Mercruiser 350 Warm Idle Problem

[Mike Wisnieski]

Hello everyone. I’ve got a problem with my 1996 Mercruiser 350 Magnum TBI inboard that has my mechanic and me stumped, maybe someone here can point us in the right direction. It starts and runs great when cold. After it has warmed up to normal operating temperature, it continues to run fine at high speed, but at idle and up to about 1200 RPM it pulsates in a steady fast-slow rhythm, belching white smoke and plenty of gasoline fumes and liquid gasoline out the exhaust. When coming down from high speed operation, it will idle fine for about 30 seconds, and then the problem mode starts. The mechanic tells me that all the basics-- compression, plugs, fuel pressure, timing, etc. are OK. There are no trouble codes showing in the computer. The sensors that have anything to do with fuel mixture have either been replaced or confirmed to be in spec—engine coolant temp., MAP, and throttle position. Strangely enough, this only happens while running in the river (water temp. currently in the low 50’s). On dry land running off a hose, there is no hint of the problem, it runs fine at all engine speeds, both warm and cold. The last time out I tried disconnecting the engine coolant temp. sensor while it was running (idling) and having the problem. It instantly began to run fine, but within a minute it reverted back to the pulsating and the fuel in the exhaust. Thanks very much in advance for any ideas you can offer.

 

[Don S]

Re: Mercruiser 350 Warm Idle Problem

Did he check the fuel filter for water? Maybe water getting into the exhaust at idle due to manifold/riser gasket leak?

 

[TilliamWe]

Re: Mercruiser 350 Warm Idle Problem

you mention some sensors, but does yours have an Idle Air Control sensor? If so, it's been checked ok/replaced?

 

[Mike Wisnieski]

Re: Mercruiser 350 Warm Idle Problem

Thanks for the replies. I don't know if the mechanic checked for water in the fuel filter. Regarding water in the exhaust, I do know that I can hear lots of water rushing in through the exhaust openings after I turn the engine off. This is my first boat with through-hull exhaust, so I don't know if that's normal or not. The flappers on the ends of the exhaust pipes don't seem to seal very well at all (do they deteriorate with time?-- last season the engine ran fine). Regarding the Idle Air Control sensor, yes it does have one, and I believe that one also checked out OK, but I'll confirm with the mechanic.

 

[K Hultgre]

Re: Mercruiser 350 Warm Idle Problem

Mike, Here is some info from a previous post (Mercru 350 mag mpi ski) that might come in handy.<br />One statement that caught my eye is "cleaning the muffler for the IAC with a little carb cleaner"<br /><br /><br />"Don, correct me if I'm wrong, but I believe that the cam and crank sensor fault codes can only be set while cranking. If the ECM is not finding a problem while cranking a fault code may not be set, but a fault may still exist. "<br /><br />"Also, were you just looking at the active fault codes with your scan tool or were you able to retreive the fault history as well?"<br /><br />"Just out of curiousity have you checked to see if the IAC values change as you start it. If the IAC was stuck you could always pull the PCV hose and see if it will stay running by regulating the vacuum leak with your finger."<br /><br />"I ended up popping up the rails and cleaning each injector as well as the muffler for the IAC with a little carb cleaner.The thing fired off like a roman candle."