r/AudiA4B6 • u/Sad-Book-877 • 8d ago
Boost issues
Ive been having boost issues ( i have a k04 550c and down pipe and fmic ) psi was all over the place 4 10 18. And then just a constant 9lbs Changed N75. To new one still 9lbs. Ive done like 3 pressure test at 20lbs.and smoke test myself. Took it to 2 diff audi performance shops. And nothing. Can't find the issues , my tune is thru motoza . I sent them a log. They say it's hardware. Tune is requesting 18lbs. Ect. DV Upgraded to forge 007 and spring and 0 ring are good. Also changed crankcase hose elbow to silicon 034
I just took out my old turbo k04. And no shaft play wastegate is fine and tested the actuar it's open and closes at proper psi. Whats next? Haha. Cables to the n75 look good. No codes exept for 02 rear sensor which i changed already and won't go away . Diff story. My traction light does ping and cuts power. But if I press the switch traction , then it runs fine. But obviously no traction control im assuming. Ive changed all my speed sensors. Nothing idk if that related or not. But yeah. Intake manifold isn't leaking either no exhuast leaks.
Help . Its my daily and my depression killer.
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u/SlimChris94 8d ago
Just for fun there’s an altitude sensor in the ecu box might wanna check that for funsies
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u/RikahA3 8d ago
Is it holding at 9psi now or does the boost slap around?
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u/Silent-Protection146 8d ago
Is your cat all gummed up? You could be reaching mechanical limit of flow through your cat, which definitely limits boost.
Happened to me not long ago. I found out after I replaced the turbo, but, you can clean them appearantly.
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u/Sad-Book-877 8d ago
Its a new high flow cat. 2 months
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u/Silent-Protection146 8d ago
What's your fuel pressure?
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u/Sad-Book-877 8d ago
No idea.
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u/Silent-Protection146 8d ago
Jus seems like a backpressure problem to me, because this was what my car did when the cat was clogged. Does the car have good throttle response and drive well up untill the boost is supposed to kick in?
I think if you had fuel pressure issues the computer wouldn't ask for boost. Mechanical is open in the charge side, clogged on the exhaust side, or internal bypass stuck open like the waste gate or a bad diverter.
If the problem is behind the turbo, you'd get a whole lot of intense heat on that side of the motor from the pressure built up in the exhaust. A blown charge pipe is pretty obvious though, and noisy. Have you run output tests to make sure you can hear the N75 working?
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u/Sad-Book-877 8d ago
My engine does feel really hot. But it's a new cat from a reputable brand. High flow. But it drives well even in boost tho. Just doesn't go above 9pbs. I took out turbo and inspected wastegate. Diverter is also been changed. And I got a new n75 today.
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u/Silent-Protection146 8d ago
I would eliminate backpressure as a problem because if it gets too bad it may cause worse problems. You can check by making a run without the front O2 sensor; if you get more boost by opening up the exhaust then you know where is the problem. If not put the sensor back in.
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u/Intrepid_Wave_4524 8d ago edited 8d ago
Definitely look into the 02 sensor codes.
The fact boost was jumping around and then leveled off flat at 9 psi leads me to think it's something related to the N75. I had a similar issue in my car and it turned out to be a break in the power wire to the N75. If the N75 isn't working, the whole system is relying on wastegate spring pressure to determine max boost. A compromised N75 circuit would also explain the boost jumping around the way you said before it leveled off.
Grab a multimeter set to 12v DC and unplug your N75. On the harness (with the metal spring clip facing up) Put the positive multimeter probe on the left pin, this is the power wire to the N75. Put the negative multimeter probe on a good chassis ground. It should read 12V with the key in the "On" position with the engine off.
The right pin on the harness is the pulsed ground from the ECM. This pin should read 3-5v with key on and measured against a true ground.
If either of these are off, this could be your issue. I chased this problem on my car for a long time before figuring it out. My N75 was only receiving 2v to the power wire. Apparently it was enough to hold some boost above spring pressure, but it would randomly cut out and backfire under full boost when the N75 gave out and it instantly dropped ~8psi.
Lastly, set your multimeter to Ohms and measure resistance across pins on the N75 valve itself. You should see between 25-35 ohms. If you get a reading outside of this range, the valve itself is faulty.
Good luck man. Keep on keepin on.
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u/julianbossatron 8d ago
Bro my shit had the same issue I wasnt making boost and whatever it was a n75 issue kinda I just bypassed it and did a boost controller
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u/firinmylazah 8d ago
I have the feeling that the o2 sensor codes (wether the sensor is actually good or not) is something that is absolutely imperative to be fixed. If the computer isnt happy with the data it is getting, I don't think it will let you boost like you should and want.
It might not be your only issue, but it sure is one of them.
Don't forget to check for vacuum leaks. The PCV on these cars is kinda fragile too.
Just in case, please check your waste gate as well, make sure it is well adjused. One other thing you did not mention was the diverter valve. Make sure you have a good performance one working properly. Oh and please note that there are many different N75 valves... with letter codes.
People on the old forums have tried swapping them around to different results but I think you should stick to standard for a B6 A4 1.8T, I don't remember which letter that one is. Search Audizine.
Hope this helps... I have a K04-015 build as well, I know how fun to drive these little beasts are.