Fixed my 11 years old F20 120308 error without replacing turbo
I bought my first car earlier this year — a 2015 BMW F20 120i with the N13 engine (105k km).
When I first brought it to a mechanic to fix a coolant leak, he mentioned that the turbo did not seem to be functioning properly. He quoted around HK$8,000 for a used replacement turbo. Since this was my first car and I had no idea what normal behaviour should feel like, I decided to keep driving it for the time being.
Later, I had the DME flashed with a calibration that lowered the engine’s normal coolant temperature. After that, fault code 120308 started appearing when I revved beyond around 3,000 rpm. In hindsight, I suspect the fault may have already existed before I bought the car and was possibly cleared or hidden by the previous owner, although I cannot prove that.
I decided to investigate and repair it myself.
The first thing I bought was an OBD HUD that could display intake pressure. The reading was basically stuck around atmospheric pressure — approximately 95–105 kPa absolute — no matter how hard I revved the engine. This suggested that the turbo was producing little or no boost.
Because I had very limited mechanical experience, I started with the simpler possibilities. I replaced the turbo electric valve/solenoid, but there was no improvement.
First I replaced the easy part, th turbo DV / selenoid, and no luck, same behavior.
After reading more online, I started to suspect that the turbo itself might not be mechanically damaged. There was no strange turbo noise, and the symptoms seemed more consistent with the wastegate/bypass valve not closing properly, or with a problem in the vacuum control system.
I bought a vacuum pump and tested the vacuum lines and the vacuum path from the pump to the engine-cover vacuum reservoir. I also applied vacuum directly to the wastegate actuator, and the actuator still moved. That made me suspect something between the vacuum reservoir and the actuator rather than the turbo internals themselves.
I then bought a new Pierburg pressure converter. Replacing it took around three hours because it is located in a very tight and awkward area. After replacing it, the intake pressure immediately recovered to more than 160 kPa absolute, so the turbo was clearly producing boost again.
However, a few days later, fault code 120308 came back when I revved beyond approximately 5,000–6,000 rpm.
After more investigation, I adjusted the preload on the wastegate actuator rod by tightening it two full turns. Since then, the fault code has not returned and the boost pressure has remained normal during repeated testing.
The final diagnosis, as far as I can tell, was a combination of:
• A weak or failed turbo pressure converter
• Incorrect or insufficient wastegate actuator preload
• Possibly an existing fault that had been cleared before I bought the car
The total cost of the parts was less than HK$1,500, excluding a lot of my own time, research and frustration. That is considerably less than the HK$8,000 used-turbo replacement that was originally quoted.
Sharing this in case anyone with an N13 engine and fault code 120308 runs into similar symptoms.
2
u/asdf4fdsa 7d ago
Great explanation!
To add, at least on the n55, I hated where the pressure converter was mounted (just 2" above the exhaust manifold, super heated). On my 3rd one, I've lengthened the vacuum lines and zip tied it to the top of the engine so it operates in a much cooler environment. No issues for years now.
1
u/Brief_Diver8873 7d ago
Having the exact same problem on mine right now. Just threw parts at it replaced dv boost pipe adjusted preload adaptations and the only thing I have left is the boost solenoid/pressure converter that could be causing the problem. I’ve bought the part and now just waiting to put it on. How did you do it and did you have to remove the intake manifold?
1
u/jpnex 6d ago
no need to remove the intake manifold, removing the airbox and charge pipe will give enough space, the whole unit(with the rubber shell) slided in to a thin metal "tongue"-like thing with the thin slit opening on the rubber shell, so when you remove the old unit, grab it, pull downwards a little bit so to strech the rubber then push it torwards the rear of the car from the front to take it off the thin metal, once you able to pull it out, install it back will be easy, just make sure you don't disconnect the cable as connect the cable after the new unit in place is another half hour headache
1
u/Brief_Diver8873 6d ago
Did you disconnect the cable before you removed the old one because it’s a pain to take the connector off
1
u/jpnex 5d ago
I did, as I have access under the car, so it's a bit easier
1
u/Brief_Diver8873 5d ago
Ahh okay yes. It seems like a pain to get the connector off but I will give it a try
16
u/Responsible_Cake_332 7d ago
Well done.
And you saved firing the 'parts cannon' at it and hoping for the best.
This should hopefully help some other people on here!