these cars, you need the exact hardware a tune calls out for or else the car may run like crap. Usually this would entail the following parts:
-MAF (mass air flow) sensor housing
-injectors
-FPR (fuel pressure regulator)

It's hard to start pinpointing problems when we can't necessarily get past the basics... but some things to look into:
-spark plugs (should be copper, level 7 heat or colder, and gapped no bigger than 0.028" [such as NGK BKR7E regapped])
-clean MAF sensor
-replace coolant temperature sensor (can also verify if it's operating properly with VAG-COM or a proper OBDII reader)
-fuel filter isn't clogged
-no misfires from bad coilpacks or ICM
-no air leaks

 

What it actually feels like it's doing is the wastegate dumping quickly. What in the system would cause that?

 

Limp mode is the only thing I can think of which would cause the behavior you're describing.

Your car is drive-by-cable, but in a car with drive-by-wire this could be due to the ECU closing the throttle plate from to too much boost being read by the MAP sensor.
This isn't your problem because your car's ECU can only control the throttle a little bit, and because your car doesn't have a MAP sensor.

Since your wastegate is only controlled by boost availability (through the MBC) the only thing that would cause it to dump quickly would be an inrush of boost pressure... when this behavior occurs does the boost level sit pretty steadily?

Also my questions above are pretty relevant. You could describe it all day long but that won't necessarily get us any closer to a solution.

 

The boost doesn't fluctuate when it happens. It runs good with everything set to 15lbs so I may just leave it all alone for now.

 

Well, these similar symptoms just starting happening on my vehicle tonight. I've been havign repeated issues with what I believe to be fueling, and have TONS of info on all of these issues so I wont go into detail again here.

Lately, I've been hitting limp mode almost daily now, has been getting worse over the past 2 weeks, even with replacing various parts. Tonight however, as I was softly accelerating as usual given the fact my vehicle isn't running healthy at the moment, I was at about 15psi or so and boom, no power like a limp mode. BUT, the only difference this time is instead of the boost stuck at 4-5psi without moving no matter how much gas or whatever, the boost would slowly rise up to 10psi towards higher rpms, but no more. While driving, no flashing CEL, but the solid CEL has been on for what seems like forever. The driveability other than no power (and what feels like a stock car) or when a MAF goes out which has happened to me years ago, there was no stuttering or anything like that. Just felt stock again.

When I hit limp mode, sometimes it's a "soft limp" where if I just continue cruising the power well eventually come back within seconds. Other times in limp, I have to disconnect my batter cable and go that route which will usually bring the vehicle back to life. Tonight however, the boost didn't come back while driving, nor when I disconnected the battery cables. Again, it feels very similar to when my MAF went out about 5-6 years ago and could be that once again.

I've had an appt scheduled for Thursday to get a bunch of other stuff installed, and I just got to work for the night. So, I guess I'll just have to drive it home feeling stock and have it re-scanned on Thursday during my appt so see what's going on.

Every since the K04/5BAR install, this car has been a pile of **** and HUGE ******* headache. Hell, even running the mild K04 tune for a couple months on the stock K03 and stock 4BAR was good having no issues, but this K04 5BAR is a joke so far. Hopefully the upgraded 255 fuel pump, MBC, and whatever else we'll need to do for this problem will make the car reliable and fun once again.