Mercruiser 4.3l cutting out.

[sal]

I have a '95 mercruiser 4.3lx with thunderbolt iv ignition, 4bbl weber carb fed fuel by an electric fuel pump. The engine runs great from a cold, starts right up. I can cruise around with no performance issues. However, after it sits for an hour or so and I go for a cruise, the engine will suddenly bog down and stall. It will start right back up but I can't put any load on it or it will stall. I suspect the ignition sensor is the problem, due to a high-heat thermal breakdown. I can count out fuel because I replaced all the old fuel and still had the priblem. I doubt its the carb because until it craps out, I can run the motor for several hours wuth no issues at all. WOT I am at 4600rpm, no problem. Yesterday, the oil pressure gauge started reading erratic. Oil level was spot on. That's a new issue but I am replacing the sender and the switch as well with new OEM parts, just to eliminate the possibilty that they are causing the stalling problem. Bottom line...since the problem only occurs after sitting following a long run, I am leaning more towards ignition issues due to thermal breakdown. The fuel pump spins ok if I run 12v to it, though the sender and switch maybe cutting off the 12v and causing the stall. Anything else to consider? Thx.

 

[sugilbert]

I suspect vapor lock. Attempt to keep all fuel lines as far from block & risers as possible. Maybe even wrap with something insulating but nonflamable.

 

[BillK]

+ 1 on vapor lock -- because once things cool down it runs fine again until it sits hot allowing fuel in the pump and lines to vaporize. Regarding the ignition sensor, if you still have the old style with the exposed contacts, I would replace it, those are a known failure point, I proactively replaced mine. While the ignition sensor could be contributing to the issue, when they fail you will get an ignition cutout or a complete no start condition, not a fuel bogging down and stalling condition.
Next time it does it, pour some cool water on the fuel lines and pump housing if you can without getting electronics wet - this is an old trick that will quickly cool the vaporized fuel.
One other thing to check since you mention an erratic oil pressure gauge is the oil pressure switch - if it is failing and detecting low or no oil pressure, it shuts off the fuel pump. Could be as simple as corroded connections on it.

 

[sal]

Thanks Sugilbert & Billk. That's some good information, advice and explanation. It makes a lot of sense. I'll need to take a look at the ignition sensor to see about the connectors. Thanks!

 

[ddurand]

I would probably just replace the oil pressure sensor to start. Pretty cheap. Check out the connector to the oil pressure sensor while you are replacing it.

 

[sal]

I replaced the cut off and oil pressures sensors. The oil pressure is about 60psi at idle. That problem is resolved. I am going over the related wiring to make sure everything is solid. A bit of rain today so later this week.

 

[sal]

Thanks for all the replies. Once I fix a problem, I like to summarize the solution in case someone else runs in to a similar issue. As it turns out, there were several things I needed to address.

For starters, the oil pressure sender was bad causing erratic oil pressure gauge readings, so that part was replaced. I also replaced the switch that sends 12v to the fuel pump once the oil pressure builds up, and the related wire harness.

As far as the motor cutting out when hot, I determined the fuel pump was bad. I tested it on the bench for 5 secs and it spun but it didn't sound smooth. Luckily I had a used pump with very low hours so I swapped them. I installed a wrap insulation on the metal fuel line that runs from the electric fuel pump to the carb just to have some protection against vapor lock, though I probably wasn't having a vapor lock issue. This was cheap, easy and precautionary. I have a spare Weber carb but the old on if fine and not the cause of any problems but I may swap eventually with the spare, since that one has very low hours and it's easy work and no additional $ cost.

Next, ignition---At one point I was no longer getting spark. I followed the Thunderbolt IV flow chart to diagnose and saw I wasn't getting 12v at the white/red pin on the distributor module. It turns out the power comes from the purple wire off the + ignition coil. That wire was frayed on the coil end so I re-crimped with a new connector and shrink wrapped. Ignition came back to life. Since the motor was cutting out when it was hot, I suspected the fuel pump after I bench tested it considering it didn't sound smooth and solid. I'd bet that was the #1 culprit but the ignition coils are known to crap out when hot so that was #2. I bench tested the coil and it was reading withing spec for resistance on both the primary and secondary but I was uneasy with that because I wasn't testing the coil hot but rather at room temperature. Rather than take a chance with it, I bought a new Mercruiser coil, which cost me 3X more than an aftermarket, but now I have some peace of mind as far as potential coil problems.

