Mercruiser 5.0 MPI low fuel pressure? Engine struggles to 3000RPM

[FliesAndFloats]

2005 Mercruiser 5.0 MPI. Engine idles perfectly, but as I advance the throttle in gear it starts to get a misfire around 1500 RPM. It feels as if the ECU transitions from idle to power at that point. At 2000 RPM, the engine runs the roughest and it will not go past 3000 RPM. The spark plugs were replaced a couple months ago (~20 hours). I pulled #1 and it looked almost new. I also replaced the cap and rotor as part of the diagnosis and pulled the fuel filter and drained it into a clear receptacle and replaced it with a new filter. The fuel looked consistent and there was no water in it. It was a yellowish color, which I assume is correct for gasoline (avgas is light blue and I know what water looks like). A friend had the diagnostic tool and did the cylinder drop test and confirmed all cylinders are firing.

I purchased a fuel pressure tester/gauge from the local auto parts store and tested the fuel pressure at the rail and it read roughly 25 psi. I've read that the engine should be at 43psi. There are a couple concerns I have with my reading. The first is that I think I screwed up by replacing the fuel filter first and I did not fill the new filter with fuel, so there lies the risk that there may have been air in the system at the time of my reading. It took a few tries to get the boat to stay running, but I let it idle a bit before confirming the reading. The second is that I think the fuel pressure tester was defective as it was leaking fuel from the back of the gauge head. Yes, bad, I know, in an engine compartment under the cowling. Blowers were on and the cowl was open; I didn't discover the leak until I went to purge it, so there's a chance my 25psi reading is from a bad gauge or how I replaced the fuel filter. Purging the gauge with the little button exhausted foamy fuel, but I think that the leak in the back of the gauge may have had something to do with that.

The fuel pump sounds healthy and the pressure comes up immediately, but only to 25psi, before starting the engine. There's a vacuum line on a T between the intake and the rail-mounted fuel pressure regulator that disappears somewhere behind the engine. It seems like there might be a little slack in it and I am suspicious that it may lead to another fuel pressure regulator somewhere else, or it may have a leak. One other thing, but I think this might be normal: if I quickly advance the throttle from idle, I hear a brief air sucking noise from the engine just before it responds to the throttle change.

I'm lost for answers and the local mechanics all seem backed up for weeks right now. I'd like to just get my boat back operational and could use any advice you guys can give. Everything in that engine compartment is extremely tight (Maxum 2400SD) and any advice or tricks on how to get to things would be most appreciated.

 

[Fun Times]

Hi, do you have an Alpha stern drive or Bravo?... Sounds like an Alpha drive...If so the fuel pump would be located down near the oil pan behind the motor mount on the Port side of the boat....Right side if facing the engine from inside of the boat.

There is also a fuel pressure regulator down there/on top of the pump area...The regulator on top of the engine doesn't regulate fuel, it is just a fuel rail damper...No need to replace it unless the fuel rail is found to be heavily contaminated then maybe yes replace it. It's much more expensive than the hard to reach regulator.

For now you'll need to work on confirming the 25 psi as that is to low. Get another gauge and re-test.

It's doubtful that not prefilling the filter is causing this issue as i'd suspect you were have this issue before installing a new fuel filter??

Also did you install new spark plugs before or after this issue was noticed? Broken/bad spark plugs maybe?,

The fuel being foamy might be an issue to suspect so you might want to take a fuel sample from the tank...Any winter time additives in the fuel? Old winter fuel?

You'll need to know if you have a low pressure fuel pump inline before the stock engine mounted water separator fuel filter or if there is another filter inline between the fuel tank and engine. A boat of your caliber, anything is possible as to what other options a manufacturer may or may not have added.

Make sure the fuel vent line is not clogged up with debris.

Next time you start the engine, try to have the scan tool again to see what the Available Power percentage reads.

 

[FliesAndFloats]

The boat has intermittently for about a year had issues achieving full RPM after sitting for 2-3 weeks. It would "clear itself" up after running for about 10-20 minutes back then and I assume it was fouled plugs. This was prior to the new plugs. Somewhat recently, I noticed that at max RPM I'd hear a rattle that I thought was spark knock, so I'd back off on the throttle a bit. It seemed to be correlated with max throttle more than RPM.

The current issue first kept the RPMs up to 3500-3800 and now they're limited down to around 3000 so it seems to be something that's failing progressively. One other thing that I consider to have possibly been affecting things is that I had a bilge pump float switch failure (I keep the boat on a lift and it is impossible to reach the plug underneath the extended swim platform). The bilge filled with water up to near the harmonic balancer when it rained for three days when I was out of town so I suspect the pump and regulator may have been submerged for a couple of days. I've since fixed that float switch and ordered a second and a different design of a pump and float switch for redundancy.

I also replaced the entire distributor before this started happening, but I am really leaning towards these two issues being unrelated unless it's possible for a bad hall effect sensor on the new distributor could cause an issue with proper spark advance; I'll know for sure once reconfirming the fuel pressure. I have confirmed twice that the line on the harmonic balance is aligned when the little triangle on the distributor is aligned properly the the rotor per the service manual.

 

[FliesAndFloats]

One more question: by "difficult to reach" for that lower regulator, do you mean, "take your time and you'll find it isn't that bad," or "inf*ckingpossible and even the professionals would rather pull a motor mount and lift on the engine a bit to get to it?" I am a firm believer of using a cheap borescope and my phone as a remote screen for difficult to see/reach things.

 

[FliesAndFloats]

Ok, I tried a new fuel pressure tester and here's what I observed. Upon turning the switch to "ON" with the engine off, the pressure immediately went to 30PSI. Disconnecting the vacuum line off of the T fitting that I assume runs to the fuel pressure regulator, starting the engine and purging the air from the tester the needle jiggled at 30-32 psi at idle. I revved it maybe to 2000 RPM and it appeared to hold that pressure. Turning off the engine the pressure dropped to 28psi and held steady there until bleeding the pressure prior to removing the gauge. The pressure sure behaves like I'd expect a working system to behave, though if it's supposed to be 43 psi, then something is definitely wrong; I've seen discussion that some Mercruisers are supposed to be 30PSI. The new tester has a long hose and I could take it out and sea trial it to get it to the 3000 RPM and see what she does. The symptoms I'm experiencing are a good healthy idle, heavy misfire from 1800-2200RPM, lighter misfire from 2200-3000 RPM with surging, and at WOT (3000 RPM with the issue) it surges, pops and backfires.

I also discovered that the engine has the "wrong" spark plugs. It has some AC Delco DS--something plugs with a large electrode instead of the ITR4A15's that I've heard that it's supposed to have. The plugs were replaced about 20 hours ago. I pulled the #1 and it looks almost new.

 

[toy4two]

Exact same symptoms to a tee, even your readings and I KNOW I should be 43 psi as I pulled the cool fuel pump and the sticker says 43 psi.

what was your ultimate fix??