What Was Your Worst Wrenching Misdiagnosis?

Illustration for article titled What Was Your Worst Wrenching Misdiagnosis?

One of the biggest nightmares in wrenching on cars is misdiagnosing faults. You spend hours and hours on what you are sure must be the problem, only to realize that you wasted an entire afternoon.


Sometimes all the symptoms seem to add up to one thing. “Surely, the head gasket is blown,” you think. You’ve got white smoke from the exhaust pipe, milky oil, a wet sparkplug, low compression in two adjacent cylinders. It must be the head gasket!

Then you remove the head, only to find out that somehow, some way, you misdiagnosed your problem, and wasted an entire day of wrenching.

I’ve been there, and I’m afraid I might be there right now.

Illustration for article titled What Was Your Worst Wrenching Misdiagnosis?

I’m in Colorado Springs, on my way home from Moab, Utah after Project Swiss Cheese did a superhero’s work driving cross-country and then off-road. My friend’s parents let me crash at their place for the night, as I snake my way through the U.S. on my return journey to Michigan.


But I didn’t stay for only a night; I’m staying all week. Why? Because in the dark corner of their garage, I spotted an XJ, one that my friend’s parents said was misfiring and burning coolant.

And since I can’t bear to see a wounded XJ, I resolved to fix their gorgeous ‘97 Jeep.


So I looked at the spark plug in cylinder 1; it was clean. Too clean. I looked at the oil level; it was high, too high. I fired her up; sure enough, a misfire in cylinder one. Too misfire-y.

So I yanked the head, and the gasket was absolutely perfect. Now the head is at the shop being checked for cracks, and by golly, if they don’t find one I’m going to cry.


So I may have misdiagnosed this one, but we’ll see. What was your worst wrenching misdiagnosis?

Sr. Tech Editor, Jalopnik. Owner of far too many Jeeps (Including a Jeep Comanche). Follow my instagram (@davidntracy). Always interested in hearing from engineers—email me.



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...