Rough Idle

Boat is a 2007 Sanger V230 with 5.7 Scorpion engine and 140 hours.

Three weeks ago we pulled it out of for the first time of the year (terrible spring in the NW). I put 30 gallons of new gas in, and hit the river. Didn't have any problems with the engine and everything was fine.

Fast forward to last weekend, I take the boat out and it starts a little rough. The RPM's were jumping around and it died about 30 seconds after starting it up. I drove around for 5 minutes or so to bring the engine up to temp. It drove fine and I thought all was good. As soon as I brought the engine back to neutral the RPM's started jumping around between 300 and 800 (roughly). After about 30 minutes of running the RPM's leveled out and all was good while in neutral.

Wednesday of this week my parents took the boat out and said they didn't have any problems but that the RPM's varied a bit when in neutral. I took the boat out again this AM and it died 3 times on me while in neutral. It drives perfect once the RPM's are up a bit. I tried not running any accessories and it didn't make a difference.


Any thoughts on what could be causing this and how to fix it?

I'm assuming from how new the boat is that is is fuel injected rather than a carburetor? Honestly I don't know which is why I am asking. Sounds like a choke issue if it was a carb. How warm or cool was it when you went out? Do newer boat have a Mass Airflow Sensor like a car? Had similar idling issues in my car that were caused by a sensor?

My apologies for asking questions I have a 96 with a carb so it's what I know.

Clean the Idle Air Control (IAC) valve and cavity if it's EFI. Should be a fat plug held in by a couple screws, somewhere in the throttle body, with wires coming out. If that doesn't fix it, I would change the plug wires next.

The IAC was mentioned by someone else as well. I will look into that. Thanks for the info.

It's fuel injected.

Same problem here. I cleaned the IAC, but problem is still there. It's a mercruiser MPI, and the IAC seems to have a actuator connected to it. I'm thinking it might be defect. Changing the IAC would do it?

Scan tool is a must for diagnostics. Throttle Position Sensor or Idle Air Control come to mind.

There are quite a few potential issues that may affect your engine's idle:

Bad gas or water in gas. Get blue Sta-Bil and use every tank.
Dirty air cleaner or back-fire flame arrestor
Fouled spark plugs
Loose spark plug or ignition wires
Dirty components (as previously noted)
Overfilled engine oil (some motors)
Dirty throttle plate (do not immerse)
Air leaks in intake system

trouble shooting your condition requires some patience and a systematic approach. Get a manual and follow the procedure to avoid chasing the problem around for weeks.

This is an old thread!

The problem with our boat turned out to be the plug on the IAC valve. After all of my research and trying different things, it turned out that the plug was loose. Easy 'fix' but frustrating too

What plug are you mentioning? I think I pretty much sorted out the problem, but since i'm not sure, any other opinion or advice is appreciated.

In the throttle body there was some kind of mesh that i removed (not sure if it was supposed to be there). Idling is absolutely fine now, the only issue is that after reving the engine, an getting back to idle, it appears like the valve does not react fast enough. The engine nearly dies, and finally picks up to get back to normal idle.

I'm thinking about changing the IAC

Am i on the right track?

I believe it's called a canon plug, and its located on the port side of the engine just under the lip of the air intake manifold, attached to the IAC. On the black scorpion engine it's in a weird place compared to most engines (from what I'm told).

I had a similar problem a few years back that turned out to be a bad distributor. I could adjust it to get proper ignition timing at idle or at throttle, but never both at the same time. I understand this is a pretty uncommon failure, but just throwing another possibility out there.

Idle is good, and throttle too. It's just the after releasing throttle, and getting back to idle, I nearly dies, and then picks up again. I get this out of the water on fake a lake, but with the transmission engaged, I'm pretty sure it will completely stop.

My take on this is that the IAC valve is not reacting fast enough and that the actuator is getting old. I'm not thinking it could be anything else since idle and throttle is mostly OK.

Am I missing something? Changing the IAC will do?

Can't hurt to pull the IAC and clean it. Shouldn't take too long to do. If you want to PM your e-mail address I think I still have some PDF's showing where it is and how to do it (I'll have to look later to be sure).

Yep found it and cleaned it, but it did not seem to do the trick...

Whens the last time you changed the fuel filter.

Its technically a fuel/water separator, but when it gets full of water your motor will starve for gas because it will not let gas through with it being full of water. They will typically surge like that when they are full. Not a 100% sure on your motor but it will look like an oil filter. Replace it and fill the new one up with fresh gas or cycle the key on and off for about a minute to prime it. Id check this first before you go tearing off and replacing electronic parts.

Yes, did this. Changed it with a brand new mallory filter. I've also changed the inline fuel filter.

hmmm. Sounds like its a fuel mapping problem almost. Does it have a TPS on it.

Yes it has a TPS on it. I doubt it is the the TPS because if turn the intake knob the engine will rev normally, and keep it's RPM and go smoothly.

I changed the IAC valve and now everything is good!!