Help with Check Engine - E30 M42

[av8drvr]

Morning all. I am dealing with a somewhat frustrating issue that has me baffled. The motor is in a 91' 318iS with unknown mileage, 2nd engine swapped by PO. The car runs like a top (it is my 2nd M42 E30, and is much healthier than my first). A few months ago I began getting an intermittent CEL, just flickering on and on off, mostly during throttle lift and idle (stop lights). It is never present at start-up and I can't associate it with cold/hot engine. The stomp test delivers a consistent 1223 (coolant temperature sensor). When I first diagnosed the issue I swapped a known working sensor in (the one closest to the front of the engine, that feeds ECU) and that seemed to eliminate the issue. It is, however, back to its evil flickering ways. It will often come on steady now after a stop but extinguishes with any throttle input. I notice it won't flicker again until I reach a steady speed on the motor (@3k rpm at highway speed, for example). It is also pretty consistent on throttle lift and decel but still exhibits enough random pattern that I can't for the life of me give it a good incidence profile. Any thoughts? Just another bad sensor (I think it would be a sold light if this was the case, with crappy idle?) harness problem? Thanks for the read and hopeful ideas.

[Geoff]

I've seen this (on my '91) when I had vacuum leaks.  where the air intake bellows attaches to the throttle body, I had a hairline crack, that would open and close under different conditions.  replacing the bellows cured it.  there are also a number of other places where the m42 can develop vacuum leaks, like the mess of hoses under the intake manifold.
                                                                        Geoff

[av8drvr]

Thanks for the response. Would a vac leak consistantly cause that CTS fault code? That is what I am curious about, why that code? But the more I think about it, the more it sounds vac related, I was just getting hung up on that sensor fault.

[jrobie79]

vacuum leaks might cause this, I had the problem and did the throttle heater delete....but  I remember cleaning off my crank and cam position sensors too and it went away after that (dunno if it was the vacuum leak fix or the sensors)

[av8drvr]

jrobie, thanks for the input. I was planning a complete "mess" fix in the future, hoping that will eliminate the issue.

On a completely unrelated note, I grew up in Newbury, MA and have a very dear friend from Andover. I spent a lot of time right down the street from Phillips Andover in the late 80's during highschool. I miss the area (just not in winter!).

[Geoff]

my personal feeling is that,  there not being a code for vacuum leaks per se, but a problem in that area will throw a couple of different codes as the ecu probably does not know exactly whats up.  when I had a vacuum leak it would throw the O2 sensor code.   maybe there is more than one problem, as well..not a fun thought, but it happens
                                                          Geoff

[jrobie79]

my folks live in Andover, I've since moved to medford, but I am up there regularly