Twice now my '89 300SEL has cranked fine, but showed no sign of catching when I tried to start it. Once was in the morning after a cool night. The next time was on a hot day after a long drive (so engine temperature seems irrelevant). For 20 (or so) starts in between, it was fine.

Both times I had the starting trouble, I first checked spark. To do this I used a screwdriver inserted into the distributor-cap end of the coil wire, and held the metal of the screwdriver close to metal on engine. Spark was great. After I performed this check (at the first incident) I put the wire back on, and the car started right up.

The next occurrence I pulled the wire, then just plugged it back into the distributor cap. There was no change (still didn't start). Then I did the spark check (like the first time). Plugged the wire back in and the car started right up again.

I checked the resistance in my coil wire and there is pretty much none, and it didn't change at all as I bent and wiggled the wire (I thought it might be a faulty coil wire).

Now I'm stumped. Is it, perhaps, a sensor starting to go bad?

Any thoughts would be appreciated.

Brett