Okay so I’m new here. Always been able to come to forums and find info to help and not have to post. Well I have a 2001 Audi TT Quattro 225 amu. It is tuned I’m not sure what tune it is. Bought it ran fine for a week. Figured some things out. So here’s new parts. New map, maf, iat, ect, upstream o2, brake light sensor, got rid of mbc and hooked up n75(bought cheap from oriellys) oem will be here tomorrow. Cleaned throttle body and took injectors out and cleaned them. New vacuum line under intake. 2.0 red coil packs. Bkr7e plugs gapped at .028(might be forgetting a few things) it has sai delete and forge dv and catless. I have vcds and only codes I get are read o2 codes and occasionally evap leak p1476 which I’m told happens sometimes for sai delete. Finally thought had everything worked out finally. Drove the car and was spiking boost at 23lbs so realized it is tuned cause before problems was 17-18ishlbs then mostly 5-10lbs. So drove for about a hour boosting fine everything looked good on measuring blocks on vcds. Went home to get my wallet. Then went and got gas and was fine. On way home would randomly hit 5-10lbs boost max. Then it would kick in and hit 23lbs and boost great. Then it would go back to 7ishlbs and so on so thought throttle needed to get used to my driving so country roads pull after pull running great and come home still doing pulls and starts the low boost again. Parked it and was stumped. Drove today and wouldn’t boost past 5-7lbs again wouldn’t open up randomly like yesterday. So confused with this car. I read anything n75 besides oem is garbage but wasn’t my only problem at first so bought cheap cause could get it right away. Does that sound like my only problem? Or should look elsewhere while waiting on part? I can’t find a boost leak but haven’t done proper test but that doesn’t make sense with back and forth boost problem. Still no new codes on vcds

 

Since you’re not getting a response from the guys who probably have an answer, I’d look for leaks, especially in the two boost hoses. Could have cracks in the hose or cracks or leaks at the connections. You might also check to see if hoses are partially collapsing under boost. Good luck!

 

That cheap N75 could for sure be causing some issues. Do you have a aftermarket diverter valve? because the stock one cant handle those kinds of boost pressures and may be showing signs of it failing and prematurely releasing boost. if you dont have one i would highly recommend getting a aftermarket one (and a good one too not a cheap one) because even if that is not the problem now it for sure will be in the future.

 

When I first built my car I had similar issues but had no tune and was attempting to set my mbc to 14psi. I found out that the stock ecu has a boost limiter and if you exceed it (sometimes), youre put into a limp mode that limits boost to 6-8 psi. This was on a 180hp 00 that has a boost limiter of 10.5psi, so maybe its different parameters for the 225? Possibly the tune you have also has a boost limiter (these are to protect from wastegate/boost controller malfunctions) and your build is exceeding that.

 

Ok after re-reading your issue I'm positive you're passing the boost limiter and putting your car in limp mode. Whatever changes you made to the vacuum system is probably preventing the wastegate from functioning properly (possibly incorrect n75 install or a bad n75) and you're not cutting boost when you should be. ECU takes over and limits your power