6v92 Guru help needed

[Bob Bradley]

Last week while transporting my boat from New Haven CT to Westerly RI, I noticed a fairly sudden loss of about 200 rpm on my stbd engine. This was about 2-1/2 hours into a 3-1/2 hour trip. I had been running at 2050, and had bumped it up to 2150 as I was fighting the tide and was trying to pick up some time and beat sunset. She ran at 2150 fine for about 20 – 30 minutes before the stbd engine dropped to around 1950.

Both engines had fresh primary and secondary fuel filters (<10 hrs) and fresh oil changes. These are 6v92’s making 435hp. The engine was acting as though it was starving for fuel.

Today I checked both filters and found no water or sludge, but changed the secondary anyway, but it had no effect. I still cannot get the stbd engine up past 2000 or so. They both normally make 2300 – 2350 under a load without any problem. They usually both make 2500 – 2520 or so with no load. The offending engine today would only make 2480 no load – not much of a difference, but a difference nonetheless.

I’m pretty much at a loss on where to start looking. Any suggestions would be appreciated. Thanks.
Bob

BTW, the engine is not smoking or showing any symptoms other than the rpm loss.

[Genesis]

Check your throttle linkage first and make sure that lever is going all the way to the stop.

With the no-load shortage its unlikely a fuel restriction is responsible but you can't rule it out without checking both vacuum on the suction side and pressure on the rail.

There are a host of possible causes but start with the simple one - throttle lever. If no joy check fuel pressure and vacuum levels and we'll go from there.

[captddis]

You don't say if you lost speed or not? If no speed loss then I would say tach.

[REBrueckner]

"The engine was acting as though it was starving for fuel.."

Can you explain what symptoms you observed.

Absent anything else, another simple and inexpensive action is to change the primary fuel filter again. Especially if was a choppy trip, muck may have been stirred up.

[Bob Bradley]

I'll check the throttle linkage tomorrow. Last fall I did have to remove the shutdown solenoid to get at the water pump when I replaced it. It's possible that I may have changed the throttle setting as well without realizing it. That said, this did come on farily suddenly (over a 5 min period) while running, 2 hours into a trip.

I do believe that the engine was not making top rpm to begin a week or two previously. I blamed that on possible running gear fouling and had the bottom cleaned last week. My diver told me that the gear and hull were pretty clean to begin with, so the problem may have already been there before this trip, and just got worse.

There was a corresponding loss of speed, so the engine rpm did actually drop. I checked the Racor and secondary for water and/of sludge and found none. Just to be sure, I replaced the secondary filter yesterday and that had no effect.

The reason I describe the problem as "acting as though it was starving for fuel.." is that I encountered a similar problem a few years ago when my secondary filter had become clogged. This is not the case this time.

[sgharford]

Similar problem, differnet engines, last year on my port engine coming back down LI sound. Out of desperation of limited things I could do with passangers on board 2 hours away at 17 knot cruise (I remeber the lump in my throat like it was yesterday) I switched fuel manifold so everything coming out of and into starboard tank - Bingo! problem solved. Funny thing was I checked connections day later on all port plumbing and everything tight, so I switched manifold back and everything OK since then.

[hibanx]

Have you checked the prop? Maybe you picked up a small line, or hit a small object and bent a blade? Maybe a clogged fuel pickup in the tank, try like the previous post suggested and switch the manifold to draw and return to the tank that the "good " engine is using.

[kwerges]

Have you checked the prop? Maybe you picked up a small line, or hit a small object and bent a blade? Maybe a clogged fuel pickup in the tank, try like the previous post suggested and switch the manifold to draw and return to the tank that the "good " engine is using.

X2 on checking the fuel pick-up (switch tanks at the manifold)...

[Bob Bradley]

I actually did switch the fuel lines at the engines, but it didn't change the no load rpm. I'll try it tomorrow and run it a bit to see if it has any effect.

[krush]

Check your throttle linkage first and make sure that lever is going all the way to the stop.


Agree, check the simple things first. I've had to do this after an "expert" overlooked the obvious.

[Dan Mapes]

Also make sure the shut down lever return spring is solvent and that the lever is on it's rest. Been there dun dat on a 6-71.

[Bob Bradley]

Also make sure the shut down lever return spring is solvent and that the lever is on it's rest. Been there dun dat on a 6-71.

Dan, please tell me more. I did have to futz with the shutdown solenoid recently. I had removed its mounting plate last fall to swap out the circulating pump, and when I first re-mounted it, the engine would not shut off. I had to put a wedge under the mounting plate to make the plunger rod line up correctly, as it was binding.

Is the shutdown system an "on-off" system, or is it gradual, requiring adjustment. If it does require adjustment, might it limit my top end rpm if not adjusted correctly?

Today, I checked the basics as best I could. The throttle linkage does not seem to be a problem, and remember, this symptom came on gradually over a 10 minute period after running for 2 hours or so. I also bypassed the Floscan system entirely to eliminate that variable. Still no love. Port runs up to 2300 ok, but stbd only hits 2030 at the most under a load. Stbd did hit 2500 no load ok today. I also switched the manifold to run the stbd engine off the port tank - no effect there either.

I hate to start digging into injectors, but I'm running out of ideas. Any other suggestions?

[yachtsmanbill]

This is in the run position on a 12-71N... ws

http://i53.tinypic.com/14tx2mf.jpg

http://www.youtube.com/watch?v=OojMdgb0HDg

Solenoid is ON/OFF but you can slow down by hand

http://www.youtube.com/watch?v=UNFGwPgbEOE&NR=1

steaming at about 1500 turns

[Genesis]

The stop lever has only two legitimate positions - fully pulled to "stop" and fully not (up against the "run" stop.)

If it is off the "run" stop by even a fraction of an inch it will prevent the engine from reaching proper RPM. If you've recently worked on that check and make sure it is FULLY in the "run" position when the solenoid is not energized.

[Passages]

I had the same problem you describe. I also followed the same sequence you are - eg- Simple Stuff First.

Sorry to tell you this Bob - It was a bad injector. $95 part and a couple hours labor, all was right again.

[krush]

Funny, I experienced the same problem with the engine stop not fully going to the run position on the same boat that I helped a buddy out on. Previous experts had overlooked it as well.

[Bob Bradley]

Thanks, Guys. I'll check out the stop solenoid linkage before I go any further down the injector path.

[Glory]

air intake?

JM

[Bob Bradley]

Air intake is clear. Just been too busy to get to Rhode Island this week to check on the stop lever. Should be able to do so tomorrow. I have a good feeling that will be the problem.

[Seabrooke]

Is there any manual adjustment for an speed governer? This may be an unlikely but possible cause.