Just yesterday I had a 6.2 MPI with low idle and stalling issues. With the engine at operating temp at idle, IAC Duty Cycle was stuck at 99.98% (and getting very hot), and timing stuck at 15 degrees with no active faults. Clearly, the ECM was doing everything it could to try to get to it's desired idle speed (600 RPM), but it could only achive around 500 RPM.

The engine had 172 hours and the IAC had been replaced at 115hours. I pulled the IAC and checked the pintle movement. It moved in and out with a light push. Checking resistance, there was 10.3 ohms across the terminals. The IAC valve appeared to be good.

I then pulled the flame arrestor and found a clogged IAC muffler. Replaced the muffler and the engine was now able to achive and maintain a normal idle, with a IAC duty cycle of about 37% (and much cooler) and timing between 0 and 5 degrees advance. In other words, the engine was now running normally, the clogged IAC muffler being the only problem.

I wonder how many of the IAC failures on these engines are just due to a restricted IAC muffler causing the valve to run at a high duty cycle and overheating. I am going to start changing them for my customers on every 100 hour service.

Dave

Merc can also send another throttle body to you if the hissing is too much. They redesigned the butterfly to help reduce the hissing problem. i think that it was a warranty issue in our shop. Might want to chech it out.