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Old 08-25-2017, 12:17 AM   #11
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What is the compression of the other cylinders?

130 is low but not too low for the power loss you mention.

Being that cylinders 2 and 4 are low my guess is the head gasket is leaking between them but adding oil into the cylinder and the compression getting better suggest valve or ring issue. May be getting some water into these cylinders.

Still think it's a fuel system issue. Did you actually measure the fuel pressure at the rail?

The exhaust system is as follows: exhaust exits the exhaust ports on the cylinder heads into e exhaust manifolds then into the riser where the raw water is mixed to cool the exhaust gases. It then enters into the y-pipe and through the transom assembly into the exhaust bellows then into the outdrive and exits thru the prop hub. If a blockage occurs it's typically a broken off exhaust shutter that is mounted at the top of the y-pipe. You need to remove the riser to see if this is the issue.
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Old 08-25-2017, 02:17 AM   #12
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Mike,

He told me the other cylinders were at 180 psi. The auto mechanic said that the rings or valves could develop a leak if the engine overheated (which it did but 2 years ago) or if it was running rich and causing carbon buildup. I have had the engine running rich, or at least, the higher fuel flow rate I experienced suggests that. He said there are ways to clear carbon buildup (spraying cold water or a product called "seafoam" into the intake) and that should be done before major engine work is attempted.

But, I agree with you I'm leaning more to a fuel issue. The auto mechanic suggested pulling a fuel line and seeing how fast the fuel fills a mason jar. Since I have a Floscan I would think this tells me the same thing, maybe more accurately. If I'm remembering correctly I was getting about 12 GPH that day when the RPM stuck at 3000. Normally when I'm going up on plane it flows a lot faster, like 18-20 GPH, while it's climbing out of the hole, but of course the RPM is higher, more like 4000.

OK so if the exhaust gasses merge with the raw cooling water in the exhaust manifold, then wouldn't a blockage downstream, like in the Y pipe, cause the boat to overheat? It showed no signs of doing that.

BTW, I'm pretty sure the marine mechanic measure the fuel pressure at the rail and it was fine. What would you expect if this were the problem? Too low or too high?

Tom
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Old 08-25-2017, 02:40 AM   #13
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180 is rather high, 150 is more normal and all cylinders should be within 10% of each other.

Never heard overheating causing rings or valves to leak. It can warp heads or block throwing tolerances out. It can also make head gaskets leak.

Sea foam will work and can be added to the fuel tank. Using water can be dangerous as if not done just right you can damage valves. Burning water increases exhaust temps which can melt the exhaust valve and or seats. Old school mechanics also used ATF. Lead free gas has less chance of carbon build up.

Measuring fuel flow rate is not the same as pressure. You'll need to look up the rail pressure in the mercruiser manual, look under Documents to find it.

There is a regulator at the end of the rail that manages pressure and returns fuel to the tank. If not keeping the pressure high enough it will just allow fuel to flow right back to the tank.

Yes a blockage in the y-pipe typically results in high operating temps.
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Old 08-25-2017, 03:15 AM   #14
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OK so it looks like the pressure at the fuel rail has to be between 34 and 38 psi. I'll ask the mechanic what he measured. Sounds like you're saying the FPR up there could be faulty, and if it can't maintain the pressure high enough the fuel will return to the tank. So I'd be looking for a pressure less than 34 psi.

Any chance the injectors could be a problem? Clogged either partially or fully?
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Old 08-25-2017, 12:29 PM   #15
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Dirty injectors usually start showing signs as a rough running engine and not loss of rpm.

Now that you found the engine manual there is a troubleshooting guide in it you may want to look at.
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Old 09-09-2017, 07:12 PM   #16
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Hello.My name is Don. I'm new to this site and didn't mean to jump in on your guys post. I'm having simular issues that I hope you guys might be able to help with. I bought myself a project boat and have an issue that maybe someone can help me with? It's a 5.0L mpi serial number 0W616784, we thought that it was electrical and may still is, not sure. Have done a complete tune up new cap, rotor, and wires. Runs great at idle and fuel pressure is 40psi at test port, but start to die off at full throttle, pull back to idle and doesn't stall its very consistent, when gauge starts to bounce at test port is when the issue shows. We are going to try alternative fuel delivery option today to see if it's possible that fuel pump might be falling off at high throttle, also have two injectors that when we pull power source off make no change to engine, they all test correct power as test. I hope someone out there can point me into right direction. Compression is 160 ish across all very even.
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Old 09-09-2017, 08:53 PM   #17
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Hi Don,
Believe it or not I'm still trying to diagnose this problem. In my case I've had the fuel pressure gauge connected while running out on the water, and the pressure is 40 psi at idle, and goes up to 44 or 45 psi under load. Engine RPM tops out at about 2500 at this point regardless of how much I throttle up. So I'm not sure we have the same problem.

I found some black grit in the fuel pressure regulator and also in some fuel injectors so I had the injectors cleaned. I'm reinstalling them and will then relaunch the boat for a test.
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Old 09-09-2017, 09:42 PM   #18
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Don S welcome aboard

What year is your boat? Is it MPI or throttle body injected?

So you loose power at higher rpms? Where does it fall off, rpm?
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Old 09-09-2017, 11:18 PM   #19
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Hi guys thanks for responding so fast. We did not get chance to retry with alt fuel source, will try to get into water tomorrow and will see if it's pump related. Engine is mid 2000 and is MPI. When we has gauge set up on engine to check pressure while on lake test, gauge started to bounce when power fell off. I know nothing of history of this boat, except he was chasing either fuel or power issue. He had a bad coil and we found bad spark wires, he replaced fuel map sensor, crank sensor, and throttle response sensor as well. Now we're going to check with the alt fuel source to verify if possible pump issue.
Will let you guys know if we get any answers.
Thanks again for your quick response. TTYL
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