So it all wound up with a frayed wire causing intermittent ignition failure, a very questionable fuel pump ( luckily I had a spare, I couldn't believe the cost of a new Mercruiser fuel pump), I replaced both components of the oil pressure sensor and sender and threw in a new coil just to know that part is new.

The result is when I turn the key, the motor starts instantly without even pumping the throttle to prime things up. It seems to be hitting nice on all cylinders and I am optimistic that my shakedown run on Friday will give me back some confidence. Aside from my own time,and I devoted quite a bit, the adventure ran me about $300 in total for parts. I could of saved a good chunk of that buying aftermarket part,s but I want to be able to enjoy boating and while there is a significant price difference, it well worth it to me to be boating more and playing marine mechanic less. I may eventually spring for a set of new ignition wires but for now I'll run with what I have since I'm not noticing any leaks or miss fires, etc.

Hope this summary helps some else out.

 

[ddurand]

You have regular dollars, boat dollars and fun dollars. Fun dollars have no price. Cannot measure the fun.

Boat dollars cost more than regular dollars but you knew that when you bought a boat.

As long as you don't have to open up outdrive or engine it will not cost very much. Your into thousands if you have to open the outdrive. Same for pulling the engine.

Summer is here enjoy the boat.

 

[sal]

UPDATE: I thought I resolved the stall problem but it turns out there's a small wire mesh screen inside the fuel tank pickup tube. At idle or low speed, there isn't much fuel being demanded, so no stalling. When planing off or higher rpm cruise, the draw through the tube is enough to pick up debris from the bittom of the tank, hence the motor stalls. When the engine stalls, the crud getting caught in the screen falls out, so the motor restarts. Whenever I checked the antisiphon valve and the screen, the screen was clean and the ASV was ok. I pulled the screen off and I'm no longer stalling. I see new replacement pick up tubes come without the screen. I have the mercriuser water seperator/fuel filter so any debris will be filtered before fuel enters the pump. There were ither issues that were repaired but the mesh screen jept me busy. May need to get the tank cleaned out. It's a 27 year old aluminum tank.

 

[ddurand]

I have never looked into a boat gas tank but I did pull the tank of a 10 to 15 year old Suburban and it was pristine. The only dirty thing were my hands. The fuel tank had an in-tank fuel pump and the fuel pickup has a "sock" on it to prevent debris from being picked up.

 

[BillK]

UPDATE: I thought I resolved the stall problem but it turns out there's a small wire mesh screen inside the fuel tank pickup tube. At idle or low speed, there isn't much fuel being demanded, so no stalling. When planing off or higher rpm cruise, the draw through the tube is enough to pick up debris from the bittom of the tank, hence the motor stalls. When the engine stalls, the crud getting caught in the screen falls out, so the motor restarts. Whenever I checked the antisiphon valve and the screen, the screen was clean and the ASV was ok. I pulled the screen off and I'm no longer stalling. I see new replacement pick up tubes come without the screen. I have the mercriuser water seperator/fuel filter so any debris will be filtered before fuel enters the pump. There were ither issues that were repaired but the mesh screen jept me busy. May need to get the tank cleaned out. It's a 27 year old aluminum tank.

Mine has the same screen I believe, the pickup actually lays on the bottom of the tank. I was having similar issues awhile back, for me it turned out to be two things:
1. Even though the anti-siphon valve looked / seemed fine, it was not. I replaced it and most of my problems went away.
2. The other issues was simply a loose connection on the fuel line - it was sucking air at high RPM and I would get a surging and sometimes it would stall. The air leak didn't happend except under heavy load, could run all day at cruise, but open it up and it would surge and cut out.

Once I corrected the above no more problems.

 

[sal]

Thanks Billk. I thought the problem was the ASV too so I replaced it but tge stalling persisted. I belive my fuel lines are tight. I replaced the line when I changed the ASV, includibg new SS clamps. I'll certainly keep an eye out as you advised.

 

[BillK]

Thanks Billk. I thought the problem was the ASV too so I replaced it but tge stalling persisted. I belive my fuel lines are tight. I replaced the line when I changed the ASV, includibg new SS clamps. I'll certainly keep an eye out as you advised.

This problem drove me nuts for a whole summer - sometimes it's the smallest things that cause problems.