Merc 350 sputters, dies, won't restart after heating up on 2000 Tige 21i direct drive

[LonghornBoarder]

BLUF: Merc 350 starts, runs for ~10min, then stalls & will not restart for ~30min. Owner seeks troubleshooting suggestions.

Chasing an issue on a Mercruiser 350 inboard motor. Starts up fine, idles smooth, runs at speed and under load (e.g. pulling a wakeboarder or skier), then if speed is reduced or brought back to idle speed, it will begin to run rough, sputter and die. Starter will crank, but engine will not restart. The first couple of times, if it sat for 20-30 minutes, then it would restart and run (during which time it would be driven straight back to dock).

Based on the handful of occurrences, it seems to be getting worse; by worse I mean it starts fine but begins to run rough & then die sooner after launch. Also seems not to "recover" following a period of "rest" like it did previously. On the last outing, it would not restart, even after an hour. Towed back to dock and am now very motivated to solve this mysterious problem.

Based upon the sputtering fashion in which the engine dies, my first thought was that it was starving for either fuel or air. But the fact that it starts up upon first launch and then starts again after cooling down made me think perhaps it was just fuel delivery once hot, with the fuel pump changing it's operation once the engine heated up. After some research, I think it may be electrical - while still being adversely affected by temperature.

Engine details:

• Mercruiser direct-drive model: MIE 350 MAG MPI SKI
• Serial: 0L393706
• Motor is circa 1999 (from a year 2000 Tige 21i) and has ~900 hours.

Steps taken since the behavior first occurred:

1. Replaced fuel filter (fuel/water separator)
2. Compression check - good
3. Replaced distributor cap, rotor, and spark plugs - one cracked plug was noticed when the old ones were pulled, but behavior was consistent after this set of maintenance replacement activities.
4. Thoughts on possible root cause:
   1. Fuel tank filter sock - trapping sediment during extended operation, then clearing while the boat floats and sloshes back and forth?
   2. Bad ignition coil - operating differently once hot?
   3. Crankshaft position sensor - faulty? More finicky at temp?

The challenge has been performing any diagnosis when it happens, because it has happened 20-30 minutes after starting a day of watersports, so we're usually with a boatload of passengers in and/or behind the boat and the focus shifts to just getting back to the dock. Hopefully now that it happened sooner, it will mean I can go on a test run and bring some tools to check / diagnose a few details. First order of business is to get a new, working thermostat in so that operator can watch that gauge and read accurate temperatures. Next I'd like to try to troubleshoot the ignition coil. I'm open to other hypotheses and/or troubleshooting suggestions.

tags: Mercruiser, 5.7, inboard, stall, no-start, temperature problem, rough engine, motor dead

[boatwakes]

If you can start it applying throttle in neutral, replace the IAC (Idle Air Control valve) and it will resolve the issue. On those boats, pull the silver button out, just forward of the shift/throttle lever, and then apply throttle like you would if you were shifting into forward. If it starts and runs with throttle applied but stalls when you bring the throttle back to idle, it's the Idle Air Control valve gone bad. Mercruiser part number 803149. The button detaches the shift cable from being activated and will allow you to apply throttle. Next time it stalls, try this. Once again, if it resolves, replace the idle air control valve.

[LonghornBoarder]

Thanks for the input. Unless I find something further on a subsequent outing, this behavior was resolved by replacing the ignition coil.

• Ignition coil kit:
  • Mercruiser part# 0300-8M0079202

I am keeping the 500+ Hour Bullet-Proofing checklist and will continue to address other elements from that list, but for now I think we're back to smooth operation with just this corrective action.

[LonghornBoarder]

Vapor lock?

Have you already done the Mercruiser TSB and added the additional fuel pump?

TeamAllen I cannot find any such TSB. Do you have a link or can you provide any details about this? There were several sites with historical Mercruiser TSBs and Tech Handbook, but they're no longer available.

[boatwakes]

Early 350's (through serial 0M310009) did not have a boost pump with the Gen II cool fuel system and were prone to vapor locking. The fuel boost pump addition eliminated the problem. Mercruiser diagram here: https://www.perfprotech.com/mercury-.../60?model=4201

[freeheel4life]

boatwakes just wanted to pick your brain on what was causing the issue on pre 0M310009

Would the fuel on the vacuum side of Cool Fuel boil in the line after shutdown when hot and adding the low pressure/boost put pressure in the line pre cool fuel to solve it??

[boatwakes]

Apparently the fuel in the rail boiled from engine/intake manifold heat. Because the fuel return went back to the fuel filter and not the tank the vaporized fuel was being returned directly to the high pressure fuel pump which was unable to clear it. The new boost pump was connected directly to the incoming fuel line from the tank and would low pressure force the vaporized fuel out of the high pressure pump. I had one of those engines without the boost pump in my 99' but never encountered the vapor lock problem, only heard about it. It only happened with the Gen II cool fuel systems as well, the Gen III module addressed the issue with a return-less fuel system.