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TOPIC: Loss of engine power; Mercruiser 5.0l MPI/Bravo 3

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 28 Aug 2017 03:35 #26

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OK so regarding compression, we measured an average of 180 psi +/- 5% in all cylinders. This test was done by another mechanic a month ago and he measured 130 in cylinders 2 and 4, but when he sprayed some oil in the cylinder, they went back up to 180 psi.

I've heard that 150 is considered normal. Strangely the Mercruiser service manual only has this spec for compression:

Compression Pressure Minimum 100 psi with this note: "minimum recorded compression in any one cylinder should not be less than 70% of the highest recorded cylinder."

So if that's really the spec we are well within that. So I don't think a leak down test will help us. Maybe there has been wear and tear but the engine only has 220 hours on it.

I am starting to agree the vacuum may not be easily fixed, and is not likely the source of the power loss. At this point with the manifold off we have no choice but to put in new gaskets, so we'll see what that does.

We are probably going to pull the injectors and look at the spray pattern. I've seen enough crud in the fuel lines over the years that it's not hard to imagine there's been some blockage, at least partial. I will also pull the fuel hoses and take a look for internal degradation.

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III
Last Edit: by iceman11.

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 28 Aug 2017 06:21 #27

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All the plugs should have been pulled then the compression test preformed, plugs out you can read them, one at a time.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 02 Sep 2017 05:49 #28

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Tomorrow, among other things, we're going to pull the fuel injectors and check them out. There are 2 O-rings per injector for a total of 16. Mercury charges $200 for a set of O-rings!

Ruffryder, or anyone who knows, are these O-rings really that special and expensive? Can they be bought from another source, like say an auto supply, and work just as well? $200 just seems like price gouging for O-rings!

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 02 Sep 2017 07:16 #29

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 02 Sep 2017 15:57 #30

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O-rings can be bought from another source, nothing special about it.
Injector tester can be found easily on eBay for less than $30 & I really recommend one.
Hook up a fuel pressure gauge at the rail, key turned on, fuel pressure pressurized, injector tester is connected to each injector... this will simulate/pulse each injector like your ECM 555 would, the pressure drop will tell you exactly.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 02 Sep 2017 22:54 #31

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No resolution today. Put the intake manifold back together with new gaskets, and the vacuum still a steady 12 in at idle, below what the troubleshooting table says (15-21 in). We verified the rotor location is OK, and therefore timing is OK.

Working theory is that engine is running lean, causing vacuum to be low.

Pulled the fuel injectors and found the same sort of black crud inside some of them that I had seen on the FPR. Problem found right? Maybe not - cleaned the fuel injectors with a homemade ultrasonic cleaning system. Injectors looked clean afterwards, so we reinstalled them on the rail and checked that each had a spray pattern that looked OK. Mounted rail on the engine and started it. Vacuum is still 12 in, which is below spec.

Is it possible 12 in is OK even though the manual says 15 -21? We could launch the boat can test it under load, but it's not clear we'd get any different result (other than the injectors are cleaner).

very frustrating...

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 01:54 #32

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some high performance engines run lower then 15.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 03:12 #33

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Again perform a leak down test on the cylinders. However I don't believe this is the cause of the loss of power. 12 psi is still a decent vacuum. Don't recall you posting the compression measurements other than saying they were ok, what are the Numbers?

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1997 Maxum 2400 SCR 5.7LX Bravo II

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 05:32 #34

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Mike please scroll up to post 26, that's where I posted the compression test results. I may end up having the leak down test done, but I'm very skeptical that this will fail. Remember, the low 12 in. vacuum is "Steady" at idle, it does not fluctuate. I should also point out, it's 12 in Hg, not 12 psi. 12 in Hg is around 9 psi.

Chief Alen, yes we are starting to think the 12 in. Vacuum is not the root problem of the power loss, however 12 in. is below the Mercruiser spec of 15-21. That can't be good, can it?

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III
Last Edit: by iceman11.

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 08:55 #35

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Might just be how the vacuum was from birth of the engine.

No base reading I assume, written down.

That's why I urge a base reading written down, compression and vacuum test.

Some run a vacuum gauge installed in the dash.

Don't really want to beat a dead horse, what is all this black crap, blocking the injectors?

I am very interested in your next run.

Please keep us informed.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 11:33 #36

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iceman11 wrote: Mike please scroll up to post 26, that's where I posted the compression test results. I may end up having the leak down test done, but I'm very skeptical that this will fail. Remember, the low 12 in. vacuum is "Steady" at idle, it does not fluctuate. I should also point out, it's 12 in Hg, not 12 psi. 12 in Hg is around 9 psi.

Chief Alen, yes we are starting to think the 12 in. Vacuum is not the root problem of the power loss, however 12 in. is below the Mercruiser spec of 15-21. That can't be good, can it?


