Mercruiser 5.0 MPI fuel problems

[blibe1]

Just some background, have a 2012 Bayliner with a 5.0 Mercruiser MPI sterndrive

After the winter we tried to start the boat and it wouldn't crank over. Realized we didn't hear the fuel pump kicking on. Tested electrical and all was good. Tried starting the motor with starter spray and it turned over but died soon after. Sent her to the marina for diagnosing and they said it's a bad fuel pump so they replaced it with a quicksilver OEM pump.

After repair the engine was starting no problem. Put the boat in the water, warmed her up good and took it for a spin and it died on me after about 15 minutes. Engine just turned off without any warning. After a few cranks I had it going again and all was well after that, until the next day.
Started the engine no problem, warmed it up and was on my way. She didn't make it 10 minutes without dying, again just completely shut off with no warning. After a few cranks it started but anytime I put the engine over 1500RPMs the engine just cut off. Same thing happens when I rev the engine in neutral. Between 1200-1500 RPMs they rpm starts bouncing eratically then dies if I give too much gas, just cuts off without warning. Was able to cruise back to shore keeping RPMs as low as possible. Checked fuel filter for water and it looked okay. I'm out of ideas now.

Marina is telling me it cannot be the pump since it was replaced with OEM and it has to be something else, but nothing else makes sense aside from a bad fuel pump. Electrical is 100% good because the pump always has a current going to it, yet the pump doesn't turn on every time.

If there anything that is preventing the fuel pump from turning on?

 

[dubs283]

Possible issue with the fuel pump relay/associated circuitry

My thought is based on your info provided there is a problem with the fuel supply to the pump. Vent line partially blocked, anti siphon valve sticking/failing, fuel pick up issue in the tank, etc....

 

[blibe1]

After some more digging I think it could be the oil pressure sensor. I want to order a new one (part # 881879T11) but first I want to test the wiring to it and I cannot find the location of it on the motor?

I looked at some part diagrams, but none show it's actual location on the motor. Any assistance?

 

[alldodge]

Before you start throwing parts at it, get a fuel pressure gauge and connect to the fuel rail. Can get one on loan from a local auto parts place maybe. Watch the pressure to find out if its loosing pressure or something else.

The MPI does use the oil pressure switch to keep the fuel pump running like a carb motor. Might be a bad connection or relay dropping out

 

[Rick Stephens]

No one, especially these days, should assume that just because it is a new part, even OEM, that it works properly. Only way to know is to test the line pressure off the pump.

 

[blibe1]

Before you start throwing parts at it, get a fuel pressure gauge and connect to the fuel rail. Can get one on loan from a local auto parts place maybe. Watch the pressure to find out if its loosing pressure or something else.

The MPI does use the oil pressure switch to keep the fuel pump running like a carb motor. Might be a bad connection or relay dropping out

Fuel pressure was checked before the pump was replaced. Also wouldn't explain why the fuel pump doesn't turn on half the time when the key is turned to power.

 

[alldodge]

Fuel pressure was checked before the pump was replaced. Also wouldn't explain why the fuel pump doesn't turn on half the time when the key is turned to power.

That's my point, it cannot be the oil pressure switch. Fuel pump is powered ON by the ECM applying a ground to one side of the relay coil. The ECM my apply a ground but if the main power relay (MPR) is not supplying power to the other side it won't energize.

So if it actually is "the pump turning off" or not turning ON at all, then there would be no pressure.

If key is turned ON before start and you do not hear 2 beeps, then motor is not powered on. If 2 beeps are heard then motor is. If fuel pump does not run for 2 seconds, then check relay connections to and from ECM along with to/from pump

DOn't know your motor serial number but will guess it is a 555 ECM. This may be how yours is connected. Note MPR must be ON so FPR can get power and both are turned ON by the ECM

 

[itsathepete]

the oil pressure switch is likely on the port side below the exhaust manifold at the rear of the block. Just above where the oil filter is if it doesn't have a remote oil filter. Or possibly on top rear of block just behind intake manifold. port side

 

[blibe1]

That's my point, it cannot be the oil pressure switch. Fuel pump is powered ON by the ECM applying a ground to one side of the relay coil. The ECM my apply a ground but if the main power relay (MPR) is not supplying power to the other side it won't energize.

So if it actually is "the pump turning off" or not turning ON at all, then there would be no pressure.

If key is turned ON before start and you do not hear 2 beeps, then motor is not powered on. If 2 beeps are heard then motor is. If fuel pump does not run for 2 seconds, then check relay connections to and from ECM along with to/from pump

DOn't know your motor serial number but will guess it is a 555 ECM. This may be how yours is connected. Note MPR must be ON so FPR can get power and both are turned ON by the ECM


View attachment 364313

Thanks I'll check the wiring.

One last thing, if I hear the fuel pump turn on and it cranks over, i'm getting a double beep whenever I rev the engine. I don't have a scan tool but is it possible this could also be related to the fuel relay issue? or would this be a separate issue (oil pressure sensor) ?

