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Old 04-04-2020, 08:54 AM   #1
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DD 6-71 issue

I have Covington DD 6-71 ti's on our Chris Craft. On our last cruise last fall I noticed some unburned fuel in the water from the exhaust of the starboard engine while at the dock after a 6 hour run. Didn't do anything to investigate at the time as we were preparing the boat for winter in the water in Scottsboro Alabama. Winterizing consisted of plugging in the block heaters to keep the ER warm while we were away.

We returned to the boat for a week in January for some projects and fired up the engines one day just to circulate the oil a bit and check them. The port fired as normal but the starboard cranked more than normal and then surged quite a bit before smoothing out. Still had the fuel in the water. All other fluid levels are normal. It's been about 500 hours since the engines were last "tuned up".

I plan to get someone to look at the issue as soon as it's practical given the current situation with lock-downs going on. In the meantime I'd like to get some thoughts on the problem.

My theory is a bad injector that's just leaking fuel and not misting it into a cylinder. I assume this would cause the cylinder to not fire and just dump fuel during the exhaust cycle. Does this sound right? Other possibilities??

Also, is it an issue to run the engine while this is going on? If I can't get someone to look into it where we are I'm thinking of running a couple of days down to Aqua Yacht and have it serviced there. It's about the closest full service marina to where we are in Goose Pond. Haven't tried Diesel Don yet so don't know what his work status currently is.

Thanks in advance!
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Old 04-04-2020, 09:37 AM   #2
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Your fuel cooler is bad. Bypass the fuel lines going into the cooler and see if it clears up. Leaking injector lines would just add to your oil.
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Old 04-04-2020, 01:23 PM   #3
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Thanks for the quick reply. This makes sense and is an easy test before calling in help. I can easily handle the replacement if this ends up being the cure. I'm guessing I don't want to run the engine then. Even though the fuel pressure should be higher I'd imagine there is a chance it could back-flow into the fuel tank also?

On the injector question I was thinking of fuel coming from the tip not the lines. I'm not making any oil so that doesn't seem to be the issue.
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Old 04-04-2020, 01:37 PM   #4
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You can run the engine to test. It would take a while for the fuel to get to hot. Not firing on 1 cylinder would be noticeable. The fuel pressure going to the injectors is 155 PSI, more than enough to enter through a pin hole in the cooler. Good Luck
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Old 06-01-2020, 09:20 AM   #5
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While this thread didn't gather much traction I thought I'd report back on the outcome for anyone doing a future search.

It turns out it was indeed the injectors causing the unburned fuel in the exhaust water. We pulled the fuel/transmission cooler and sent it out for testing which was fine. Injectors were mostly original and all were sent out for rebuild including new tips (both engines). We finally got out for a test run yesterday and the issue of unburned fuel is cleared up.

I was lucky to find that we are docked right behind Diesel Don's boat and he was able to come onboard and do the work in under a week. For those that don't know him he's well respected here in the southeast and hard to get at times, I was very fortunate to have him do the work.
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Old 06-01-2020, 10:02 AM   #6
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Thanks for the follow up. It adds greatly to the knowledge base.
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Old 06-01-2020, 11:53 AM   #7
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I'd be really interested to hear how this occurs from a technical point of view.

Were some of the injectors just leaking continuously while the engine was running?

If that was occurring, I could see the leaking fuel being blown right out into the exhaust by the incoming charge air from the blower while the intake port is exposed and the exhaust valves are open.
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Old 06-01-2020, 12:56 PM   #8
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Most likely cause of the unburnt fuel was worn injector tip holes. When the holes get bigger, proper atomizing of the fuel isn't happening. Spray drops are too big and don't fully combust. In the bottom of the injector is a spring loaded valve that is closed unless fuel is being sprayed.
The holes get worn because of heat, flow, and small debris in the fuel. The injector tips are cooled by the constant flow of fuel thru the injector. Depending on the model engine about 30 to 60 gallons an hour pass thru and return to the tank. If the flow gets restricted, tips can burn quickly.

You'll probably see better mileage with rebuilt injectors. The spray pattern should be uniform around the piston top with perfect atomizing. Depends on how many bad injectors there were.
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Old 06-01-2020, 12:57 PM   #9
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The way Don explained it to me is that over time the tips on the injectors wear and the holes get a bit larger. This allows the fuel to exit more as a drop than a mist and it does not fire completely.

I'm sure some of the diesel gurus here can explain to better.....

(There you go, it was being answered as I typed my reply!)
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