I have a 2004 Mercury 40 horse power 4 stroke (three cylinder) with EFI. The tag on the motor states an idle speed of 775 rpm in gear. Mine idles at 1000 rpm. I had the tach checked out and it is correct. I've had three technicians tell me it cannot be adjusted, yet I have read somewhere that you can file a brass throttle stop or drill a hole in the plastic to access an idle adjust screw. After looking at some diagrams, it appears to me that this adjusting screw and brass stop are the same part, and that it could be accessed by drilling a hole in an emboss on the plastic intake manifold. Has anyone out there tried this and does it work?
The idle speed on this engine is ultimately electronically controlled. There are 2 throttle valves on this engine, the main throttle body, and what is called an IAC valve. IAC stands for Idle Air Control. Click this link for a diagram -
https://www.mercruiserparts.com/bam/subassembly/32557/11848/30 In the diagram locate #21 (It's down towards the bottom of the picture). #21 is the IAC. The way it works is when you are at idle speeds, the engines computer will open up the IAC valve and allow air to enter the engine via the IAC. As you start to throttle up, what you are doing is opening up the main throttle body. As the throttle body opens up, the IAC will start to close. Once you get to about 1500 RPMS, the IAC will be fully closed, and all air enters via the main throttle body. When you throttle back, the main throttle body closes, and the IAC will open up. Because the IAC is an electrically operated air valve, they are considered a wear and tear item, and they do eventually fail. When they fail they will fail in 1 of 2 ways. The IAC will either stick closed, and the engine will not idle at all. Or they stick open, and the engine idles high. All you need to do here is just check and make sure that the main throttle valve isn't sticking open for any reason (like a hung up cable or corrosion). Other than that, if the main throttle valve is opening and closing properly, then you would be looking at a bad IAC valve. Does that all make sense to you. I do want to make sure you are happy with my service before you go. If you had a further or more specific question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.
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