Merc MCM 5 L: Strange Restart Issues

[OxfordBoater]

New to this wonderful forum.

Boat Info:

Sunchaser 1901 XL Deck/Bowrunner Model
Alpha one O/D

Engine Info:

Mercury Marine
Model: MCM 5.0 Litre/LX
Displacement: 305 CID (Cubic Inch Displacement)
Rated HP: 230
Ignition Timing: 8 BTDC (Before Top Dead Center)
Cylinder Firing Order: 1-8-4-3-6-5-7-2
Spark Plugs: AC-Delco MR44T
Engine Rotation: LH (Left Hand)
Max WOT (Wide Open Throttle) RPM: 4200-4600
Spark Plug Gap: 0.035
Hours: 715

So with the brand new battery I recently bought, she starts up great. Get out in the water and can idle around or go all out 50 mph and we have no issues. The problems come after cruising around for an hour or two (or 6) we decide we wanna find a nice cove for swimming and turn off the boat for an hour. We don't use any electronics while stopped. When we decide we want to get back to skiing, she starts back up a little slow but will start. We'll get about 5 minutes in and it'll die. Doesn't matter if we're going no-wake speed or fast...the engine will just stop. We can get it started another time or two but finally it becomes unstartable.

We've had the issue probably the past 5 times we've been out. Only happens on the restart after the hourish swim. Aighnt a big deal because we'll just have our jetski tow us in...but it's getting old.

Every time we go out, we change a little something. Things we've tried:
-Added two blowers for more air flow
-Changed out gas
-Only use high octane fuel
-Cleaned carb
-Allow it to warm up before beginning again
-Try traveling fast and slow
-Used Seafoam to clean things out
-Check for negative pressure in gas tank


Out of ideas because it's difficult to test when it starts and runs great the first run of the day.

Any suggestions would be greatly appreciated.

 

[OxfordBoater]

Re: Merc MCM 5 L: Strange Restart Issues

Also, all gauges are within operating norms throughout the day. Including temp, oil pressure and volts.

 

[alldodge]

Re: Merc MCM 5 L: Strange Restart Issues

Sounding like vapor lock. When it has difficulity starting, have you removed the flame arrestor and pump the throttle to see if your seeing gas? Higher octane fuel makes no difference with vapor lock. How are your fuel lines routed to the carb, and how close to the engine surface?

Oh and :welcome: to the forum

 

[OxfordBoater]

Re: Merc MCM 5 L: Strange Restart Issues

Thanks for the warm welcoming.

I was under the impression vapor lock only occurs when the gas in the lines has gotten warm over continued use.

The engine is shutting off only after the engine has had an hour to cool off while we were swimming. And, does not shut off while cruising around a while for the first couple hours of the day.

I will check the lines tomorrow in the light and may have to wait till we go out on the water again to know better about whether gas is getting squirted or not. It is right now.

Thank you!

 

[alldodge]

Re: Merc MCM 5 L: Strange Restart Issues

Vapor lock occurs when the engine is hot and the fuel lines soak up the heat from the engine when turned off. Fuel in the lines start to boil and vapor is what is left. You can read other post about folks once they shut down to open their hatches, leave blowers on, idle for longer time to help the engine cool down before turning it off. If you can find non-ethonel fuel is best to use.

This is not to say that you don't have another issue, it's just so far from what your saying I'm leaning this way.

 

[OxfordBoater]

Re: Merc MCM 5 L: Strange Restart Issues

I did check today and it does appear the fuel lines pass directly next to the hot engine. And makes more sense about why it only happens upon shut down. How sensitive would the fuel system be to lengthening the lines in order to route them around the hot spots? Would newer, better insulated lines be another possible solution?

Next time i go out, i'll run the blowers and engine for a while idling before shut down to see if it runs better on start up.

Thanks for the help guys. This sounds like it very well possibly could be the issue when looking at the lines. I was going crazy trying to figure out other possible causes because the engine runs spectacularly for hours.

 

[alldodge]

Re: Merc MCM 5 L: Strange Restart Issues

For the most part fuel lines can be failly long with no impact. Merc started putting the fuel pump (electric) under the engine to help keep it cool. They have gone through several renditions and the thing that works the best is to use non-ethonal fuel. The fuel which is put out during winter is the worste. In older days they use to put fiberglass insulation around the lines and wrap with foil, it's an issue and hope you can find a work around. I use nothing but high dollar marina fuel because of no ethonal, cost sometimes a buck more a gallon but no problems so far.

Another possability is the coil breaking down, if it's not fuel pull a plug wire and see if you get spark.

 

[CharlieB]

Re: Merc MCM 5 L: Strange Restart Issues

Seriously suspect heat soak into the fuel.

Alcohol reduces the boiling point of gas.

Reroute and insulate fuel lines.

Open hatch or make sure you have a second battery and switch to run blower to vent heat and cool engine bay while swimming.