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!

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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.

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.

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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?