The two new codes are interestingly for a dual sensor consisting of the intake air temperature sensor and the boost pressure sensor.


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Here’s an interesting observation: The car will idle fine, the errors only start after the engine rpms are increased. The other day the car idled fine for 20’ while I charged the a/c system and the problems only began after I took it out for a test drive. Similarly, the car idled fine two days ago until we started revving it to 3K to check the air flow values, after which it started the stalling behavior. And last night I had some work to do in the garage so I started the car and just let it idle while I worked, and could not get it to stall.

 

Fault codes triggered by revving?

Do the IAT sensor readings look normal or abnormal?

 

Is your scan tool reading -40C for the IAT sensor? If so, in this thread read the first post and also netstable's posts and mike1967's replies to netstable.

This information from Pelican Parts may also prove helpful.

 

This car is going to drive me to drink! There are two sensors that might affect the MAF, 0261230134 which has pressure and temp and is on the left side of the engine bay (facing the car from the front) and 0261230135, which is under the airbox on the throttle body housing. I ordered both sensors, and first replaced 026123013. I checked the car with my scanner and still got a MAF error code. I cleared the codes and installed 0261230135, started the car and idled, no codes! Looking at the MAF air flow, I got similar results idling as before (20 lbs/hr) but when I revved to 3K rpm I now got about 130 lbs/hr, so it appears that the MAF is working now.

I took the car for a 20’ drive and ran it through the gears and rpm range, all seemed well. I got home and shut the car down and found no error codes with my scanner. Then I tried to start it up again, and it reverted to the stalling behavior, with no DTC codes at all.

I am at a complete loss as to where to look next. Could this be the JBE acting up, even though it was freshly repaired?

 

Mr. Toad's Wild Ride!

 

These are the two sensors I replaced.

 

I presume that the one on the left is the dual air temp and pressure sensor. What's the one called behind the intake manifold?

I wonder whether all DME adaptations must be reset after replacing so many important engine sensors?

 

The other sensor is called the same (manifold absolute pressure) but I don’t think it has temp.

I did reset all the adaptations before I drove it. I wonder if that open port is potentially involved? I still have not found a line that is loose that might connect to it, but it bugs me that a port that clearly can take a hose doesn’t at least have a cap on it if it’s not used. It’s not coolant, as nothing is leaking.

 

Ah, the MAP sensor. That's an important one.

 

So, you've replaced the sensors in the intake pipe and intake manifold... Have you done anything with the mass airflow sensor in the turbo inlet pipe from the air filter box? What are the state of your PCV pipes and cam cover? Are you sure you have all the air piping connections between the turbo and inlet manifold secure and sealed? Some of the connections could be fine when cold, but become loose when hot and under boost.

 

I’ve replaced the MAF at the airbox upstream of the turbo, the throttle body, and smoke tested multiple times. I haven’t smoke tested while the problem is occurring, so I can try that.
The valve cover with its integral PCV is only 3 years old, and appears fine. How would I test the PCV valve in the valve cover?

 

The car has thrown 4 intermittent fault codes, 2 for the MAF sensor, 1 for the IAT sensor, and 1 for the boost pressure sensor (all three sensors have been replaced).

One of the MAF sensor fault codes is a plausibility issue with air flow too high.
All three of the remaining fault codes above indicate electrical issues.

 

I use this site as a reference for interpreting DTCs:
https://bmwfault.codes/

With the codes surrounding MAF and pressure, seems there is a plausibility issue with your intake system. And quite possibly only when hot. This can be frustrating to troubleshoot, as you have found.

The PCV valve is integral to the cam cover, so that can be difficult to troubleshoot, unfortunately. However, there is that plastic hose coming off the back corner of the cam cover. That plastic hose can become brittle with heat, then crack. That hose / tube can cause all sorts of issues. If it hasn't been replaced, might be something to consider:
https://www.ecstuning.com/Search/Sit...h/11157605186/

 

I found the video below, which shows that the plastic cover over the PCV diaphragm may be removable. I also earlier posted a method to test for PCV leaks.


Given that 3 of the 4 intermittent fault codes point to an electrical problem, it should be considered that the engine may have two separate issues (unmetered intake air and one or more electrical problems with the affected engine sensor circuits).

 

Update: Mystery solved! As suspected, there was an air delivery problem (which was getting reported as a MAF problem) and an electrical problem. I discovered the electrical problem after I got a code for the accelerator pedal sensor. I replaced the accelerator pedal and the car ran fine in the driveway, but then started the dying routine again. I confirmed that the PCV system was functioning correctly, and while I was at it I smoke tested the intake system (again) by using the PCV hose as the inlet. I eventually got smoke coming from under the driver’s side front of the car, and when I jacked up the car I found the charge tube from the turbo to the intercooler had blown off. I had pushed it back on several times without thinking, but this time I took a good look at it and figured out that it was not seating all the way because the intercooler mount would flex and the hose would only seat half-way when I pushed it on. This was probably causing my problem, as if I got the hose on it would idle fine but as soon as the pressure from the turbo increased as it would on a drive, the hose would slip off and start to leak. Eventually it would blow all the way off and start running rough and die.

I could not get the hose to seat at first, but before ordering a replacement hose I lubed the intercooler inlet and the hose with dish soap and it slid on with only minor convincing. Cleared the codes and have been driving the car for the past week, and all appears well.

Thanks for all the great suggestions!