Engine bogs under full load

[BIGALF]

My 5.7 LX EFI Mercruiser engine is skiping/stumples/hesitates at full throttle after it warms up for awhile. It sounds like it actually stops for a second and starts again. The fuel pump and regulator and water seperator have just been replaced and the fuel injected throttle body nozzles have a good spray. Fuel supply and pressure lines are all to spec. I think there might be a short somewhere that shorts after the engine has been running for a while.
I have a EST ignition and I am going to replace the ignation module and pick up coil and a new distribution cap.
Any other ideas on what the cause of the problem might be. I'm running out of things to change.

 

[havasuboatman]

Re: Engine bogs under full load

Have you checked to make sure the main engine wiring harness plug is seated and firmly connected? I had a boat with similar symptoms, and that was the fix. I pluged it all the way in and placed a hoseclamp around the plug and GENTLY snugged it up.

 

[BIGALF]

Re: Engine bogs under full load

I will do that tommorrow. I know something is loose or shorting out. I just can't figure which one it doing it.

 

[AZSenza]

Re: Engine bogs under full load

Mine acted the same way when I got it 97 Merc ECI 5.7, cap rotor and plugs fixed it right up! WOT with no bog.

 

[BIGALF]

Re: Engine bogs under full load

Thanks, That is my next move. I think the problem is electrical not mechanical.

 

[BIGALF]

Re: Engine bogs under full load

I'm really confused now. I did check and replace the distributer cap and replaced all the rest of the stuff inside.
I took the boat out today and it ran fine for the first hour at full load. However, after 1 1/2 hours of operation it started to heat up past 175 degrees but under 200 degrees and started to buck (lose power then pick it up again) I cruised back to the ramp going 1000 rpm and it was ok. I stopped at the ramp and put it in neutral and pushed the throttle down and it worked good. It seems that load definitely has something to do with the problem.
Any ideas?

 

[CharlieB]

Re: Engine bogs under full load

You haven't mentioned spark plugs, how old are they, what do they look like?

When plugs begin to fail it is only noticed at heavy loads at high speeds, they will run fine under 3/4 throttle and rev perfectly in neutral.

 

[BIGALF]

Re: Engine bogs under full load

The engine is a rebuilt Mercruiser engine manufactured by Mercruiser and installed 1/07 the plugs should be good. I am now thinking the fuel from the tank to the fuel pump might be the problem. I need to find out if there is anyother fuel filters other then the fuel filter. I want to know if there is a screen of some kind in the tank that mnight be accumulating stuff the more I run.

 

[BIGALF]

Re: Engine bogs under full load

Just fixed the problem. It was the fuel line inside the fuel tank. There was a small screen filter that was clogged. I don't know why there was even a screen inside the tank. My water sererator is good enough . Put in a new line and the boat runs fine.
Check you boat by putting another fuel tank in line instead of your main tank. This could save a lot of money instead of changing fuel pumps, nozzels, etc.

 

[nrf414]

Re: Engine bogs under full load

Big Alf,

I was having some bogging problems. I did the plugs and adjusted the carbs, still couldn't get it to run right. Well then I decided to check the fuel lines / filter and change the Fuel Water Separator.

Check out this pic of clogging! I was surprised the engine even ran at this rate... NASTY!

 

[AZSenza]

Re: Engine bogs under full load

GOOD FIND! Glad you found it, FUNNY thing though, I used to build/run Mustang GTs, my 82 would do the same thing, after about 20-30 miles it had zero power but would idle fine, I changed EVERYTHING...Finally I dropped the tank, the screen was clogged, a few years later with an 84 GT, when the same thing happened one night at the drag strip, I pulled the tank FIRST, same issue-clogged screen. I took them out and ran an inline filter pre pump as well as post.