Mercruiser 7.4 MPI won't idle, surging and racing - mechanic stumped!

[brad614]

The port engine started racing and surging when I started it Saturday. It goes from almost stalling to around 2000 RPM, and then fluctuates back and forth. It is also spewing out gas "flavored" smoke when it does this. The fuel pressure is consistent at about 35 psi and the computer when plugged in shows no fault codes. We have swapped the MAP sensor and ECM from the other engine (which is fine), and also replaced the module in the distributor. Trying to accelerate with the throttle causes stumbling, and doesn't really go above 2000 RPM. New plugs, wires, distributor cap, and rotor were just installed. Any suggestions??

 

[alldodge]

To Start with you need another mechanic. First does the "mechanic" your using actually have a scanner that will read codes on a Mercruiser? Next, the MAP I don't see as the problem, maybe the Throttle Position Sensor (TPS), has it been swapped?

 

[brad614]

Update - it wasn't the MAP sensor it was the IAC.

 

[brad614]

The mechanic is a certified Mercruisers guy and he used the factory computer system.