C280 crank but no start

[1971 250/8]

My 97 C280 has been sitting awhile. I have a fully charged battery (13.38 volts) nice warm day. She cranks over, I hear the fuel pump energizing from the rear, slight smell of gas in engine bay so it appears fuel is making it through.
I haven’t pulled a spark plug yet. What is the best visual way to check for spark? At night? Or with a voltmeter?

( two months ago during the winter I tried starting her up… long cranks of about five seconds… she did fire up but only for a few minutes. I didn’t use the gas pedal as I should have and she just idled to a stop) thought it might be cold weather but apparently not. Also the gas seems good or she never would have started I think.

[Duke2.6]

Use a spare spark plug if you have one, or if not, remove an easy to access plug from the engine.

Insert the spark plug firmly into the free plug wire, then use a jumper wire to connect the ground electrode to a good ground. If you don't have a suitable jumper wire you can just hold the spark plug ground electrode against a good ground.

Have someone crank the engine and watch for a small blue flash. It should be easy to see in daylight, but try to be out of direct sun. Try at least a couple of times. If no spark try another ground or two until you are sure that there is really no spark.

I'm surprised you smelled fuel in the engine compartment, since the fuel system is essentially closed. I usually recommend sticking your nose next to the exhaust pipe and having someone crank the engine.

I just ran these tests on a contractor's truck who was working on the house across the street. I saw him cranking, but no start, so I grabbed a loose spark plug and jumper wire and went across the street. There were a couple of other guys there so plenty of manpower.

There was definite fuel order at the exhaust pipe, but no spark, so he had to get it towed. I don't have an OBD reader because my newest car is 1991. Your '97 has OBD, and it might be showing a code that can help direct you to the problem.

He's back, today, so his truck was only out of service for a day and he had towing insurance.

Duke