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Old 03-01-2015, 02:33 AM   #1
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Default Ongoing saga with fuel burn

Some may remember that I've posted about an ongoing problem with high fuel burn. I have a Floscan gauge, and when things are going OK I burn about 13 GPH on plane at ~3500 RPM. When things are not OK it can be as high as 20 GPH.

I believe the root cause is the Mercruiser Gen III Cool Fuel module, which is apparently painted on the inside and the paint ends up breaking away from the fuel passing through and clogs things up. Last season I had this problem and I eventually found the fuel pressure regulator was clogged so I replaced that, and things went back to normal.

So this season we took our first trip 3 weeks ago, and again the fuel burn was high at about 18 GPH. Today I removed and replaced the fuel filters in the Cool Fuel module, and the lower one was pretty clogged. Strangely, there was some sort of brown flaky debris in it, which is different than the black sludge and chips that I see from the paint. I've posted a picture of it. Has anyone seen anything like this, and maybe know where it comes from?
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File Type: jpg fuel filter with debris 1.jpg (197.0 KB, 6 views)
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Old 03-01-2015, 12:38 PM   #2
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If debrie in the fuel system is causing your problem you are not buring extra fuel but instead the regulator is sticking open and flowing a higher volume to fuel that gets returned, the flowscan is just measuring this higher flow.

I have heard of the paint issue but in most cases it starves the engine. As far as the brown flakes it sounds like sluge from the gas tank.

Does the boat seem low on power?
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Old 03-01-2015, 03:43 PM   #3
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Mike, there hasn't been a noticeable loss in power, although the first time this happened I had a hard time starting the engine. I agree the extra fuel probably isn't burning, it's just probably going out the exhaust as raw gas. The floscan does accurately reflect the fuel used, since I fill up the tank after every trip and the fill matches the floscan readout within a 1/2 gallon or so.

I wondered if the brown flakes could be from the tank. Is there some sort of filtering done at the fuel pickup? Or is this filter the first line of defence?
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Old 03-01-2015, 04:54 PM   #4
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I was thinking the fuel was either being returned to the tank or recirculated and not going into the engine.

There should be a screen on the tanks pick up tube.

When reading higher fuel flow how long does it occur?
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Old 03-01-2015, 06:10 PM   #5
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The high fuel rate is stable most of the time, although it does fluctuate. On this last trip, it was 18 GPH for most of the hour we were on the water, but it did jump to 20 briefly at times. Strangely on the trip back 3 days later, it was back down to about 14 GPH. But from past experience I know once I've seen the high rate, I know there's a blockage. I'm guessing the debris is loose and moves around.
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Old 03-01-2015, 09:51 PM   #6
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What rpm do you average as 14 GPH seems high for a 5.0 as I would expect it to be more like 10 GPH running at 3500 rpm on average.

Also gear and number of passengers come into play.

Your economy is more like a big blocks.
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Old 03-01-2015, 11:49 PM   #7
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When I get the best efficiency, I burn about 13 GPH at 3500 RPM running about 20 knots over smooth, flat water. I've never seen anything close to 10 GPH on plane. I've been told the 5.0 MPI is small for this size boat (27', 6800#) but I don't know if that's true.

I do know the Cool Fuel module paint problem has been giving me grief for the 5 years I've owned the boat. There is a thread from a few years back on this forum that I posted when I was first trying to troubleshoot the problem. And when I called Mercruiser they blew me off because the engine was out of warranty, so I'd have to cough up the $1000 to replace the part. I find that to be unethical on their part but I guess ethics and business don't always go together.
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Old 03-02-2015, 12:17 AM   #8
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IMO the 5.0 is marginal for this size boat once you add gear and more than four passengers.
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Old 03-02-2015, 02:41 PM   #9
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This is a well known issue.
Mercruiser Gen III Cool Fuel Module Service Bulletin attached
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File Type: pdf GenIII_CoolFuel_Module-SvcNotice.pdf (319.9 KB, 5 views)
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Old 03-02-2015, 04:17 PM   #10
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Shrew, that bulletin addresses a different known issue where corrosion was occurring in the raw water cooling connection that blocked water flow.

As far as I know, Mercruiser has yet to officially admit that the fuel passages were painted, although there are many reports of this on a number of forums.
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