3.8 omc bogging

[kwebster1]

Alright guys, I'm finally at a loss. or at least I think I am. I have a 1984 omc sea sprite with the 3.8 omc 800 stinger. It idles fine, and it will rev up fine, but when I back off the throttle it goes into a weird bogging almost dieing state and just chugs like poo. I've check the fuel lines for debris, found none. I have checked and replaced the ignition coil, distributor cap, rotor, point, and condensor or conductor? I cant remember what the little silver tube in there is called. pretty sure it starts when a c though. I have gapped my points to .018 like spec calls for and it still runs the same. I do know that the dwell is off. It says between 37 and 41 degrees and I am floating around 34 to 35 degrees. I have tried to close the points gap to .016 to increase the dwell to the 37 degrees and it stayed just the same at 34. Im not sure if that is whats causing the problem but I dont think so. to me it still seems like a fuel issue. The fuel water separator has been replaced as well as the fuel filter to the carb. The only thing I have not touched is the fuel pump. The guy before me said he rebuilt it. However I do not know his skills so it could still be jacked up. Anyone have any thoughts before I throw in the towel and take it somewhere?

 

[Howard Sterndrive]

Re: 3.8 omc bogging

might want to watch the ESA switch on the shift converter while someone else drives. The way you describe a connection to the throttle on decel might be the cables sticking or misadjusted causing the ESA to activate.
Good to just watch everything in the engine compartment while it does it anyways.

can just temporarily unplug the ESA module and see if that cures it

also unplug the tach...never hurts to eliminate the tachometer

Also kinda sounds like failing exhaust risers or manifolds gaskets etc... might want to pull spark plugs when it does it and look for evidence of water in the combustion chambers

 

[kmarine]

Re: 3.8 omc bogging

I have had simular bogging from both fuel pump issues and water in fuel. First you should get about 8 to12 psi on a fuel gauge installed between fuel pump and carb. Do you have a water seperatin filter? Dump it into a coffe can and look for water in it. I have had occasions where the filter will allow enough fuel for low rpm, but not enough for higher speeds.

 

[Howard Sterndrive]

Re: 3.8 omc bogging

12 psi sounds high to me - I thought 6-9 was proper pressure feeding a Rochester carb

but that is always a good test

 

[kwebster1]

Re: 3.8 omc bogging

I was able to talk to a boat repair place that I didn't know about around here who was actually helpful, unlike some of the other boat places I call that just say "well you're the mechanic, you tell me." I hate that. Anyhow, if the ESA swith you are referring to is the little black switch that (correct me if im wrong) does something to the sort of misfires the engine a little while you shift into gear for easier well...shifting into gear, then that is the same thing this guy told me to check out. I did notice that prior to all of this, shifting into reverse was insane and kicked you around like crazy. The guy before me adjusted the shift linkage so that it went into drive better or something but never adjusted the little plastic pieces that push on that switch to make the engine run low while shifting. So I have adjusted the cable back to where it normally is...i think he had it like 4-8 notches to the forward gear. Ive got it around 2 in forward and then moved the plastic piece down to where it lightly presses on that switch to get into reverse. I may need to adjust it back a little further if that doesn't hit it hard enough. I couldn't get the boat to act up in the driveway so far today to unplug the ESA and see if that is causing the problem. I did unplug it however just to see how it all came apart before trying to unplug it while the engine was running bad. Maybe that movement cleaned something up in there and now its functioning but im going to try to get it to the lake later and put it under a load and see what she does and unplug it if necessary.

On to the other replies, I would love to check to see the fuel pressure going to the carb but with a metal fuel line (or whatever it is) I do not have the tools to do a proper repair and checking it with a gauge myself would lead to needing a new fuel line. At that point i'd soon just rather replace the fuel pump and see if it fixes it. I don't mind fixing things that could potentially go out vs replacing a fuel line that should shouldn't necessarily have to replace.

So thats the latest update. I will let you know what happens tonight or tomorrow whats going on. I would love it to be that stupid, however useful, little switch.

 

[Howard Sterndrive]

Re: 3.8 omc bogging

but with a metal fuel line (or whatever it is) I do not have the tools to do a proper repair and checking it with a gauge myself would lead to needing a new fuel line. At that point i'd soon just rather replace the fuel pump and see if it fixes it. I don't mind fixing things that could potentially go out vs replacing a fuel line that should shouldn't necessarily have to replace.

but checking fuel supply pressure diagnoses more than just a fuel pump - tank issues, anti-siphon, all that can be ruled out when you see proper fuel pressure... even fuel pump drive cam lobe wear can be eliminated...
you can just remove the steel line, insert your flexible hose with gauge and replace the steel line after testing.

probably even a way to tee in a test fitting into the NPTF fitting at the fuel filter outlet by just bending the existing steel line.

 

[kwebster1]

Re: 3.8 omc bogging

This is true, if the switch isnt the problem perhaps that will be the next line of attack

 

[kwebster1]

Re: 3.8 omc bogging

Alright here is an update. When I was looking at the shift assembly by the engine, I noticed that the guy before me had set the forward reverse gear setting more into the forward side. From what I can understand, the module switch on the f/r gear is almost like a bypass for the ESA module. That switch known as a overstroke switch I believe, gets pressed in when you are fully engaged in forward or reverse by little plastic pieces at the far end of each gear. When that is engaged it actually bypasses the ESA module making it ineffective if tripped by the cam (looks like a small horseshoe) surrounding the ESA module. When the overstroke switch is not pressed in by being in gear the ESA module is effective when tripped by the cam which forces certain cylinders to misfire making it easier for the boat to get into gear. That said, I adjusted the f/r gear back toward neutral and adjusted the plastic overstroke pieces that push it in. Since I have done this I couldn't get it to run like poop like it was. However between my buddy and my dog we maybe only had 300 lbs on the boat in the water so the load may not have been enough to get it to run like it was under a load. I plan on going back out tonight with more friends to get extra weight on it and see if it continues to run ok.

 

[kmarine]

Re: 3.8 omc bogging

The esa switch has always been an issue with omc. I find the boat must be in the water to properly adjust.
My expierience is about three rounds of adjustment before proper allingment in both foreward and reverse. be patient, many profeshional marine mechanics have been driven crazy with omc esa adjustments. good luck!

 

[Howard Sterndrive]

Re: 3.8 omc bogging

many profeshional marine mechanics have been driven crazy with omc esa adjustments.

and it's not usually too far of a drive