engine knock under load

[greentahoe]

I have a 1984 OMC 3.8 v-6. It gets used very little. Last year it went in the water twice. I had to change the distributor on it last year, and I very carefully set the points and timing to spec, after which it ran very well on the trailer, but I didn't have a chance to try it on the water. This Spring I've run it a few times on the trailer and it's run pretty well. It always starts up quickly and runs very smoothly. The engine has a total of about 100 hrs. on it since a total rebuild. This week I took it to the shore after adding 13 gallons of gas to that already in the tank, which was probably about 50 gallons. At the launch ramp it started fine and idled quietly and very smoothly. We headed out and I barely got it up to plane when the engine started losing power. I added more throttle, up to just over 3,000 rpm, but the engine slowed down even more. It slowed to a max. of about 2500 rpm and after about a minute or so developed some knocking noises. I checked for overheating and water pump operation, both were good. I checked the oil level, which was a little low but still enough there. Couldn't determine the exact location of the noise. The engine ran fine at low rpms, under 1300 or so, but for the rest of the day couldn't plane or if I tried anything over 2000 rpms made lots of noise. Again, no knocking at idle or low rpms. Just for information, all of the plugs, wires, distributor cap, rotor, points, condenser, and coil are new, less than an hour usage and all of that on the trailer. Also, the water separating fuel filter and the carburetor filter are new, less than an hour usage. <br />I checked the timing today and it's set at 10 degrees BTDC at 550 rpm. The oil pump is putting out 50 lbs at idle. The noise sounds like very advanced timing, but that apparently is not the case. I've checked my repair manuals and I'm growing suspiscious of my fuel, some of which has been in the tank for two years, some from last year, and some new this week. It doesn't smell sour, but could it have too low an octane and cause this problem? The manual lists low octane as a cause for this type of noise. <br />If low octane is the problem, will adding an octane booster help the situation or, and I really dread this, is it necessary to drain the tank of it's approximately 70 gallons of fuel and start fresh? <br />Thanks for the input.

 

[lilmandavis]

Re: engine knock under load

fresh is the answer i think. i dont know if id eat a bait/sushi joint?

 

[Gary H NC]

Re: engine knock under load

I'm with lilman about the fresh fuel.I would pull the plugs out and see how they are burning too. <br />Bad gas can cause a knocking in the engine..<br />I hope it's not a bearing knock..

 

[cobra 3.0]

Re: engine knock under load

That's why super or premium fuel is recommended for inboards. It will lose octane over time, but still have enough octane rating to run your engine after sitting for long periods of time.<br /><br />In all likliehood you will need to get rid of your fuel. That's quite a bit of gas to try and burn slowly in your car or truck a gallon or two at a time with fresh gas.<br /><br />You may want to try ovctane booster and give it a shot. Heck you have little to lose. Add the required amount of octane booster and give it a try.<br /><br />If you are timing your engine with fuel which has too low an octane, you'll need to retime again for the actual octane of the gas. A temporary trick I have tried with success is to time the engine by "ear" as it's running on the water. Get someone to drive and you adjust the timing slowly one way or the other until it sounds and feels smooth and powerful.

 

[bjcsc]

Re: engine knock under load

Pull your engine and change your distributor cap. Worked for me! <br /><br />Seriously though, you may want to check out my thread.

 

[greentahoe]

Re: engine knock under load

Thanks for the ideas everyone. bjcsc I did read your tale of woe the other day, made me almost cry because I've been there. There's the story about the list of experts who suggested everything they could think of when the engine would run for a couple of minutes then make a sucking sound and die. It took a while to discover that it was a bad condenser. Then there's the time the engine didn't have enough power to drive the boat but it would rev up to 2,000 plus out of gear. Took a while to find a pinhole in the bottom of the coil where the oil had leaked out. That was a $400. tow job.<br />This time I disconnected my fuel intake line and ran the engine this morning out of a can of fresh gas. As the engine ran for a while I heard a distinct knock even at low rpms. After studying the situation I decided that a check of the main bearings was warranted, so the engine is now ready to be plucked out as soon as I can find a suitable chainfall to lift it. I'm disappointed that it wasn't the gas, but I'm sure now that that's not the case. On the bright side of things at least I'll be able to clean out the bilge without the engine being in the way.<br />Thanks for all the ideas,<br />Jack

 

[bjcsc]

Re: engine knock under load

Just wanted to make sure you checked everything I checked and didn't pull it unless you had to. I super cleaned my bilge once I pulled my engine, too. I really have learned a lot about my engine and its systems. Strangely enough, I'm glad I did it. I'll be less likely to be held hostage by the slackass marine mechanics around here. Make sure you have a helper (preferably with only 1 "X" chromosome) when you pull it and go slow...

 

[KaGee]

Re: engine knock under load

RE: Gas...Keep burning and add fresh everytime out. Get you a qrt of a product called Pri-G and add the appropriate amount to your tank. That 3.8 will burn it. <br /><br />A friend of ours runs a radiator/A-C/gas tank repair businees for nearly 30 years. All ASE Certified. The only Gas they recycle is contaminated. All else gets reused.

 

[Almeja]

Re: engine knock under load

>" I added more throttle, up to just over 3,000 rpm, but the engine slowed down even more... after about a minute or so developed some knocking noises"<br /><br />Losing that much power is a worry... If bad fuel is the cause, then it's really turned to complete crap. <br /><br />With as long as the boat sat idle, you may also have varnish in the carb, partially plugged high speed jets, water contaminated fuel, a plugged fuel filter (if present) or perhaps another issue.<br /><br />My 2 cts.<br /><br />Drain the current fuel into jerry cans. Put in 10 gallons of fresh high-octane fuel with an appropriate amount of quality carb cleaner.<br /><br />See how the boat runs. <br /><br />Radical improvement? Give the old fuel to the hippie down the road. No change? Other issues are present.<br /><br />My lesson gleaned school of hard knocks...<br />Bad Fuel + Good Fuel = Bad Fuel

 

[greentahoe]

Re: engine knock under load

OK, here's another thought. My brother-in-law is a pretty good mechanic. Last night he told me he heard an engine with a fairly loud knock that sounded like a bearing, just like mine. Turned out it was the mechanical fuel pump. I'm thinking that if it was the fuel pump that might also explain the loss of power. I've had a bad bearing before that didn't cause the power loss. Any comments? I'm going to pull the fuel pump before I remove any covers.<br />Thanks

 

[Boatist]

Re: engine knock under load

I would set timing to 4 degrees BTDC and see what happens.

 

[greentahoe]

Re: engine knock under load

The pump was faulty. I took it out yesterday and found some play in the arm. When held in one had and hit with the other it made the exact noise I'd been hearing with the engine running. I've never been so glad to have a bad fuel pump!<br />Thanks for the help.