Check Engine Light, aka CEL, is the ruin of many a shady tree mechanic, but it’s really not all that bad.  Modern OBD cars, especially OBD-II cars can provide you with some very valuable information via codes that can be pulled via an OBD reader.  I, not being a professional mechanic, have gone the affordable route and have just an inexpensive Bluetooth OBD reader, which allows my Android phone to read what’s wrong with the car.  I highly recommend everyone who knows how to work a smart phone invest in this handy tool.

So what brings me to this subject?  Well shortly after purchasing my ’98 M3 Sedan I had the engine stumble and a CEL popped up.  I knew the previous owner had mentioned a few problems with the secondary air-pump system, which primes the catalytic convertors so that they reach their efficient operating temperature more quickly during cold starts.  That, however, would not cause an engine to stumble or run rough.  I noticed that the issue occurred immediately after a hard acceleration and hard shift.  This seemed mechanical, or at least something physical had gone awry rather than electrons not arriving in the anticipated sequence.

The OBD reader… my phone that is, called out misfires on several of the cylinders as well as the generic multi-cylinder misfire.  That doesn’t exactly narrow things down that much, but at least I knew it wasn’t something in the emissions system (including the previous SAP problems.)

I had recently installed new spark plugs, so that was my first guess.  The OEM-spec four prong electrode plugs can’t be gapped like a traditional spark plug, so nothing I could do there to check or adjust.  I then decided to clean out all the coil boots, to ensure I had good continuity.  Everything reassembled, codes cleared, I took the car for another drive.  This time an hour long jaunt to a remote spot for work.  I decided to replicate the circumstances surrounding the initial problem as best I could, so I gave it a hard acceleration… nothing.  So I bucked it in a low gear and the engine stumbled, ran rough, and there was my CEL.

So this had something to do with engine movement, something was loose or not all the way connected.  I had recently replaced the valve cover gasket to cure a small oil weep, which means I had removed the crankcase ventilation tube (aka PCV.)  I pulled to the side of the road, where I read the same codes from the DME on my phone.  I popped the hood with the engine still running, then wiggled the vent tube.  Ah ha, there was my issue.  The plastic clip that is integral to the tube was cracked one side, allowing the tube to open just enough to form a leak.

Cracked Vent Tube Vent Tube Diagram

This tube sucks dirty crankcase gases (blowby etc) into the intake manifold via pressure regulating valve with an integrated air/oil separator that drains collected oil back to the pan.  This small crack in the vent tube was allowing unmetered air directly into the intake, therefore making the mixture swing wildly lean, creating a misfire.

I pushed the vent tube on as best I could, cleared the codes, and then drove the rest of the way using the throttle as gingerly as possible.  Once I had a chance to stop again, I was on the phone with the local BMW dealership to order a new $17 vent tube.  A few days later I was installing the new tube, lubricating the two o-rings first of course.

Since this little fix I’ve done plenty of hard driving (including a track day) without a stumble, much less any CEL.


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