Removed and replaced one 1993 previa fuel injector now engine sounds "seized".Doesnt even turn Could it be vapor lock?

No vacuum lock on fuel injection
You have to diagnose the problem
with a sequence of proper testing
Check fuel pressure at the motor with a gauge
Check for injector pulse & spark when it fails to start

To promote easy starting, all gasolines contain volatile constituents that under some conditions, such as high ambient temperature, tend to produce more vapor than the fuel-system vents can handle. The action of an engine-mounted fuel pump, in decreasing the pressure at its inlet, tends to vaporize the fuel. If the vapor forms faster than the pump can draw it from the fuel line, the flow of fuel to a carburetor is effectively stopped and the engine stalls. Vapor lock is much less likely on a fuel-injected engine with an electric pump in the fuel tank. However, an engine with port fuel injection may experience vapor lock if the injector or fuel overheats, or if the Reid vapor pressure of the fuel is too high.

Read more: http://www.answers.com/topic/vapor-lock-1#ixzz2Pf13O7aL

sounds like it needs a good tune up and new airfilter will sort it out

Vapor lock? That might happen in a diesel engine, but I have never seen that in a gas engine.

It is more likely there is something obstructing the flow of fuel from the tank to the pump. That is not a very long distance to travel considering the pump is in the tank. This "over-working" can cause the pump to draw too much power tripping relays/breakers.

Just because the pump was replaced doesn't mean it is not bad. An obstruction could have also damaged the new pump if it is not a manufacturing defect.

I do understand that Nissan parts are kind of expensive and I would have the pump replaced just on principle since it is probably still under warranty.

I have seen a few similar situation with residual gas tank contaminants (sugar) damaging pumps. (fluid pumps tend to burn up when they don't have a liquid to cool the internal components) No flow is very bad for the life of a pump.

Furthermore, if such vapor lock does exist and can easily be bled off by relieving pressure from the last injector in the series. (gasoline fuel injectors are different from diesel injectors, so trapped air is typically forced through the system without any priming necessary)

first remove the plugs and try to turn the engine by hand(slowly), if the engine locks stop, remove valve covers and check each valve keeper, i think you may have dropped one , if not a nut or washer may have found its way into the piston chamber

It's possible that the fuel system is slowly leaking down somewhere. Nissan injectors are known for leaking at the intake where they connect to the rail. Also, if the check valve in the pump is bad, fuel could be running back to the tank. The engine needs 30 psi in the fuel rail before the engine will start so if the system leaks down somewhere while the car sits, it will take a little while for the pump to build pressure again. If you can connect a fuel pressure gauge to the fuel rail, you'll be able to watch the pressure after you shut the engine down. It should hold residual pressure for a long time, if it doesn't, you will need to find where it's leaking down to.

try running an injector cleaner thru the system.
you may have a dripy injector.

When this truck dies,does it have spark,or fuel pressure?Check it for no spark,if it has no spark,replace the ignition module.These gm year models are bad to go out ,and the distributor pick up coil also.Replace the module first,if this does not help,replace the pick up coil.NOW ON A FUEL RELATED PROBLEMit could be fuel pump,fuel pump relay,or the pick up coil.The pick up coil pulses the injectors,and the spark.I hope this helps,if not let me know.

Does it spin at all? If not either the starter is bad or the engine is seized. Try to turn engine with a socket wrench on the crank pulley bolt. Anything happen to the truck just before you had starting problems (drive through a deep puddle, overheat the engine, lose oil pressure, blown head gasket)? If it was deep water the engine could have hydraulic lock from water in the cylinder(s), remove the spark plugs and try to turn it over with the starter. If it turns over clean, dry and re-install the plugs. If it overheated, ran with low oil pressure, or the oil was contaminated with coolant the engine is probably seized.