Misfire after getting hot

capecodcruiser

capecodcruiser

Active Member
Aug 1, 2008
525
Falmouth, MA
Boat Info
2001 310, 350 HO's/Vdrives.
Engines
Mercruiser MPI 350 HO's
Hello, tis the season for issues with our boats! My 01 310 with 350 Mag/vdrives has been put back in the water after being out for 5 years. Everything has been gone thru maintenance wise. Engines run flawlessly. All pressures and temps are right where they should be. And they ran perfectly when last in the water 5 years ago. Now, after running hard(3500ishRPM) for 30 min or so, the port motor starts to lose power and back fire if I feed throttle. It will idle fine and run up to RPM's for a while and than start to misfire again. This has happened twice in the last week. I replaced the fuel filter to no avail. I am trying to figure out if it is fuel or ignition. I think these motors are too old for the paint chip issue on the CFM. Any help would be greatly appreciated. I was planning on my first weekend on the boat this weekend! Hope to resolve. Thanks
 
Espos4 said:
Is the gas 5 years old?
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Good question, I should have made it clear that no it's not. The tanks had maybe 20 gallons in them and I filled with fresh. So dilution should be fine. And as I said, it runs perfect for 30 min or so at 3500+. And today I idled around for 1.5 hrs with no issues. It is either fuel or ignition for sure. Thanks
 
I believe you're port tank has contamination that chokes of the fuel flow when the boat is up on plane and the contamination flow to the rear of the tank and blocks off the flow of fuel. I've had this happen once and was cured by adding "BioDiesel" fuel additive that will kill the algae contamination and remove the blockage. Give it a try it is cheap.
 
A partially Or completely clogged fuel tank vent can possibly be the cause. Next time it happens, remove the fuel cap and see if the issue goes away.
 
Backfire is typically an electrical issue. Most overlooked items are plug wires and the coil. Coils will be fine until they get warm, then they begin to breakdown. Also check distributor cap.
 
cfd95 said:
X2 backfire is Ignition.
Sounds like coil is failing when it heats up.
Click to expand...
I swapped coils between motors. Will be heading out today and will report back what I find. Have to admit the inside of the distributor cap on said engine was not pretty. Corroded terminals. Ordered 2 new ones with wires.
 
Just a follow up-I exchanged the ignition coils between the engines. Headed out yesterday and the "problem" switched to the other engine...score! Water was too rough to do on the open ocean so turned around and headed in. Than said engine decided to throw the serpentine belt and overheat....Not sure what is up with that. And, follow a new thread, but limped back on one engine. 3 hours later, went back out to deal with coil, etc and bilge was running every 30 seconds. Shaft seal leaking like crazy. Pulled it out and now have to deal with that. F$%^!
 
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