Slow rough idle

[longbeard]

Crew, as my previous post stated, I just made a jump to a 2004 Tige 22v. I noticed that at idle is is hovering around 400-500rmp and seems like it is kinda just loping. It has Mercruiser 350 Mag MPI. I think it may be the IAC that needs replaced, but wanted a second opinion.

[Cap305]

Does the problem happen when the boat is cold, warming-up, or hot? (all three?) I ask because I had problems at idle but only while the boat was about half way to operating temp. Ended up being a coolant temp sensor from what I was able to deduce. Cheap fix after I threw a TON of parts at it with no luck. Please give a few more details as to when it's happening and if it feels like the idle is going up and down or if it feels more like a cylinder miss. Mine felt like it was missing.

[ibelonginprison]

I had a similar issue this weekend after riding around for 3 hours straight. (using that much fuel is an expensive way to charge my batteries, but whatever)

After having it on for that long, I shut it off and let it sit for about 30-40 minutes while we chilled. No water cooling the block/exhaust manifolds meant that engine locker got super hot, and the fuel pump suffered vapor lock. I went to leave and the boat started, then idled really, really low and rough for about 30-40 seconds (350-400rpm), then shut off and wouldn't really start and run after that.

Poured about 5 cups of cool lake water down the side of the pump, cooled it off nicely, and everything ran smoothly from there on out.

Not saying that's your issue, but it is certainly something to keep in mind.

[longbeard]

Does not matter the temp, cold, warm, hot, just kinda lopes along when at idle speed. I have had a vapor lock issue before. This feels different.

[oceanbu]

is it running rich at idle too?

[Jimmypooh]

Check fuel pressure! I've seen several fuel pressure regulators clog cause a very rich condition.

[justgary]

And before you throw parts at it, grab a can of carb cleaner and clean inside the throttle body, including all the little passages that may be inside. Turn the engine off and open the throttle to spray the edge of the butterfly and the small vents where it hits the wall of the body.

If the idle improves, great. If not, at least you can rule that out and start throwing parts.

[Bturner]

I'm with Jimmypooh on this one. We fought this problem for years before it finally got so bad the boat wouldn't run at any speed. The pressure regulator on engine packages from this era are a well known problem.

[oldjeep]

Since you just got it, I'd pull a couple plugs and check the cap and rotor too. You notice a lot of ignition issues at idle that go away at higher rpms.

[longbeard]

Thanks for the input. I will check it all out. It doesn't seem like it is running rich, it seems like it is just choking.