When the fuel runs out, the engines just keep on running on both the Cessna AgTruck and the AirTractor although the tank is empty.
First thought that there is a conflict with the assignment of “liquid” and “fuel”, but the bug persists independent of the loading level in the hopper.
Do you have unlimited fuel selected in the Assistance options?
Did the following test (unlimited fuel disabled):
Spawn at a runway
Set fuel tanks to 0
Increase RPM and wait for the engine to shutdown
cessna-188-agtruck (2500 RPM): 7:30 minutes
airtractor-at-802 (1500 RPM): 3 minutes
I don’t know what should be the real figures once you run out of fuel but time for the cessna-188-agtruck seems too high.
Hopefully users with knowledge will be able to provide some input.
OMG I just tested it again and you’re absolutely right!
I thought I was patient enough, having waited 5 minutes… 
Well I agree, these values seem waaaay to high, but at least they don’t run indefinitely.
Did the same test with the C172, 1800 RPM and had to wait 15:45 minutes…
I don’t know what should be the expected time for each one of them.
I had tried it with the Cessna 152 and the Pitts S2S - both engines quit within a minute.
