What The He**

[masbury]

1991 Black...

I am stumped. Took care of the typical vacuum leaks around the intakes, smoke tested and no leaks - the engine is pristine and has no issues except this...

On bank 1-6, had an error for the O2 sensor, 7-12 would be clear (both are new OEM). CE light is on as well as Trans Fail Safe reported on the mid. Occasionally, it would throw the O2 sensor pre-heater error in just for fun. Also, it would do the "Bank 1-6 O2 sensor can't richen any more and Bank 7-12 can lean any more" did the O2 sensor wire swap.

Clear codes. No check engine light, no codes. Though it did not report it, Trans Fail Safe was still in operation as it would go into reverse very hard, but...

Wait for it...

Check engine light would come on again, O2 sensor code about Lean/Rich and occasional pre-heater code as well as Trans Fail Safe on the mid. The kicker is that it changed sides. Bank 1-6 was clear, but 7-12 now had the codes or the Rich/Lean would reverse.

No matter how many times you cleared the codes, everything would switch sides.

What is going on with this thing?

I really don't want to tap out on it. I have a soft spot for E31's.

I'm open to any and all ideas or suggestions. All I know is we need some help.

Thank you

[m6bigdog]

I assume with the statement" Took care of the typical vacuum leaks around the intakes" you replaced the intake manifold gaskets and therefore disconnected some vacuum lines and electrical connectors?
With that, the engine has some crossover connections!! i.e., Left (driver side) throttle body is for cylinders 1-6, etc.
Hence, the Pressure Regulator (injector rail for 1-6 cylinders) vacuum line run to the correct (driver side) throttle body.
Make sure the DME wiring (O2, donut and crank sensor) are all connected to the correct bank.

[Timm]

....and also check that the Lambda sensor wiring isn't crossed, that would cause those exact symptoms

[Albert850i]

The intake manifolds have cross-over connections with the other banks anyway via the PCV-valves.
Someone suggested earlier to connect both throttle bodies using a small air hose, which seems like a good idea to me (as the vacuum levels should be the same).

[m6bigdog]

The intake manifolds have cross-over connections with the other banks anyway via the PCV-valves.
Someone suggested earlier to connect both throttle bodies using a small air hose, which seems like a good idea to me (as the vacuum levels should be the same).

Seems like a good idea? You guess?
If the vacuum levels are the same! then the hose connecting the throttles does NOTHING but waise a piece of hose.
The BMW engine designers functionally did the engine Bank-1 & Bank-2 cross-over connection electronically through the DME's with inputs from the individual bank MAF and O2 sensor!! Why mess that up!!

BUT, for the rest of us, it is more appropriate to fix what actually went wrong, rather than compensate for failed components with a hack of unknow actual effects.
IMHO, any change that makes it runs better - is not a good repair strategy.

FWIW:
That is not what I was referencing!!
I was pointing out, the physical component crossovers; hence, parts for Bank-1 are physically closer too Bank-2 and can be an issue at re-assembly, after repair!

[masbury]

I assume with the statement" Took care of the typical vacuum leaks around the intakes" you replaced the intake manifold gaskets and therefore disconnected some vacuum lines and electrical connectors?
With that, the engine has some crossover connections!! i.e., Left (driver side) throttle body is for cylinders 1-6, etc.
Hence, the Pressure Regulator (injector rail for 1-6 cylinders) vacuum line run to the correct (driver side) throttle body.
Make sure the DME wiring (O2, donut and crank sensor) are all connected to the correct bank

M6: Apologies for being so screwed up. This has been making me crazy for so long that I'm losing brain cells.
Yes, all the intake seals are brand new. Has been smoke tested.
All vacuum lines that cross are connected correctly.
The plugs for the Crank Sensor and Cylinder ID are plugged correctly. The only thing not checked is continuity between the plugs and the DME's (Hmmm)

....and also check that the Lambda sensor wiring isn't crossed, that would cause those exact symptoms

Timm: We did check that the correct O2 sensor was connected to the correct DME.

Still trying to figure out why, without changing anything other than clearing the codes, the codes switch banks and it displays TFS on the MID. This, mind you, is while the car is running the whole time.

Process goes...
1. Car is running
2. Shows codes on bank 1-6, CEL is on and Trans Fail Safe displays.
3. Clear Codes on 1-6 (Car still running)
4. Codes are clear on 1-6
5. CEL goes out and Trans Fail Safe does not display, though trans is still in TFS mode.
6. Blip the throttle.
7. CEL displays
8. TFS displays in MID
9. No codes on 1-6
10. Same codes now show up on Bank 7 -12.
What?

[Albert850i]

IMHO, any change that makes it runs better - is not a good repair strategy.

I would do anything to make it run better.

[m6bigdog]

I would do anything to make it run better.

Anything? Maybe that is why it doesn't run as it did out of the factory; as designed!!
Not me! I will only passionately diagnose and R&R with OE parts to correct a performance problem.
IMHO:
A finely tuned engine is not receptive to MacGyver conceived improvements to maintain peak performance.
Enough said..

