If your 2005 Mercruiser 496 HO is idling rough, surging, or even stalling occasionally, there could be a few reasons behind it. Since your engine runs on fuel injection, addressing these issues involves a bit of troubleshooting and careful maintenance.

Dealing with Surges:

Surging usually hints at problems with air or fuel delivery. Leaky gaskets around the idle air control (IAC), intake manifold, or throttle body can mess with the air intake, causing the engine to run unevenly. It’s also wise to check the throttle valves for any wear or damage. If they’re worn out, they might not handle air intake properly, leading to surges. And don’t forget to inspect all hoses connected to the IAC or pressure regulator for wear and leaks—they need to maintain the right vacuum pressure.

Addressing Stalling Issues:

Stalling is a bit trickier and needs closer attention. If your engine stalls at idle, start by cleaning the flame arrestor to make sure there’s enough airflow. Testing the IAC is also a good idea to see if it’s opening and closing correctly; a faulty IAC can mess with your idle stability. Also, checking the fuel pressure with a gauge is crucial—it can help pinpoint any fuel system issues. High fuel pressure might flood the engine, so it’s important to inspect and possibly adjust the fuel pressure regulator.

Dealing with Misfires:

Misfires can contribute to rough idling, so it’s important to check your ignition system thoroughly. Take a look at the spark plugs, spark plug wires, and ignition coil for any signs of wear, damage, or malfunction. Replacing any faulty parts can help ensure your engine runs smoothly without misfires.

Taking these steps can help you figure out what’s causing your Mercruiser 496 HO to idle rough, surge, or stall. If you’re not comfortable doing these checks yourself, it might be worth getting help from a qualified marine mechanic—they can diagnose the issue accurately and get your engine running smoothly again.

Hope this advice helps you get to the bottom of your engine troubles. If you need more details or have other questions, feel free to ask!