So I've been battling this problem since Jan of this year. After a cold start (and usually after a longer than normal idle, i.e. sitting at a drive through or waiting for a long red light) I will lose all boost for a relatively short amount of time. Sometimes I'll notice the boost loss and then next gear the turbo spools again, and then other times I'll have no boost for 20 - 30 seconds.

I grabbed this datalog and threw some color into it to help visualize what's happening:

https://docs.google.com/spreadsheets...it?usp=sharing

Looking at the far right columns, you can see a 100% accelerator position all through 2nd gear with only 3psi of boost, then on the next shift into 3rd, 100% accelerator position yields ~18psi of boost. This data log was taken immediately after I sat at a red light for 3 mins.

I've had my car at a tuning shop for a few months trying to find the problem. They're claiming that the car is mechanically fine. In the past year, I've rebuilt the turbo, replaced the spark plugs and coil packs, replaced the thermostat housing, replaced the water pump, and had MINI do a walnut shell blast cleaning on the intake valves.

I've also replaced the MAF sensor multiple times (since the P2B5C code is MAF related) and even replaced the MAF sensor cable. After all of these repairs, this boost loss issue persists. I would appreciate ANY advice as to what could be causing this. Thanks!

 

I also forgot to mention that this issue also persists after resetting the ECU, changing AccessPort maps, and even unmarrying the AccessPort completely. Resetting the ECU makes the issue go away for maybe a week and then the boost cuts will start happening more frequently as I drive. The first day I got my car back from the shop, I had 1 boost loss for only a few second (one gear). 3 weeks later (today) and I experienced the boost loss multiple times over multiple driving sessions and it lasts a lot longer than one gear.

The shop also told me that all pressure tests have passed every time they've tested.

 

Anyone? One thing I find rather peculiar is that this problem started for me when I had MINI repair my coolant leak last year. The very first time I noticed this symptom was 1 day after getting my car back from replacing the thermostat housing and water pump.

From all of my searching, I happened to find this thread, where someone with a similar problem (P115C) tried all of the common fixes only to find out that the code went away when he had MINI repair his water pump and thermostat housing....

So my boost cuts and P115C problems started after a thermostat housing/water pump repair and this other fellow's problems were solved by the same exact repair. Is it possible there's a point of failure near either of these two components that might get affected by the repair?

 

Bump! I've been researching as much as I can about the P115C codes as they seem to be very common. One thread I found (on MA, can't link) told the story of a guy who went through the same steps as I did, replace MAF, replace intake, check boost tubes, and even replaced his valve cover to no avail.

What's more interesting is that this guy's fix for his problem was the exact same repair that caused my problem: coolant leak at the thermostat housing and water pump. The very first time I ever noticed this CEL and boost cut was 1 day after having my thermostat housing and water pump replaced. This other guy had these problems all the way up until the day he had MINI do the exact same repair: water pump and thermostat housing.

Could there be a point of failure that's near either of these two components that might affect boost? It seems like the steps to replace the water pump and thermostat housing might hold a clue as to what's causing my boost cuts.

 

Wiring harness could have a short in it if the person replacing the thermostat housing stress the wiring harness coming from the ECU which is located right next the the thermostat housing, did you check the harness to see if it had been wet. Your mechanic said everything is mechanically sound, so if you replaced the MAF sensor more than once you must start thinking about how the sensors send data to the ECU. A short in the wiring can cause any sensor to appear as if it has failed, this is only one possibility. Mini/BMW aren't capable of troubleshooting their own cars which I think is very sad, it shows they don't know what they are doing nor understand the workings of the own brand.

Over-engineering, poor designs and using cheap suppliers lead to failure which Mini/BMW experience everyday.

 

1. Is your intake tube slipping off, or off of the compressor inlet? That will cause a MAF deviance.

2. Is the vacuum source that runs over the thermostat to the wastegate actuator intact and connected at either end?

3. Can you see any oily buildup at any of the charge piping connections that would indicate a boost leak?

 

That's exactly the type of thing I was hoping to hear. I will try to check this tonight if I can squeeze in there without disassembling too much.

 

My intake tubes are as secured as they've ever been. They have been on and off a handful of times between when this started and now as well. I'm not sure about the vaccuum line near the thermostat, but I can say that the tuning shop who has been working on it did completely replace my turbo and did pressure and vacuum checks without any problems. I'm pretty sure the mechanic said they tried reseating any vaccuum lines they saw and that hasn't made a difference.

I do have an oil catch can, but I do still notice buildup at the turbo inlet and inside the air intake tubing. I don't know how a leak would have made it past my mechanic and dealership multiple times, though.

 

I'm going to assume your have an N14 engine which has two PCV ports, one near the throttle body and the second one near the turbo inlet. If you have plugged up the throttle body PCV port or your PCV system is bad that would explain why you're seeing oil in your turbo inlet pipe, intercooler and intake plumbing, the oil vapors will enter the engine regardless if you have a OCC or not.

Dealers are hopeless, they excel at replacing parts but fall short when it comes to diagnosing problems.

 

I just wanted to update this thread to let everyone know that this was caused by a failing throttle body. My dealership service center said they've seen a few 08-09 models throwing MAF sensor codes that ended up being throttle bodies. Sure enough, after replacing mine, everything seems normal again.

 

Same here - new MAF sensor didn't fix it, but new TB did.