[masbury]

Cannot believe this has stumped you guys! Over the years I have seen the people on this forum come up with some of the most amazing suggestions and ideas and in most instances they worked! My only hope is, maybe there is something completely off the wall that I'm missing to get this car to run right.

I bought it 8 years ago. Let me rephrase... I rescued it 8 years ago. In that time, I have put maybe 100 miles on it, due to rebuilding all of the suspension, resealing the engine and bunches of other stuff. I have kind of a soft spot for it, so if anybody has any ideas, I'm certainly open to them.

Thank you

[Timm]

We're not stumped - we're just not there! With the given error codes did you check the fuel/air mixture? Do you have INPA?

[m6bigdog]

If it was my car I'd just fix it.
Guessing rarely yields good results!
However, I can't fix a car with my knowledge and tools when it is not here with me and my tools.
Diagnostics, is not about someone describing a problem and guessing a solution, it is about using knowledge and diagnostic tools to locate exactly what is causing the fault.
Sometimes you need to start with the basics leak-down/compression test, electrical systems connections/grounds, fuel pressures, injectors and test/replace some parts to locate the culprit.
Diagnostic systems can only get you in the ball-park some of the time!!
However, I would not get started without doing the diagnostics first and continuously until the problem is solved.

Last CEL I had with my M73 was an intermittent and recurring cylinder #8 misfire. DIS fault code. Engine performance was flawless!!
The misfire CEL would not occur while driving at speed or with mileage or drive time as the engine could be driven for hours and the CEL would only occurred after the engine was at operating temperature and the engine was now at a low RPM's or idling
Engine has excellent & even cylinder compression, good plugs wires, Cap & Rotor and good Bosch Copper plugs.

So now diagnose what was the actual problem?

Answer: the #8 cylinder spark plug socket/boot [12-12-1-710-707] needed to be replaced, as the internal spark plug socket had failed!!
I must have driven the car 500-800 miles and reset the CEL 10 times, before I "found" the problem. (notice I didn't say diagnosed the problem).

Hence, the spark plug socket - internal socket snap-ring had failed and would not make positive contact with the spark plug post.
I found it because, I had a sense that the spark plug socket/connector when inserted on the #8 spark plug did not positively seat with a "click" and when a test plug post (a tool I made 30 years ago) was inserted into the spark plug socket it did not engage my test plug post.
I installed/crimped on a new spark plug wire circular connector (12-12-1-705-656) as I could not get the spark plug socket off the plug wire without cutting off the plug wire connector and the circular connector is inserted into the new spark plug socket.
The #8 cylinder misfire has been gone for over a year and several thousand miles now!!
Who would have thought!!

[iceman2007]

If it was my car I'd just fix it.
Guessing rarely yields good results!
However, I can't fix a car with my knowledge and tools when it is not here with me and my tools.
Diagnostics, is not about someone describing a problem and guessing a solution, it is about using knowledge and diagnostic tools to locate exactly what is causing the fault.
Sometimes you need to start with the basics leak-down/compression test, electrical systems connections/grounds, fuel pressures, injectors and test/replace some parts to locate the culprit.
Diagnostic systems can only get you in the ball-park some of the time!!
However, I would not get started without doing the diagnostics first and continuously until the problem is solved.

Last CEL I had with my M73 was an intermittent and recurring cylinder #8 misfire. DIS fault code. Engine performance was flawless!!
The misfire CEL would not occur while driving at speed or with mileage or drive time as the engine could be driven for hours and the CEL would only occurred after the engine was at operating temperature and the engine was now at a low RPM's or idling
Engine has excellent & even cylinder compression, good plugs wires, Cap & Rotor and good Bosch Copper plugs.

So now diagnose what was the actual problem?

Answer: the #8 cylinder spark plug socket/boot [12-12-1-710-707] needed to be replaced, as the internal spark plug socket had failed!!
I must have driven the car 500-800 miles and reset the CEL 10 times, before I "found" the problem. (notice I didn't say diagnosed the problem).

Hence, the spark plug socket - internal socket snap-ring had failed and would not make positive contact with the spark plug post.
I found it because, I had a sense that the spark plug socket/connector when inserted on the #8 spark plug did not positively seat with a "click" and when a test plug post (a tool I made 30 years ago) was inserted into the spark plug socket it did not engage my test plug post.
I installed/crimped on a new spark plug wire circular connector (12-12-1-705-656) as I could not get the spark plug socket off the plug wire without cutting off the plug wire connector and the circular connector is inserted into the new spark plug socket.
The #8 cylinder misfire has been gone for over a year and several thousand miles now!!
Who would have thought!!

I would check ecu for leaking capacitors. Problems with traveling errors between banks and trans errors along with CEL often are caused by capacitors fluids corroding circuit board traces along with faulty capacitors. 1990's was the worst decade for bad capacitors hitting auto manufacturers, specially MB, BMW, LEXUS etc.