Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 - Maxum Boat Owners Club

iceman · 07-03-2017, 06:30 AM

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

mmwjr · 07-03-2017, 12:04 PM

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.

iceman · 07-04-2017, 09:33 PM

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??

mmwjr · 07-04-2017, 09:36 PM

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?

mmwjr · 07-04-2017, 09:49 PM

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.

iceman · 07-05-2017, 06:05 AM

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.....

mmwjr · 07-05-2017, 11:49 AM

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.

tnrivrunr · 07-09-2017, 04:57 PM

How many people in the boat? What pitch prop?

iceman · 07-10-2017, 10:29 PM

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.

iceman · 08-24-2017, 11:32 PM

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?

mmwjr · 08-25-2017, 01:17 AM

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.

iceman · 08-25-2017, 03:17 AM

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

mmwjr · 08-25-2017, 03:40 AM

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.

iceman · 08-25-2017, 04:15 AM

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?

mmwjr · 08-25-2017, 01:29 PM

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.

Don S · 09-09-2017, 08:12 PM

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.
Thank you

iceman · 09-09-2017, 09:53 PM

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.

mmwjr · 09-09-2017, 10:42 PM

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?

Don S · 09-10-2017, 12:18 AM

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

iceman · 12-12-2017, 06:08 PM

Just want to close out this thread with the final resolution. The reader's digest version is: the Crank Position Sensor was bad (intermittent), leading to incorrect timing and thus loss of power. The mechanic's theory is that the sensor was damaged 2 years ago during an engine overheat incident, but didn't fully fail until this year. I've posted more details on other forums, so if someone's interested I can repost here.

As always, thanks for your help and advice!

Tom