I have a 2000 GMC Savanna, and I've had this problem twice.
If we have a particularly cold few days, then a warm humid front moves through overnight, the engine won't start. I can see quite a bit of condensation all over the engine block, wiring, etc.
The engine turns over, the fuel rail is pressurized, and there is a spark.
The first time this happened I was in a hurry, and had it towed and had them perform a tuneup (belts, spark plugs, spark plug wires, etc). They reported no problems, and suggested the condensation was sapping the spark, but that the new wires should resolve the problem.
However it happened again yesterday morning. I waited a day, and this morning it starts fine - and the engine is dry on the outside, no signs of the condensation that were there yesterday.
The spark plugs have a satisfying 'pop' suggesting they seal against the spark plugs when removed, but there's no dielectric grease or anything inside them.
I'm wondering what I can do to eliminate this problem once and for all, and what else I should check for?