Shock cooling a motor can be bad, but unlikely in my opinion by washing it at the local pressure washer. We all drive during rain storms and over puddles and water splashes up in the engine compartment, also, I have been washing my motors for years with good results and no problems by first misting the motor from far away, then getting closer with the pressure. I find that with the pressure washer, I don't need to use any degreaser - try it both ways and you'll see what I mean. Avoid electrical connections like the other gents said as well. good luck!