After replacing the # 1 engine start switch on a B737-300; a Line Mechanic attempts to perform two additional functions of the start switch that were not required; but led to an inadvertent engine start and high EGT overheat condition. The engine was subsequently replaced.

Narrative

Aircraft had pilot write up; '#1 engine start switch sticking; will not cutoff.' I replaced the switch. While doing functional test 'audible test of ignition system;' I tested all switch positions to verify ignition system worked correctly and before the final step of returning plane to usual condition (returning start lever to cutoff position); I thought that I would check two other functions of the switch. 1) the holding solenoid; and 2) start valve operation (note these 2 checks are not required check; but I was thinking I didn't want the (flight) crew to come out and not have this working on the morning flight; it was incorrect to do it all this time). I checked the solenoid by cycling the circuit breaker and next I started the APU to check the start valve. I put the switch to the 'GRN' (ground) position and checked the start valve light to see that it opened. (I was not looking at the engine instruments because I was not expecting it to start.)Unknown to me; the engine started because I had inadvertently left the start lever up. As I was shutting it off; I first noticed Engine Indications; but it was too late. I don't know how high EGT was; but it was 580 degrees and decreasing when I was able to focus on it. As a result; I suspected engine was overheated. I called the Powerplant Engineer and told him what I did. He said a borescope was needed and after borescope they decided to change the engine before further flight. I made a very poor decision; even though I knew the correct procedure. Although my motive was to be thorough; thinking on the spur of the moment I decided to 'Ad Lib' check the start valve; which was not required. If I had followed correct procedures and returned aircraft to normal after checking ignition; I would not have caused damage.