Guess I was too tired when I posted last night, yes manifold vacuum is in Hg. When the compression test was made was the engine at normal operation temp? 180 is high. Since you added oil to two cylinders to get there compression up I suspect they are contributing to the low manifold vacuum but still don't think this is the source in power loss unless these cylinders are not firing. A leak down test will give you a better understanding of why these two cylinders are lower than the others.

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1997 Maxum 2400 SCR 5.7LX Bravo II

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 16:36 #37

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Sounding more like there might be a faulty MAP sensor or vacumm leak.
intake mani leak could have been detected with some carb cleaner easily.
Yes, 12 in Hg is about 8.7 PSI.
Merc Service manual for the engine, the manifold pressure seen by a vacuum gauge should be 15 to 21 inches HG at idle. The manifold pressure is reported in absolute PSI, 15 to 21 in from a gauge should translate to about 7 to 4 abs PSI as reported by MAP.
First, make sure intake manifold vacuum is within specifications at idle. vacuum reading can be unusually low due to a vacuum leak, retarded ignition timing, an exhaust restriction.... when was the last time both exhaust shutters were checked?
A low intake vacuum reading or excessive backpressure in the exhaust system can trick the MAP sensor into indicating there is a load on the engine. This may result in a rich fuel condition.
No code thrown this time?
What is the MAP sensor value reading, voltage output?
A good MAP sensor should read barometric air pressure when the key is turned on before the engine starts & should be compared to the actual barometric pressure to see if they match.

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Last Edit: by Ruffryder.

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 18:50 #38

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there is a vacuum line on the GEN III fuel module pressure regulator must be connected to intake manifold, has that line inspected for splits, kinks and proper connections?
that will play an important role since fuel pressure varies with engine vacuum.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 19:09 #39

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Ok, I think we got it now.
I just read your post 16 & 22 again.
Your fuel pressure is too LOW, SPEC on your engine is not 34-38.
All ECM/PCM 555 runs on 43PSI regulator, SPEC is 43PSI +/- 2 PSI... aren't you working with a mechanic? he should've known all these specifications :pinch:
You keep bringing up a lower pressure regulator also... FYI that upper-pressure regulator at the fuel rail is a DUMMY ONE.
Wrong pressure regulator you put on most likely!

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Last Edit: by Ruffryder.

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 21:08 #40

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RuffRyder, I really appreciate your thoughts above. I have been working with mechanics: my son who is a precision equipment mechanic and previously an auto mechanic (but not marine), and a local marine mechanic who I have not worked with before. The marine mechanic is convinced there is a problem with a valve seat, and wants to do a leak down test. He has collected engine telemetry with his scanner both at idle and while the boat was on the water under load, and said nothing looked wrong in that data. My son is the one who checked vacuum and is focused on that aspect. He also checked the exhaust shutters (OK), timing (OK) and a few sensors including MAP. He replaced all the gaskets on the intake manifold and that made no difference. He cleaned the fuel injectors - we don't have a tester for those but all of them sprayed fuel.

So regarding fuel pressure - assuming you're right about the 43 psi, I have been misleading myself by using the older Mercruiser 5.0 Service Manual . My son measured 38, and that is definitely below that spec. I don't know if the marine mechanic looked at this, he was focused on compression.

The pressure regulator I replaced was the lower one mounted on the cool fuel module. I'm 99% sure it's the right part, since I ordered is based on the Mercruiser engine diagram. I looked at the upper FPR but I've never replaced it. I did notice that the return from that FPR is intentionally blocked rather than routed to the gas tank, which I found to be strange but again the engine diagram shows that as well. So I guess you're right, it doesn't really do anything.

My next steps are as follows:
1. Check the fuel lines for ethanol degradation (as Chief Alen has suggested).
2. Check the FPR vacuum line (as you suggested)
3. Launch the boat on Tuesday to see if cleaning the injectors helped at all.
4. If all of the above fails, bring the boat back to the marine mechanic for a leak down test

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 21:30 #41

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Of course, I'm right! :lol:
two separate issues going on possibly.
Yup I'm glad you looked at the fuel rail FRP found out to be strange, but you know why now.
Give me the cool fuel module FRP #, I can tell you if it's the right one or not... 42.5 PSI FPR to be exact on your engine.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 21:38 #42

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FPR I ordered and put in engine: 892681 REGULATOR (Mercury Marine)

I'm wondering if there is interplay between manifold vacuum and fuel pressure where one root cause is affecting both. At least I hope so.

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 21:48 #43

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That is the CORRECT FPR for your engine.
Yes, there is... On PCM/ECM models, fuel pressure varies with engine vacuum.
Fuel pressure will be high during cranking, low at idle and increase proportionally as the throttle is opened to the
wide-open position. Disconnect the vacuum line (on the cool fuel regulator) to find the
regulator’s rated pressure, then reconnect the vacuum line to make sure the pressure
drops at idle. Pressure specification is 43 psi (+/– 2 psi) on all models. Pressure usually
drops about 6-8 psi at idle (from the regulator’s rated specification).
There are a few tests we can perform to determine this issue, but I'm afraid to say it is going to take a new module or new pumps which are not cheap.

