MAS related problem?

[aslan.de]

I've been trying to figure this one out for a while and thought someone here might have the ability to provide some insight.

I have a 1993 BMW 525 Touring produced in 9/92, German spec. It has the VANOS engine.

A few weeks after I got it (I've had it over two years), it started having problems starting and going when cold. I worked through a number of issues it had (CCM, burned out capacitor on the environmental control board causing a .65A leak, bad speedometer, etc.) but this problem persisted until we disconnected the MAS. After that, it runs alright, although the idle is a bit rough at times.

So, we thought we had a MAS problem. We replaced it with a known good one, no changed. We changed the ECM out with a known good one, no change. We tried them both, no change.

Next, we targeted the wiring harness. An over drive to 12V (yeah, I know, not recommended) on the return line has the vehicle running, but consuming larger than normal amounts of gas. grounds both look good (chassis and ECM ground at MAS) and the return line has good continuity. Power is 12V at MAS. So, it looks like the MAS system is completely okay. No issues. But, when the MAS is connected, the car won't idle, it just dies.

It seems the only thing left is the limp home mode. When some systems fail, the engine will still run using defaults. My question is, when the car is in limp home mode, what else does it ignore? These could be the real problem components.

One thought I had was that if the idle speed control valve is part of list, it may be the actual problem, but I thought you guys might have further insight or ideas concerning what is wrong. BTW - I've heard of one other vehicle, same engine, with the same problem. They were asking me if I'd figured it out yet!

[whiskychaser]

Can you plse confirm what you mean by MAS? Limp mode is on autos. What fault codes do you have on the engine or transmission ECUs?

[aslan.de]

Can you plse confirm what you mean by MAS? Limp mode is on autos. What fault codes do you have on the engine or transmission ECUs?

MAS == Mass Airflow Sensor.
You are right that my terminology might not match with that used here. I'm referring to the Bentley manual for the E34, section 130-10 "air flow sensor, checking" which states:
"If the hot film breaks or if there is no output from the air flow sensor, the ECM automatically switches to a "limp home" mode and turns on the Check Engine light."

Interestingly enough, the check engine light is not on, nor does the ECM complain about the MAS not being connected up (even though the ECM should not be receiving a signal from the MAS). The car is driveable without the MAS hooked up, idles at around 650 RPM but seems to idle a bit rough and the idle differs based on the air temperature and weather, etc, although the car doesn't stall. Its a real mystery as to what is going on, though someone here might have some insight that could help figure it out.

-DN

[whiskychaser]

Have read the section in Bentleys and see what you mean now. Did you do the tests it lists? The wiring diagram for the MAS only shows 4 lines going to the ECM (ECU) plus a ground wire. But the text also refers to the main relay. Did you check power from that? (You say An over drive to 12V (yeah, I know, not recommended) on the return line has the vehicle running,)
If you have checked all the wiring and replaced the MAS and ECU with known good ones, no wonder you are scratching your head
By the way, does your CEL actually work - the stomp test doesnt work on Euro cars and my CEL certainly isnt wired up

[aslan.de]

Have read the section in Bentleys and see what you mean now. Did you do the tests it lists? The wiring diagram for the MAS only shows 4 lines going to the ECM (ECU) plus a ground wire. But the text also refers to the main relay. Did you check power from that? (You say An over drive to 12V (yeah, I know, not recommended) on the return line has the vehicle running,)
If you have checked all the wiring and replaced the MAS and ECU with known good ones, no wonder you are scratching your head
By the way, does your CEL actually work - the stomp test doesnt work on Euro cars and my CEL certainly isnt wired up

Sorry for the late reply.
The engine in the car is the M52 VANOS or M50TU (IIRC). The car is actually a 1993 Model year, not a 1992. Learned that from the diagrams in Bentley. The 1993 diagrams match, while the 1992 doesn't. As such, the tests listed in the section only consist of testing the harness, which we did.

We also tapped into the wires at the sensor. We tested the grounds, power and return while the car was running (cutting the return line so the car would actually run). Everything looks good.

Since we've looked at the sensor in detail, swapped parts, etc, our thoughts have started to turn to "side effects". With this "limp home" mode mentioned in Bentley's, what else would the ECM be ignoring? Perhaps one of these other sensors is faulty. As such, the car won't idle if its being paid attention to, but will if ignored. Anything that might fit that description (preferably easy to test)? Anyone with an idea as to what might be "turned off" or ignored when the vehicle is in this condition?

Scratching my head is right, grasping at straws as well.

-DN