Pontiac Repair: Misfire in cylinder, independent mechanic, cause failure

Question
I was running very low on gas. Not long after getting gas van start missing. when I'm in park you can hear the mis. I took it to an independent mechanic and they claim the misfire is a cylinder. which will cost a lot to replace. with  the van having over 120,000 miles they suggested that I should consider replacing engine for $1800.00. I know that they are very honest people but, wondering if this could be a misdiagnoses! This problem didn't o cure until after putting gas in while the tank was about empty.

Answer
Hi Mel:

I'd get a second opinion! I's want an exact cause/failure before I spent any money. If this van has this many miles on it I'd think twice before I sink and large amount of $$ into it. It could be something very simple like a fule injector that's plugged or dirty. If you ran the tank down to the bottom it may have sucked up some dirt that made it past the fuel filter and got stuck in one of the injectors. Just a though.