V8 Mercruiser Cutting Off During Idle

[jjedmund]

I have a 21 foot Nautic Star 2007 with a v8 Mercruiser. Bought it last July and put a lot of hours on it from July to November. Stored it for the winter and when it got warm again, took it back out of dry storage and it wouldn't crank. Looked at a few things and the first issue that we found was a diode for the fuel pump was bad. Replaced that and it cranked right up. Well next time I took it out, it wouldn't crank. This time, we found that moisture was building under the distributor cap, so we replaced that. Cranked fine. Next time I tried to run the boat, wouldn't crank. Figured we probably fouled out the plugs from all the times we tried to get it to crank the last two times it didn't run. Replaced plugs, cranked fine.

Took it out this last time and I had it in neutral in the marina and it was idling kind of weird. Went out of the marina and opened the throttle and boat ran fine. Once I hit a no wake zone and slowed to idle, the boat stalled on me. The only way I could get it to crank back up was giving it throttle while cranking. It refuses to stay running unless it's basically wide open.

The boat ran so fine when I bought it with literally no issues. Now every time I fix something, it's just something else immediately wrong. What would cause it to stall during idle? What's weird is when I had it running for 10 mins in the marina, it didn't cut off at all in neutral. My first guess is the IAC valve or maybe my fuel pump is bad? Anyone experienced this and any recommendations?

[Bt Doctur]

Any particular mercuiser v8 engine you refering to?

[o2batsea]

Now every time I fix something, it's just something else immediately wrong

Ha ha ha ha ha welcome to boat ownership!!! Honestly tho, it seems like it's "running out of gas" which probably means the IAC is not working properly. It may be a loose vacuum hose or wire disconnected. These are easily addressed issues in any case. When stalling at idle it's usually because there is too much air and not enough fuel. If a hose is cracked or a port is open that should be capped, the ECU has no way to know that there's a very lean condition from this extra air. When you open the throttle, more fuel is introduced and it runs smoothly.
Your first assignment is to check the entire fuel system for good flow. If you didn't put on new fuel filters for the season, do so. It doesn't take much glop to make the engine run poorly.