671 Detroit Sputtering

I would first start with the filters, make sure they are all clean. Second I would check for something in the tank that could be surging back and forth that might be clogging the fuel inlet line.

Ok this looks like it might be a by the book job.

Have you had the fuel system maintained?

When was the last time the fuel filters were replaced?

Have you checked/serviced the water trap/s?

Is there any evidence of adulterated fuel? I.E. Water in the fuel?

Are the fuel lines clear internally?

Are the strainers on the fuel tank pick up clean?

Is there any "algal" sludge in the fuel tank?

Are all the unions on the fuel line system properly tensioned?

Is it sucking air into the fuel lines?

When was the last time the injectors/injector pump was looked at/serviced?

I would look at or do the basic maintenance first before ripping the injectors and injector pump off. However it may well be a case of giving it some luvvin if it has been neglected. If you have had water in the fuel and it has gone through the injector pump, then you will have to take it off (with the injectors) and send it to the man to service it. As your operating "outside" it would be good insurance to catch up on all the maintenance of your motor.

Note: All it takes is to get one dud dose of fuel to upset the apple cart. Your making a living from you boat and if the motor ain't turnin, you won't be earning.

Also, check the oil level because I worked with a Detroit engine that had a low oil shut off and when the oil would get low the engine would surge until it finally stopped.

Last several posts said it all. As you probably know by now, fuel cleanliness is essential for a good running engine. Unless you have already done or had done, fuel system service is the first consideration in my opinion also, and should be a routine regularly with these engines. Until these basic maintenance items are attended to, it would not be appropriate to suggest any additional procedures.

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Filters first. After that try looking at the pick up in the fuel tank. Extra throttle = extra speed= lifting bow=uncovered fuel pick up. It would be worth a look.

Could the governors setting be low, and the engine be bouncing off of it?

Tapping a fuel pressure gauge into the port on the head should show weather the fuel delivery is sufficient. Good luck.

Thanks all for the insightful comments

The filters are changed regularly (about 30 hrs since last change)

I use a fuel treatment with every fill up and put in an algalcide for winter storage.

I will check all the other points (as soon as I find out where they are) Strainers on the fuel tank pickup?

Once I go through the excellent check lists provided I'll move to other suggested sources such as governor and low oil. I also like the idea of a fuel pressure guage.

I'll dip the tanks to check for water. Thanks for all the excellent input I'll post my progress.

just a shot in the dark.

try removing the exhaust at the manifold if poss, i know it may not be easy on a marinised engine or just slack off the exhaust at the manifold to see if the silencer is blocked

All the other posts are very good

Is this 671 blown? An injector or upper cylinder issue in all probability but the pump rack could be at fault as you say it was surging, pop the valve cover off, move the rack, make sure there is no bind.(the rack is a long metal tube that is connected to the fuel rod in the injectors). If it is stuck or binding, i would have someone who knows these engines fix and adjust it.

Many of these engines used a bridge on the exhaust valves. A bridge that has broken, or an exhaust valve spring that has broken might cause a build up of exhaust pressure, leading to an oxygen depleted environment in the cylinder. perhaps one of the exhaust valve springs. Pull the cover(s) off and let it idle, and watch for any non-moving parts.

Update #1

I started with the strainer; what a mess. lots of sludge and crap. Don't know when that filter was ever changed. Cleaned the housing, slapped in a new filter. Opened up the air filter; the main filter was clean (blew it out anyway) The prefilter (air) screening (layered mesh screens) I found were significantly filthy. Blew them out and re-installed the unit. She fired up with no issues, I slowly raised the RPMs to 2000+ and held it for about 10 minutes; not a single sputter; the sound she made was one I have not heard in a while (her old self) purring like a kitten. Will test drive tomorrow under load and change the main filter and the strainer filter again to see if there is any additional sludge buildup. All fittings were tight and secure. We'll see what tomorrow brings

Thanks a bunch to all for your valuable insights; I've added all the comments to my troubleshooting guide and slipped it into the maintenance log.

What was the nature of the sludge build up? Is it algae. there are companies that specialize on fuel polishing. they run your fuel threw a series of high quality filters to remove crap. If that crap was on the strainer, where else is it? Logic says it should also be in the fuel tank. (or is your strainer in the fuel tank) Either way, what ever got into the strainer had to come from the tank. Heavy doses of algaecide and frequent filters have helped me with a series of 92s. Good luck.

Thanks Bob;

I'm sure that the filter had never been changed (5yrs) I have a partner and he's responsible for maintenance and I look after administration and logistics; however, the last few years and, in particular this year, I'm finding that I am involved with the maintenance significantly more. The more I read and discuss the more I discover that good maintenance is not being done. All that said I am handling the maintenance now and won't have a partner at the end of the season. I am going to check the filters more regularly to see if there is consistent sludge buildup; if so I will get the tanks serviced. I have been researching building my own filter bank where I can isolate my tanks and manually control flow and filtering prior to operating.

That's redundant again.

Redundant filters are a consideration when having to shut off the sole power supply is not safe. Think about changing filters in rough seas. If money were not an issue, I would run a second pick up out of the tank. A second fuel pump and upgrade to higher capacity filters. After complete dual primary and secondary filters and pumps, join them back into one single feed to the engine with a check valve and a manual shut off in each line. That would allow turning on the unused line when the engine starts to run poorly. Then shut off the first line, and the engine sees an uninterrupted fuel supply. Good luck.

I failed to mention independent switches for the fuel pumps. Electric?

I have never seen, or used the product, but it has all of the features one would want on a fuel filter. I would only add that remember that using a filter rated for 50 GPM on an engine needing 25 GPM is much better than trying to use a 25GPM filter on a 50 GPM engine. What ever the manufacturer recommends for your engine, just go at least one size bigger. You can not have too much filter.

Right-o and I think the lowest flow they produce is 60 GPH, stop into a truck service dept and ask about them some time when you're puttering around a few of the fleets use them. Times when using other forms of fuel filtration the engine didn't sputter when the filter was loaded; they'd just slow down and then quit.

I should refrain from commenting about diesel repair because what little I know about diesels in centered around driving the vehicles.

Start with your filters (fuel) and then see if there is any improvement. The old 671 is a reliable old cow and the fuel is injected by unit mechanical injectors that are fairly dependable. Go with all the simple basics first!