STARTS AND RUNS FINE BUT WHEN I START WASHING IT QUITS SPRAYING AND THEN BUILDS PRESSURE AND KILLS ENGINE

The reason it stops spraying might be a clogged gun, hose or nozzle-tip.
The reason for pressure buildup killing the engine might be a stuck unloader valve in the pump.

While you are checking each component of the "output" side, spray some penetrating lube into the "input" side of the pump (where the garden hose connects) and let it be working on the unloader valve.

Fix the spraying problem first:
Start at the nozzle and verify that it is open by using low-pressure air. Clean or replace as needed. Next check the hose (with nozzle and gun unhooked) and finally the gun/trigger mechanism.
All must be clear and open to perform correctly.

Next, pour a few ounces of power steering fluid (or similarly LIGHT oil) into the garden hose, attach it to the pump and turn on the water supply.

Leave the "output" hose, gun, etc. UNHOOKED and point the output port of the pump (where the high-pressure hose connects) towards a vacant, safe area.
Startup the engine and allow the oil in the garden hose to work its way thru. (About 1 -3 minutes of run-time.) Shut it down and attach the high-pressure hose.
Hope this solves the problem!