Surges/boggs down randomly at speed?

[Knot Waiting]

1988 23' Crown Line Cuddy. Mercruiser 5.7 350. Alpha 1

Just recently my boat has developed an issue of bogging down or surging after planing out. The problem seems to start if more fuel is demanded (ie: planing out or speeding up) and once it starts will not stop until the engine is brought back to idle or shut down and re-started. It behaves as if its not getting fuel or if the ignition is being turned on and off. Whats stranger is that it doesn't do it every time. I have had a few days where the problem never surfaced and others where it plagued us every run.

I've ran a fresh tank of fuel through, added fuel dryer, and replaced the water separator all to no avail. The fuel lines appear to be in solid form and the tank vent is clear. I dont want to get stranded (obviously) so does any one have any ideas as to what might be causing this? Thanks

Note: Since the start of the season I have noticed a lower output from the alternator. Not to the point of dead batteries or anything but ony about 13.2v at speed which is slightly lower than prior years. Not sure is this could be related or not but I figured I'd mention it.

 

[Knot Waiting]

Re: Surges/boggs down randomly at speed?

Any one have an idea? I'm at a loss and wont stray far from shore until I resolve this. Thanks.

 

[Bondo]

Re: Surges/boggs down randomly at speed?

It behaves as if its not getting fuel or if the ignition is being turned on and off.

Ayuh,... Does it stumble as a lack of fuel, or just Die, 'n spring back to life,..??

It sounds like you've covered the fuel delivery,...
Was the filter full of water, 'n crud, or clean gas,..??

 

[Knot Waiting]

Re: Surges/boggs down randomly at speed?

Ayuh,... Does it stumble as a lack of fuel, or just Die, 'n spring back to life,..??

It sounds like you've covered the fuel delivery,...
Was the filter full of water, 'n crud, or clean gas,..??

The fuel separator was clean. No debris, or water (like phase separation or anything). Best I can describe it is a bogging down to the point of stalling. If I give it more throttle while the problem occurs it dies or stumbles worse. If I back down the problem resides.
I disconnected the fuel vent hose and ran it with no change in the problem. Anything under 2500 rpm or 25 MPH runs as normal. Beyond that it only takes a few minutes for the stall/surge to begin

 

[jersigsel22]

Re: Surges/boggs down randomly at speed?

I had this exact same problem last year. next to your drive oil reservoir there is a fuel vapor seperator tank. its small about the size of a pop can it has 3 metel fuel lines going to it, a little black rubber hose, a clear hose and an electrical plug of some sort. take the 6 screws off to remove the cover. inside is a float on the square side and screen/filter on the round side. I cleaned out the inside and cleaned the screen with a small makeup brush. slapped it back together and it never did what you described again. Mine was a 1995 5.7. Hope this solves your problem.

 

[Knot Waiting]

Re: Surges/boggs down randomly at speed?

I had this exact same problem last year. next to your drive oil reservoir there is a fuel vapor seperator tank. its small about the size of a pop can it has 3 metel fuel lines going to it, a little black rubber hose, a clear hose and an electrical plug of some sort. take the 6 screws off to remove the cover. inside is a float on the square side and screen/filter on the round side. I cleaned out the inside and cleaned the screen with a small makeup brush. slapped it back together and it never did what you described again. Mine was a 1995 5.7. Hope this solves your problem.

I tried to find the component you are refering to but couldnt. Perhaps it is something added to newer engines. My set up goes: Rubber hose from tank to pump/steel line from pump to separator/steel line from separator to carb/rubber sight hose from carb to pump. Sucks because I was hoping my problem was the same cause as yours. Any other ideas? Anyone?

 

[Knot Waiting]

Re: Surges/boggs down randomly at speed?

Could it be electrical? It behaves as if the key is being turned on and off rapidly. No misfire from the engine or anything just a smooth power down and then power up.

 

[winterfunguy]

Re: Surges/boggs down randomly at speed?

Pull the distributor cap and rotor and check the pickup (the part the rotor canes pass through) in the bottom of the distributor for rust, this can cause all kinds of ignition problems. If the pickup is rusty replace it.

 

[freedomintheskies]

Re: Surges/boggs down randomly at speed?

Did you verify the shift interupter is not gummed up or sticking? It will cause your ignition to cut out and do what you are experiencing.

 

[Beefer]

Re: Surges/boggs down randomly at speed?

You changed the FWS filter, but there are other filters that should be checked/changed. And don't forget the screen inside your pickup tube in the fuel cell, and also check to make sure the anti-siphon check valve is working properly (or replace it/ remove it). I had a very similar situation last year, and it was the stupid screen in the pickup.

 

[winterfunguy]

Re: Surges/boggs down randomly at speed?

Pull the distributor cap and rotor and check the pickup (the part the rotor canes pass through) in the bottom of the distributor for rust, this can cause all kinds of ignition problems. If the pickup is rusty replace it.

Your problem sounds like ignition to me, and I'm a marine mechanic. You have not told us the year of the boat yet. If you have points replace them and the condensor. If it is electronic ignition the pickup sounds like it is failing. You have replaced a lot of parts already so do a little diagnostic work first before just throwing more parts at it. Remove your fuel supply line from the carb and put a 2 liter bottle over the end of it and crank the engine for 30 seconds, you should have about a 1/4 at the bottle filled with fuel this an easy way to see if you are getting fuel. As long as the line is off the carb pull the brass nut (3/4" wrench) and check the filter screen behind it. Ill bet my ex-wife that your problem is ignition.