Hard to start

I have a 2000 rwd astro. Has always ran great. Lately I have been having a hard to start problem when cold. When I first turn it over it doesn't even try to start. Just keeps turning over and over and doesn't seem to wear down the battery. The book says to wait 10 sec. for relay to reset. I do that and still nothing happens except oil pressure comes up to 45#'s. Wait another ten secs and then maybe it might start. Sometimes takes more tries but it always does start eventually. Had a tune up, Changed fuel filter.Fuel pump always seems to be working. Book says if you take out fuel pump relay while it's running and it stops then it is the oil pressure switch. It's supposed to have a secondary circuit for the fuel pump. But I don't know where on the block to look for the pressure sensor. Also could it be an EVAP pressure problem. Gas tank and cap not pressurizing properly? Does anybody have any ideas what I can look for? After it does start and warms up to temp it doesn't have anymore problems starting until it gets cold overnight again. No other problems with power or cooling or oil pressure, just hard to start first thing in the morning. However starter fluid sprayed in the air cleaner kicks it right over everytime......Thanks

My'97 became hard to start. I could get it to start by"priming" several times, turning the key several times, but not to the "start" position, to build up fuel pressure. My fuel pump was weak, changing it fixed it.
I measured the current draw of the old pump at the relay. Check the diagram on the relay, but I remember the contacts for the ampmeter to be the upper left, and the lower right pins of the socket. BUT CHECK THE DIAGRAM! Ampmeters can really screw things up if mis-used. The old pump drew about 4 amps, ramping up to 7, in about 15 seconds. You don't need the key, just put the ampmeter across the right pins, and the pump starts. The new pump draws almost 10 amps, and the van starts every time now. If you can get a fuel pressure guage, GM Service told me "minimum 60" psi for the van to start (that for a '97, I think yours is the same) Hope this helps.