 

[alldodge]

Thanks I'll check the wiring.

One last thing, if I hear the fuel pump turn on and it cranks over, i'm getting a double beep whenever I rev the engine. I don't have a scan tool but is it possible this could also be related to the fuel relay issue? or would this be a separate issue (oil pressure sensor) ?

The double beep can be many things. Reving the motor in neutral can be the ECM saying your not in gear. My memory is not the greatest but its some where around 2500 RPM. If motor revs above 2500 (lets say) the ECM lets you know there might be a problem

 

[QBhoy]

Initially, I’m thinking your issue is an electrical issue. Perhaps somewhere around having a reliable and constant continuity to the ecu or more in particular, the kill switch wiring perhaps. But only thing you’ve said that would throw that theory out…was the mention of it not cranking initially. Do you mean it wouldn’t fire initially or it actually wouldn’t crank over ?
So many factors to think about on this engine. The kill switch in play will allow cranking but no fuel pump…but doesn’t explain it running in any way at all or rough running. But a poor or intermittent voltage would.
Thinking as I go here…but I’d say you’ve a few things going on here. The high pressure fuel pump comes into play as the rpms increase, so perhaps around there is the issue. But the two beeps mentioned, also means one of many things too. I’d be making certain you have a cast iron sure thing with regards to the reliable supply and continuity of power feeding into her systems. Check and check again the battery connections and main cables from alternator etc etc. it only takes the slightest malfunction of just one component in the system to throw gremlins at you from numerous sources elsewhere in the system. Especially the 5v system.
Do you get one long beep when you turn the key or two ? When the fuel pump activates ?

 

[blibe1]

Initially, I’m thinking your issue is an electrical issue. Perhaps somewhere around having a reliable and constant continuity to the ecu or more in particular, the kill switch wiring perhaps. But only thing you’ve said that would throw that theory out…was the mention of it not cranking initially. Do you mean it wouldn’t fire initially or it actually wouldn’t crank over ?
So many factors to think about on this engine. The kill switch in play will allow cranking but no fuel pump…but doesn’t explain it running in any way at all or rough running. But a poor or intermittent voltage would.
Thinking as I go here…but I’d say you’ve a few things going on here. The high pressure fuel pump comes into play as the rpms increase, so perhaps around there is the issue. But the two beeps mentioned, also means one of many things too. I’d be making certain you have a cast iron sure thing with regards to the reliable supply and continuity of power feeding into her systems. Check and check again the battery connections and main cables from alternator etc etc. it only takes the slightest malfunction of just one component in the system to throw gremlins at you from numerous sources elsewhere in the system. Especially the 5v system.
Do you get one long beep when you turn the key or two ? When the fuel pump activates ?

I meant that it wasn't firing. So if I hear the fuel pump turning on, the engine cranks and fires as normal, but struggles as soon as I give it any throttle. It just starts bouncing around the RPM ranges from 1200-1500RPM. If I go past 1500RPMs I get a loud constant beep.

With the beeps, I get one when I turn on power with the ignition and then 2 beeps once the engine is on. I then get 2 beeps every minute or so when the engine is on. If I don't hear the fuel pump turn on, I still get only 1 beep when I turn the key to power.

 

[QBhoy]

I meant that it wasn't firing. So if I hear the fuel pump turning on, the engine cranks and fires as normal, but struggles as soon as I give it any throttle. It just starts bouncing around the RPM ranges from 1200-1500RPM. If I go past 1500RPMs I get a loud constant beep.

With the beeps, I get one when I turn on power with the ignition and then 2 beeps once the engine is on. I then get 2 beeps every minute or so when the engine is on. If I don't hear the fuel pump turn on, I still get only 1 beep when I turn the key to power.

Ok. Got you. So I think you have a few issues going on. Someone better versed than I will be able to tell you exactly what can make the 2 beeps. But the well known ones are voltage (may be relevant), low gear oil, IAC (think this might be one of your issues perhaps).
Other than that..the well known issue that can make your engine run rough like that, on the mpi is the distributor cap too. But that won’t usually beep.
Like I said earlier..you may also have an issue relating to or caused by the kill switch circuit…but that wouldn’t cause it to run at all. But it would stop the fuel pump coming on when the key is turned. I think you need to get it plugged in or get a vessel view mobile device for about $200 and use that to see what the fault or faults are showing. Can you remember the voltage showing when it miss behaves ?

 

[alldodge]

List of things that can cause 2 beeps and constant beep

 

[QBhoy]

List of things that can cause 2 beeps and constant beep
View attachment 364369

Just the one or two then, haha. Excellent as always AD.

 

[blibe1]

Wow that's a lot of possibilities, I'll def have to get the CPU scanned now before toying around with much else.

Greatly appreciate the input.

 

[Rick Stephens]

2 beeps looks to be same usefulness as a check engine light.

 

[Scott06]

2 beeps looks to be same usefulness as a check engine light.

Yeah basically saying log into me...