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 22:02 #44

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late valve timing, you say the intake leak possibility is out of the equation ...

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 03 Sep 2017 22:09 #45

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OK if you're saying the fuel pressure should be 6-8 psi below 43 psi at idle, then what I measured (38 at idle) is OK, right? I believe the pressure went up with RPM but I'm not sure how much. We did notice that with higher RPM the pressure was fluctuating, not steady.

I checked the vacuum line to the FPR, it looks fine.

Currently I have a Floscan unit. There's a fuel line from the tank to the Floscan sensor, and from the sensor to the Fuel module. I'm thinking of bypassing these lines before launching and going from the tank to the fuel module with a new fuel line, just to see if this makes a difference. The only other fuel line is the hybrid factory fuel line from the module up to the fuel rail.

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III
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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 04 Sep 2017 20:18 #46

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I totally agree with removing plugs and having a look. We had trouble getting to plane on our 288. We changed fuel filters and plugs. When changing plugs I noticed one was different color than others. When we still had trouble getting to plane did a compression test and found that cylinder was bad (and another cylinder was outside 10% of other 6 as well). You're not going to like the solution if that's the problem!

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 04 Sep 2017 22:11 #47

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The plugs looked OK, we've pulled them all out. If anything they indicated a lean running condition.

Today I bypassed the Floscan sensor, including the fuel lines going in and out. So I have a brand new, direct fuel line from the tank to the Cool Fuel module. Just for fun, I bought a fuel pressure gauge (my son went back home with his tools) and ran the engine. At idle I am now seeing about 40 psi.

The next step is to launch tomorrow, if conditions are OK and try to get on plane. If not, off to the shop for a leak down test.....

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III
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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 05 Sep 2017 20:24 #48

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Launched the boat today - no bueno - performance loss was the same. Topped out at 2500 RPM and couldn't get on plane. So.. cleaning the injectors and bypassing two fuel lines and the Floscan sensor made no difference. I had a fuel pressure gauge on the engine while we ran it, and it showed 40 psi at idle as before, and about 44 or 45 psi under load (at 2500 rpm). We also smelled raw fuel in the exhaust - I'm not sure what that's about since we thought we were running lean???

Called the marine mechanic to schedule the leak down test. When I explained what we did, he said that he feels we should have the injectors professionally serviced and tested. He wants to do that before we bring the boat to him for a leak down test, since I can just pull the injectors and bring them to him.

I'm still concerned that there is a root problem that needs to be fixed in the fuel system. If it is paint inside of the Gen 3 Cool Fuel module, I suppose that should be replaced too, but that's a $1000 part. Don't know how to rule that in or out other than I suppose pulling it off and looking inside of whatever fuel passages I can see.

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III

Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 05 Sep 2017 21:20 #49

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iceman11 wrote: Launched the boat today - no bueno - performance loss was the same. Topped out at 2500 RPM and couldn't get on plane. So.. cleaning the injectors and bypassing two fuel lines and the Floscan sensor made no difference. I had a fuel pressure gauge on the engine while we ran it, and it showed 40 psi at idle as before, and about 44 or 45 psi under load (at 2500 rpm). We also smelled raw fuel in the exhaust - I'm not sure what that's about since we thought we were running lean???

Called the marine mechanic to schedule the leak down test. When I explained what we did, he said that he feels we should have the injectors professionally serviced and tested. He wants to do that before we bring the boat to him for a leak down test, since I can just pull the injectors and bring them to him.

I'm still concerned that there is a root problem that needs to be fixed in the fuel system. If it is paint inside of the Gen 3 Cool Fuel module, I suppose that should be replaced too, but that's a $1000 part. Don't know how to rule that in or out other than I suppose pulling it off and looking inside of whatever fuel passages I can see.


This just really sucks, not sure what else to tell you but I'm very interested in the root cause when you find it.

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1997 Maxum 2400 SCR 5.7LX Bravo II

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Loss of engine power; Mercruiser 5.0l MPI/Bravo 3 06 Sep 2017 19:21 #50

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Fuel injectors have been sent out to be professionally cleaned. I should have them back by Friday. Mechanic says we'll be told if any were bad. Need to know that so we have confidence that they were the source of the problem.

I think I found out the source of the raw gas smell. While I was removing the injector rail I noticed the Distributor cap was loose. One screw was backed off about 1/8". I tried to tighten it up but it looks to be stripped. With an 1/8 inch gap on one side of the cap, it's no wonder some plugs may not have been firing or fired intermittently. I don't think the Cap was loose before Saturday so I don't THINK this is the cause of the power loss. Not sure what I can do to repair this - is the next bigger screw a possibility? It's a Torx self-tapping screw I think, that comes with the Distributor cap.

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2006 Maxum 2600SE with Merc 5.0L MPI/Bravo III
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