We’re all about wrenching around these parts, but the fix isn’t always so simple.
Today $600 Craigslist Jeep Moab hero David Tracy asked you what your biggest wrenching misdiagnosis was. He got a ton of great answers, but this one from StaceyS about the malaise-tastic first-generation E21 3 Series may take the cake:
Rescued a badly degraded almost not running BMW E21. Cleaned it up and started trying to diagnose a smokey, rough running M10 with no power that wouldn’t idle.
I spent a few hours adjusting the valve train. Very slight improvement.
I cleaned all the spark plugs. No change
I cleaned out the distributor cap (which was brand new). No change.
I checked all the spark plug wires (which were all new). No change.
I then started suspecting that the catalytic converter was clogged, so I dove under the car and started battling rusty exhaust system bolts. Got the mid-pipe section that houses the cat off and tried looking down the pipe to see if it was clogged or falling apart. I did see some white grit coming out the exhaust end, but nothing on the engine side.
I tried blowing the cat out backwards with an air compressor, got a bit of gunk out, put the whole thing back together and the car back on its wheels. No change.
Took the whole thing apart again and decided to bash the cat material out of the pipe, figuring that since it was shot, I’d just try running it without any cat material and then at least it would run ok and I could order new exhaust components (which was my plan eventually anyway). Bashed it all out, and confirmed that the front side of the cat was clogged up with sooty residue (most likely from not being able to run hot enough to burn that crap out).
Put it all back together and anticipated a smooth running, strong(ish) engine. No change at all. Rough running, smoking, no power. Ug.
More internet research and then I discovered this little gem of information about the M10 engines: Somewhere between 1979 and 1980, BMW changed the direction of the distributor rotation.
I went out, swapped 2 spark plug wires and the car ran like a kitten...
Well, at least you figured it out eventually! And you got a COTD out of it.