If it is not your throttle linkage, then it is most likely your Idle Air Control Motor. This stepper motor is controlled by the EFI computer and regulates the amount of air fed into the intake manifold when the engine is idling. In my experience, when this device is dying, it usually will not let enough air in an causes the engine to choke off at idle. Does it reset to a normal idle after every start or is it intermittent?
The idle air control motor looks something like this:
http://www.carolinaboatoutfitters.com/catalog/product_info.php/idle-air-control-valve-43505774l-volvo-penta-8343751-p-78710If I were you, I would disconnect the throttle linkage from the engine, start it up, and operate the throttle lever on the engine by hand. When you let go, the built-in spring should snap it back to idle and you should get something around 600-700 RPM when the engine is warm. If it does not behave correctly, then swap the IAC motor and try again.
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Jim
Richmond, Virginia
2005 Horizon 230 w/ Volvo 5.7GXI Duoprop
Towed by: 2007 Chevy Tahoe 4x4 LTZ
