Mercruiser 3.7 L stalling over wake

[bull12001]

I have a Mercruiser 3.7 L I/O w/ an Alpha One sterndrive. Not sure of serial # on motor as the tag is missing.

This weekend I took it out on the lake and it ran fine up until we had been on plane for around 10 minutes. Then it started stalling as we went over the wake from other boats. Once the wake cleared, it would run again. It's a project boat so my first assumption was that the fuel gauge was off and I was out of fuel. Puttered at idle to the fuel pump-5 gal to top off. Boat ran fine for the trip back until around 10 minutes on plane and started doing the same thing again. Throttled back and idled for a bit then she came back up on plane and never ran flawlessly over wake until we reached our fishing spot.

My first thought: fuel. The fuel pump, filter, and carb are all recently new/replaced/rebuilt. My thought however is a clogged fuel pickup tube or dirty fuel/sediment. The vent whistled a bit when topping off and you can't put fuel in at normal speed as it back up into the fill tube so possibly some restriction there. I'm shying away from a vent problem though as it runs smoothly for 10 or so minutes.

Temp is running around 155 F so not overheating.

Second thought: coil going bad. Distributor, plug wires, rotor, points, and condenser all recently new and adjusted for proper timing and dwell. Could this possibly be a failing coil that is failing upon reaching a certain temp?

So I'm going to clean and inspect the fuel tank/system and probably replace the coil. Am I on track? Is there anything else I should consider in addition to the above? Thanks in advance for the help and advice.

 

[tdshepard]

Re: Mercruiser 3.7 L stalling over wake

If your carb float level is not spot on those rough wakes will cause a fuel slosh, temporarily flooding carb........................

 

[dubs283]

Re: Mercruiser 3.7 L stalling over wake

you could try running with the fuel fill cap off, that will eliminate the fuel vent - it seems that although its not completely blocked (whistling upon fill) there may be some restriction in the vent hose that is affecting higher rpm fuel delivery along with filling the tank

your true indicator will be a vacuum test on the fuel system along with a fuel pump pressure test

a faulty coil could be your problem, they do fail from time to time but they are pretty much bullet proof - one test is to check the temp of the coil when the issue arises, if its super hot then it could be the cause

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

If your carb float level is not spot on those rough wakes will cause a fuel slosh, temporarily flooding carb........................

Could this still be the cause if the boat operated fine while going over wake while running on plane prior to this? Should I be concerned about the fuel pump not keeping up?

When it happened the first time, I pulled the doghouse to inspect and also found that the tube from the flame arrester to the head was off. Would this cause a change in mixture that would cause the carb to screw up?

 

[dubs283]

Re: Mercruiser 3.7 L stalling over wake

the tube from the valve cover is just to vent crankcase fumes into the carb, it has nothing to do with mixture, idle or anything else

if the float was off you would be experiencing symptoms at all times

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

the tube from the valve cover is just to vent crankcase fumes into the carb, it has nothing to do with mixture, idle or anything else

if the float was off you would be experiencing symptoms at all times

That's where my thought process led me as well.

you could try running with the fuel fill cap off, that will eliminate the fuel vent - it seems that although its not completely blocked (whistling upon fill) there may be some restriction in the vent hose that is affecting higher rpm fuel delivery along with filling the tank

your true indicator will be a vacuum test on the fuel system along with a fuel pump pressure test

a faulty coil could be your problem, they do fail from time to time but they are pretty much bullet proof - one test is to check the temp of the coil when the issue arises, if its super hot then it could be the cause

Would a restricted fuel vent cause it to only happen after a while of running on plane? The way it would work in my head is that it is getting enough suction to barely keep up and as it runs at high RPM's the bowl slowly empties until it's dry. Then once fuel pump catches up again, she runs like a charm. I should mention at this point that I am not working off the stock mechanical pump. It has been replaced with an electric fuel pump. What concerns me is that it is a pretty generic pump which reads 'fits most 4 and 6 cylinder applications'. Manual only calls for 4-7 psi fuel pressure. Possibility?

 

[thumpar]

Re: Mercruiser 3.7 L stalling over wake

Is the fuel pump hooked to an oil pressure switch?

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

Is the fuel pump hooked to an oil pressure switch?

It is not. It gets it's power from the hot side of the ignition system. Directly grounded to battery.

