I concur with the high idle being the culpret. And as HotRod has stated you can play with your idle settings if you prefer a higher idle. My old 79 Monte had an A/C idle up solenoid that mounted to the carb and "bumped" the idle when the air was on. You could adapt one of these to set your base idle and when the key goes off it would lower the idle too low to diesel on. You probably need a 12 volt system though,
Scott


Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!