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Old 07-03-2017, 06:30 AM   #1
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Default Loss of engine power; Mercruiser 5.0l MPI/Bravo 3

Hi all,
You folks have helped me out before so I'm hoping this learned group can do so again.

We took our Maxum 2600SE out on the SF Bay for two trips in May/June this year. The engine seemed to perform fine, with the exception of slightly elevated fuel burn (17 gph vs 13.5 gph on plane). This is a chronic problem on our boat, and I have fixed this in the past by replacing the fuel filter and cup and the lower fuel pressure regulator. A new fuel filter and cup did not correct the problem. I removed the FPR and saw that the screen was covered in black crud so I replaced it.

We launched again on July 1. Everything seemed normal at idle and displacement speeds (up to 8 knots). When I tried to go on plane, I could not get the boat RPM above 3000. I was not able to get on plane or go faster than ~13 knots. I ran the boat for about an hour trying to see if the engine performance changed. Gradually our speed dropped to 8 knots and the engine power and RPM seemed to fluctuate. I then brought the boat back to the launch ramp.

Today I checked several things. The FPR was removed to see if the o-rings had been compromised when I installed it, but they were fine. I checked the engine oil for signs of water but it looks fine. I checked the upper FPR and it looked fine. I checked the distributor cap and rotor and they seem fine. The air filter looked a little dirty but doesn't seem to be bad enough to cause this problem.

Any obvious things that I haven't checked? The boat is stored on a trailer, so there is no marine growth etc on hull, sterndrive. Thanks in advance for any wisdom you can offer!

Tom
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Old 07-03-2017, 12:04 PM   #2
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I would check the fuel pressure as the pump may be dieing, however this is counter to increase fuel use. I would look into having the engine scanned for fault codes.
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Old 07-04-2017, 09:33 PM   #3
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Thanks Mike, I agree there might be codes and the fuel pump is a possibility. Something seems to have failed between our last trip 2 weeks ago where we had a high fuel burn and this one. This time the burn was 12 GPH but I don't really know if that's normal since I don't normally cruise at displacement speeds. Maybe 12 GPH was all that the pump could produce??
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Old 07-04-2017, 09:36 PM   #4
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I would expect around 10 gph running at 3200 rpm. At 1500 rpm it should be a lot better say 4 gph. Could be leaky injectors, what do the spark plugs look like?
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Old 07-04-2017, 09:49 PM   #5
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Another thing could be a bad temp sensor having the engine think it always cold and keeping the mixture rich which intern is fowling the plugs.

All these sensors is why a scan is required.
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Old 07-05-2017, 06:05 AM   #6
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I haven't checked the plugs or injectors. At this point the game plan is to tow it to the mechanic tomorrow and get him to scan the engine. I'm hoping there will be a code.

You mentioned the temp sensor, which made me think of something. Two years ago I had problems overheating, which turned out to be a clogged exhaust manifold. In the process of diagnosing this, the mechanic replaced the thermostat with one that has a different set point. Instead of 175 degrees, it tries to maintain about 150 degrees. So the engine is running cooler. That hasn't seemed to make a difference in performance, but I wonder if it has created this problem? It's been 2 years though.....
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Old 07-05-2017, 11:49 AM   #7
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For a raw water cooled engine 150 is the correct t-stat, I don't think this is contributing to the issue. There are several temp sensors on the EFI, one for the gauge, one for the alarm and another that the computer uses.
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Old 07-09-2017, 04:57 PM   #8
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How many people in the boat? What pitch prop?
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Old 07-10-2017, 10:29 PM   #9
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tnrivrunr: 3 people on board, as always. Not sure of the prop pitch but it's been the same since I bought it 8 years ago.
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Old 08-24-2017, 11:32 PM   #10
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OK so I've taken this boat to a mechanic and he's done some diagnostics, but there is still no resolution. There were no fault codes and the engine telemetry looks normal (including under load). He's leaning towards a cylinder leak problem. He did a compression test and all cylinders were OK but 2 and 4, which tested at 130 psi, but after he sprayed in some oil they tested normal. He wants to do a leak down test, but I'm not sure he's on the right track since the engine SOUNDS fine at low speeds and even under heavy load, it just can't deliver enough power. Also the engine has been running fine up until two weeks before the July 1 launch. It's hard to believe a cylinder problem would manifest suddenly.

Another mechanic told me that with cars, typically two things can cause the engine to not get full power: a fuel delivery problem, and a blockage of the exhaust system.

The fuel pressure seems to be normal, and as I mentioned I replaced a component that was clogged. But I suppose there could be another problem that isn't detected by the engine computer.

I don't really understand how exhaust gasses leave the engine. I believe they leave via the stern drive, but I don't know how they get there. Are they routed to the exhaust manifold that is used for raw water exhaust? Or is there a separate set of manifolds/pipes? Is there something that could get blocked and should be checked?
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