 

[dubs283]

Re: Mercruiser 3.7 L stalling over wake

It is not. It gets it's power from the hot side of the ignition system. Directly grounded to battery.

it should be wired to an oil pressure switch for safety reasons

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

Would a restricted fuel vent cause it to only happen after a while of running on plane? The way it would work in my head is that it is getting enough suction to barely keep up and as it runs at high RPM's the bowl slowly empties until it's dry. Then once fuel pump catches up again, she runs like a charm. I should mention at this point that I am not working off the stock mechanical pump. It has been replaced with an electric fuel pump. What concerns me is that it is a pretty generic pump which reads 'fits most 4 and 6 cylinder applications'. Manual only calls for 4-7 psi fuel pressure. Possibility?

it should be wired to an oil pressure switch for safety reasons

So is this something that could cause the current problem?

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

Is it possible that the electric fuel pump doesn't pump enough fuel at the higher RPMs?

 

[dubs283]

Re: Mercruiser 3.7 L stalling over wake

Is it possible that the electric fuel pump doesn't pump enough fuel at the higher RPMs?

yes, but you will only know if you pressure test it, a vacuum test will show if the vac is too hig at higher rpm

wire the pump to work with the oil switch, when the oil pressure drops, the pump stops for safety reasons

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

yes, but you will only know if you pressure test it, a vacuum test will show if the vac is too hig at higher rpm

wire the pump to work with the oil switch, when the oil pressure drops, the pump stops for safety reasons

What pressure should it maintain at higher RPMs? The manual calls for 4-6 psi at 1000 RPM. What should I look for around 3000 which is right around where it happens? What should the vaccum be? I'm not sure if there is an oil switch installed on my rig. I think it's just an oil pressure sending unit as I don't have the alarm system installed. Is there someplace else it could be wired to accomplish the same goal?

 

[dubs283]

Re: Mercruiser 3.7 L stalling over wake

What pressure should it maintain at higher RPMs? The manual calls for 4-6 psi at 1000 RPM. What should I look for around 3000 which is right around where it happens? What should the vaccum be? I'm not sure if there is an oil switch installed on my rig. I think it's just an oil pressure sending unit as I don't have the alarm system installed. Is there someplace else it could be wired to accomplish the same goal?

higher rpm would demand more fuel so 6-7 steady psi @3k rpm

vacuum = less than 2 in/hg at all rpm

you would have to install an oil safety switch for your fuel pump, you can buy them online for just such a purpose

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

Thanks. Thankfully, I still have the mechanical pump installed which feels as though it's putting out good pressure. A family member of mine thought it was bad so they installed the electric pump to bypass it only to find that it was the water separator. I'll have to tear into that next chance I get. I might just switch back to the mechanical pump and see how it does and leave the electric pump installed (but unhooked for now) to get me back to the dock in case the mechanical fails. Also, for reference, I tested resistance on the coil and found .4 ohms on the primary and about 11,000 ohms on the secondary.

 

[generator12]

Re: Mercruiser 3.7 L stalling over wake

Since the problem you describe is linked to physical motion (i.e., crossing over a wake), it would not appear to be any of the suggestions above that aren't impacted by such motion, such as ignition components. If it were me, I'd be looking squarely at fuel delivery, including the carb.

A pressure gauge on the fuel pump discharge is always helpful as it will break a delivery problem down to pre-or post pump. You can make up a tee out of flexible hose and put it into the pump discharge line, then watch it when the problem occurs.

(Ever heard the old "rag in the fuel tank" stories?)

 

[bull12001]

Re: Mercruiser 3.7 L stalling over wake

Removed tank, cleaned tank, replaced tube, replaced anti-siphon valve, new fuel lines, new water separator, new vent lines.....we'll see what happens. Not a bad idea anyway. So much crud coming out of the fuel tank, I'm surprised it ran at all.....

 

[bull12001]

The fuel system rebuild seemed to solve the problem. Ran great the entire rest of the season. Now this season, I'm chasing what I believe is another fuel issue. It never ends....lol. Break Out Another Thousand....

 

[bull12001]

The fuel system rebuild seemed to solve the problem. Ran great the entire rest of the season. Now this season, I'm chasing what I believe is another fuel issue. It never ends....lol. Break Out Another Thousand....