BMW N54 Twin Turbo - Diagnosis and Repair - 0030FF Low Boost Pressure Error Code

I got a check engine light this August while on a family road trip. 205,000 km (145,000 miles) and I replaced both turbos 2 years / 15,000 km ago. The fact that it probably wasn't the turbos was comforting, but it still bothered me. I had a few other issues to sort out with the car - an oil pan leak and the replacement of both rear drive axles (leaking grease), so I figured I would get the car up on the Quickjacks and sort all this out at one. 

Low boost pressure can be caused by 3 main reasons:

1. Bad turbos (worn wastegate bushing, wastegate rattle issue). Turbo is unable to generate boost because the wastegate valve can't close completely, preventing the exhaust turbine from receiving energy, or more sinister failure such as worn or seized bearings; 

2. Vacuum control issues - turbo wastegate actuators not getting enough vacuum when demanded by the ECU - due to bad Pressure Controllers, bad Actuators, or vacuum leaks / blocked or collapsed vacuum lines.

3. Charge air duct leaks / burst charge air ducts, air duct connections have popped open, or Pop-off valves are disconnected from the charge air pipes. If boost is leaking, the pressure sensor won't detect boost. There's also a chance the charge air pressure sensor is faulty. 

A wise person once said, don't rule things out because you think they're fine, someone else said they're fine, or that you think they can't be the problem. They might actually be the problem, so make a diagnosis plan from most likely to least likely and be systematic.

Step 1 - Turbos

I had the advantage of having the exhaust system off the car (in order to remove the left rear drive axle), and the front right drive axle and bearing support removed (in order to repair the oil pan leak). So access to the downpipes, and rear turbo actuator was pretty good.

The first item to check were the wastegate bushings - I could access the rear linkage - everything was tight, nothing was loose. As expected for an almost new turbo. I expect that the front will be the same. 

Step 2 - Vacuum Control Issues

I disconnected the vacuum lines from the two wastegate actuators, where they combine and connect to the pressure controllers. I picked up a Mityvac handheld vacuum pump to help with the diagnosis. I then tried actuating the wastegate actuators - the front turbo actuator worked fine - I could develop 20 inhg of vacuum no problem, I could hear the linkage moving, and when vacuum is released, I could hear the linkage releasing. No problem with the front actuator. 

Testing the front turbo actuator in car, using the hose to the pressure controller. Holding vacuum.

The rear actuator was a different story. Apply vacuum to the line - and nothing. Vacuum would not build in the line, and the actuator would not actuate. I replaced these vacuum lines two years ago with the vacuum line supplied with the Turner Motorsports N54 turbo kit, the vacuum lines should be fine. So I decided to remove the actuator since it was accessible above the downpipe and test the actuator out of the car. 

Some gymnasitcs to remove the rear turbo actuator, but not impossible. 

When I tested the rear turbo actuator directly, it worked fine, no problem at all. It turns out the vacuum line I installed 2 years ago wasn't suited to the heat, and had essentially crumbled and collapsed. 

Rear turbo actuator out of the car

Note the breaks in the vacuum line. First issue found. 

So - I ordered 3 metres of 3.5mm silicone vacuum tubing, which should withstand the heat better. I wasn't finished with diagnosis on the vacuum system, I also tested the pressure controllers. 

Testing the pressure controllers was pretty simple, you can do this in the car. To test the pressure controllers, disconnect the two hoses and the electrical connection, apply vacuum to the inlet port (from the vacuum reservoirs) - and the pressure controller should be closed, and allow vacuum to build without releasing it. Both my controllers failed this test - they were leaking vacuum to the outlet port (to the wastegate actuators). I could tell because I could only build vacuum with the pump holding my finger over the outlet port. Second test would be to build vacuum, then slowly let it off by applying quick pulses of 12v to the controller electrical contacts (you can use battery power and test leads) - the pressure controller should allow you to slowly and evenly drop vacuum over several pulses. A bad pressure controller may let off all vacuum with a single pulse of 12v. Third - the pressure controller should be able to hold vacuum even during vibration (you can apply vacuum then tap the end of the controller with a small wrench or screwdriver). My controllers would lose their vacuum with minor tapping - when holding my finger over the outlet.

So - new pressure controllers ordered as well. That covers the vacuum control system diagnosis.

Step 3 - Charge Air Ducts / Air Leaks

This is fairly straightforward - just inspect the pressure side of all the air ducting around the engine. The only difficulty is that accessibility is poor, it's difficult to see everything. I was fortunate that I had the engine out of the car 2 years ago - I have a pretty good idea of where all the ducting runs and could feel around and use an inspection mirror to make sure everything is connected properly. Removing the underbody panels is necessary under the engine - so you can see the intercooler duct connections and make sure they're solid. In my case - I found a transmission oil cooler hose leaking onto the left intercooler connection, but all the intercooler hoses were fine. Check out my O-Ring repair for the transmission oil cooler quick connect fittings here. 

Left intercooler charge air connection. Solid, but covered in transmission fluid from a leaky quick connector. 

From above, you can inspect the charge air ducts arriving under the air filter. Both of my Pop-off valves were connected properly, but I decided to remove them temporarily to test them. If the valves are not actuating properly, they can cause a intake manifold pressure when you let off the accelerator. Using the handheld vacuum pump, connected to the Pop-off valve, you should see the valve open when vacuum is applied. In my case, only the front valve was working properly, the rear valve wouldn't open under vacuum. Another part to replace. 

Testing the Pop-off valve. You should see the valve open through the connection to the charge pipe.


Replacing all the vacuum lines was pretty simple - just go piece by piece through the system and make sure you get the inlet / outlets on the pressure controllers in the right order. The RealOem vacuum line routing image helps here. 

Replacing the pressure controllers - also simple. I went with the Piersburg OE parts from Pelican for this. Good value. I tested the new pressure controllers with the handheld vacuum pump before installing them - they held vacuum perfectly out of the box - much better than the original ones. 

New Pressure Controllers installed

Replacing the Pop-off valve - also simple, 10 minute job. The connectors are a bit tricky working underneath the air filter housing, but patience gets the connection done. Don't forget to reconnect the vacuum lines. 

Road Test

With all the repairs completed, (and once the axles were back on the car, and the exhaust replaced), I got the car out on the road and tested it out. It certainly felt awesome - the car accelerated as good as when I first got it at 78,000 km, if not a little better. 

Boost pressure data from Carly

I logged boost pressure using the Carly app - and managed to hit one peak slightly over 2 bar on the first run - better than any peak this year. Car was sustaining about 1.7 or 1.8 bar in long, sustained acceleration (0 to 60 mph). 

So - successful repair - car is running amazing again. If you have any questions - feel free to use the comments below. 


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