350 mag dieing at idle

usafaviator

Wondering what could cause my engine to shut off while at idle, it happened twice this past weekend. Cranks up fine idles initially around 1200 rpm. Underway temp and pressure are within limits after I pull it back is when it happened both times, I thought I saw about 700-800 rpm at idle after pulling it back and then with no warning shut down. Im thinking fuel pump or fuel filter? It did this maybe 3 times over about 20 hours last year. Any help with diagnosing this would be much appreciated. 

Also the boat is taking on just a little water in the bilge, still not enough to turn on the auto float switch but more than I remember from last year. Over 24 hours enough accumulated to run the bilge pump for about 3-5 seconds before cavatating. We sleep on the water and I just don't want any surprises while asleep. Just wanting you guys opinion if this is normal or if I do have a leak that needs to be addressed asap.   

rinker312

IAC valve would be my guess. Try fuel filters first if you want but if they were the problem you would be having issues at higher Rpms I would think. It is easy to change. 

You should not be taking on water and I would look for it. But that being said mine was taking in water for a few years until I finally realized where it was coming from and got it fixed.  Would not let it stop me from using the boat with a small leak like that, but that is just my opinion. 

Good luck

Black_Diamond

IAC valve for sure.

LaRea

Probably IAC, but since it always happens when shifting to neutral, it could also be a problem with the neutral cutoff switch. 

Most boats older than a couple years have some water in the bilge as a matter of routine.  If your pump runs only once a day for a few seconds, I'd call that normal but definitely worth investigating. Any chance it's fresh water from your plumbing?  If not, check the trim tab actuator cables to see if there's a drip trail.  Check your forward bilge too, in case the leak is up front somewhere. 

212rowboat

good call on the idle cutoff switch, LeRea... 

call it a stall preventer when the gear box shifts into or out of gear- spark is momentarily cut off, and the engine actually misses a beat or two... 

but... re: IAC... clean it.. use throttle body cleaner and make sure the pin is clean as a whistle... you can do it with the sensor on or off... if you take it off, be midfull of NOT altering its position, as the position is mapped by the PCM, and it's easy to do. 

a bit of gum (which they'll all build) and you've harmed your efficiency- the PCM thinks (based on it's mapping of the plungers position) there is enough air present to idle, but there isn't... it'll adjust it all by itself after it figures it out, but it's gotta be running to figure it out. 

usafaviator

thanks everyone! The boat is a 2007 with just under 100 hrs on it. Would I be better off cleaning the IAC or just replacing it?

Dream_Inn

You can try cleaning first.  I had this issue on my prior boat and just replaced.  Then I had it on my other engine a month later, had the spare already bought so I changed it as well.  They run about 100 bucks.

craigswardmtb

You can find them on Rock Auto for $44.  The below link is for a 496 but its the same part for the 350mag.  I have a couple spares on the boat just in case.

https://www.rockauto.com/catalog/moreinfo.php?pk=1319722&cc=10001422&jnid=120&jpid=0

rinker312

I do not believe that a boat that new with bravo 3 has a neutral cutoff switch. I think that was on older boats with older bravos. I would buy a new iac. Then clean the old one and hold onto as a spare. Sometimes when these things go they will give you an annoying double tone and the engine may not idle at all. So having a spare that that still runs and has no tones may save the day at some point. 

For your leak. Be sure to check behind the engine on the top of the transom plate where the steering arm goes out that is where my leak was and it took me forever to notice it back there. Ideally your bilge should be dry but it is tough. I had a problem two months ago but cleaned it up and have not had any water since. Weird. 

craigswardmtb

ditto on the R312 recommendation on the potential swivel shaft leak.  I just posted a lengthy write up on repairing this.  It feels great to finally have a dry bilge...

usafaviator

Thanks for all the help guys, going to order a new IAC and keep the original as backup like suggested.

I was initially thinking a leak in one of my bellows but the swivel shaft leak makes more sense.

craigswardmtb

Put your smart phone down by the steering arm and start snapping some blind pictures. If it's the swivel shaft you will see water where the steering arm attaches to the swivel shaft and likely some rust.

usafaviator

Just a quick update, wanted to thank everybody for the diagnoses. I did end up ordering a IAC valve off of Rockauto parts and it turned out they sent me the wrong one, (oh well). I did take out the IAC valve and cleaned it with throttle body cleaner. So far the boat has been running fine and hasn't died since. Thanks again everyone.

Dream_Inn

Great!  Thanks for the update!