2000 chevy venture mini van. was missing, thought it to be coil packs, plugs and wires--all appeared to be original, lots of rust and harder then hell to remove. replaced all. van wouldn't start. unplugged MASS sensor & vehicle runs better but misses/back fires/etc. but drives great. I understand the MASS sensor part. but what could cause it to continue to back fire and miss?

Yes if that plug he left in was bad or worse yet, if that plug that wasn't changed is actually missing. You need to check that all the plugs are actually installed first and also that the Spark plug wires are actually seated on each plug completely. Unfortunately a lot of "Shadetree" mechanics make the mistake of pulling on the wire itself to get them free and as a result damage the wires causing this problem. Another possibility is he has the coil wire loose or damaged or the firing order mixed up. The firing order is usually stamped on the intake manifold near where the air intake or carb are mounted to the manifold. The Distributor cap should have a mark indicating the number 1 position and the rest follow in clockwise fashion. The problem is this is the time frame that Chevy was switching between Distributors and Coil packs which are commonly square and a little more difficult to identify the actual position on. If you suspect the firing order and want to be sure beyond a doubt about what it is and which is which on a Coil pack I recommend you get a Haynes or Chilton's manual on your vehicle from Auto Zone or Advance Auto parts store, other Auto parts stores carry them as well anywhere from $12 to $20 dollars depending on your area.

two quick thoughts on this, is the thermostat in correctly? (copper colored end into the
engine block). And second thought is on some vechicles it may be possible to route
the belt incorrectly. Make sure that the overflow tube is correctly attached to the overflow tank as this will cause the fluid to dump on the ground and cause an air pocket
to form in the block. hope this is useful.

Remove plugs and check compression, maybe burnt valve, unseated valve

Here is the firing order for the 1994 Chevrolet Lumina APV Van with a 3.1 Liter engine with a distributor, and the 3.8 liter engine had the coil packs, and let me know if you need any help to understand this diagram, or if you require any further assistance.

This is usually caused by carbon traces in the spark plug connector. You must pull the boot off of the spark plug and look down the boot for the carbon trace.
Do not pull the boot off of the plug by the wire as this will seperate the wire from the boot.
It is very apparent on the spark plug porcilin.
Regards
Rob

Have you replaced the plug wires? If they have too much resistance they can cause the coil to fail.

If you do your plugs and wires yourself, pull the coil packs out for a much much MUCH simpler and less painful procedure.

you can use a scosche plug in that plugs directly to the wiring harness

check to make sure the security light is not on if it is the van will have spark but no injecter pulse the best way to solve this issue is to have the original igniton key and put it in the igniton in the run position and leave it on for 10 minutes then the light will flash then try to start the van

The injector is a good possibility, although you should check for spark in the wire first, as the electrical is the weakest link in the system. Put a spare plug in the end of the wire with the plug resting on metal, then have someone crank the van to see if you have spark that far, test and replace spark plug, (do not get shocked!) then look into a new injector or a good cleaning.