I just installed a new Denso starter in my 98 LS400 and torqued manifold and throttle body. Wondering is there a way to test the starter before I plug back the fuel injectors harness and other harness /hoses. Can I test without the the airbox and throttle body exposed?

 

Without knowing waht's in place and what is not, what's connected and what is not, etc...
As long as the injectors are disabled the engine can not fire. Assuming the starter is functional it should simply turn over without starting (firing)

 

I am sure the fuel hose needs to be connected else the fuel pump would simply dump the fuel in the bay. Likewise I am looking for some tips what is essential to test the starter when its plugged in the valley.
So for now the injectors can stay disconnected. What else?

 

Several ways to accomplish this.

1. Remove the fuel pump fuse.
2. Disconnect the cam position sensor under the cam gear cover.
3. Disconnect the crank position sensor...

No 1 should be obvious. No 2 and 3 will basically disable spark and fuel at the ECU. The starter will still spin the flywheel.

 

What about vaccum hoses?

 

You can ignore the vacuum hoses if you’re just turning the engine over

 

so just remove the cam and crankshaft position sensors and fuel pump relay and should be good?
Do I need the accelerator cable to be attached, coolant in tank, coolant hoses to be plugged back?

 

as you crank the engine yes the water pump will be spun as well and can push coolant through whatever passages are open. so i would at least plug up and coolant passages with a rag.

i would also make sure there are no exposed electrical connections, to avoid a short, especially around the starter since IIRC the battery or a thick cable from the battery or fuse block is directly connected to it.

 

Just one of these not all three. The fuel pump fuse is the easiest to access. You're only spinning the motor for a few seconds to check the starter is engaging and working correctly.