Mercruiser 5.0 MPI Fuel Pressure

[jwilson_46143]

2004 Mercruiser 5.0 MPI (serial number 0W029452) has had an intermittent staring issue the last year or two. It has never failed to start and runs well after starting. Another intermittent issue is stalling when coming of plane and shifting into neutral. Replaced fuel filter, distributor and rotor, and IAC valve last year which didn't really help much. I finally got around to testing the fuel pressure today after it wouldn't start after coming out of storage. When key is turned to on and fuel pump 'primes' I get 42-43psi. After finally starting, the fuel pressure read 30-35psi at idle and up to 2000rpm. My understanding is that the fuel pressure should be 42-44psi when running. So do I have a fuel pump problem? Replacement of fuel pump necessary? It is the original fuel pump (~20 years old). Thanks for the input!

 

[alldodge]

Your correct 43 psi is where it should be at 1800 RPM. Running at 30-35 is to low. Most likely problem is the regulator item 13
https://www.mercruiserparts.com/bam/subassembly/31748/8093/60

Does your motor have a boost pump above the fuel filter canister?

First connect a remote tank (small gas can and length of hose to filter). Then connect fuel pressure gauge to rail and see if you get same results

 

[Fun Times]

Could possibly be a weak fuel pressure regulator too.

Next time while the engine is running, disconnect the vacuum line heading to the fuel regulator and see if pressure goes back up roughly 4 psi...ish...

Also possibly being an Alpha stern drive the shift cable could be a bit miss adjusted or worn out, etc. to where it's hanging up and activating the shift interrupt switch killing the engine/no start/no spark type symptoms...Especially if you're shifting a bit to slow trying to be careful maybe.

Fuel Pump and Fuel Cooler

Fuel Pump and Fuel Cooler

www.mercruiserparts.com

Shift Bracket(ALPHA)(SN-0M660000 and Up)

Shift Bracket(ALPHA)(SN-0M660000 and Up)

www.mercruiserparts.com

350 MAG MPI ALPHA/BRAVO - 0M600000 THRU 0W059999

350 MAG MPI ALPHA/BRAVO - 0M600000 THRU 0W059999

www.mercruiserparts.com

Your serial number is just out of range of these issues so doubtful it's happening but just be aware just in case it's usual info, https://manualzz.com/doc/23430314/service-bulletin---marinemechanic.com

 

[jwilson_46143]

Your correct 43 psi is where it should be at 1800 RPM. Running at 30-35 is to low. Most likely problem is the regulator item 13
https://www.mercruiserparts.com/bam/subassembly/31748/8093/60

Does your motor have a boost pump above the fuel filter canister?

First connect a remote tank (small gas can and length of hose to filter). Then connect fuel pressure gauge to rail and see if you get same results

Hi alldodge, I don't see a boost pump above the water separating fuel filter. As I was retesting the fuel pressure today, I noticed a 'choppy' sound from the fuel pump as it primed when the key was turned to 'on'. The choppy sound also correlated with pressure fluctuations of a few psi below 43psi. Any thoughts on this uneven sound from the fuel pump? Maybe another sign the regulator is faulty? Thanks!

 

[alldodge]

Might be there is a restriction causing an issue with the pump. Restrictions can be caused by Antisiphon valve on tank, collapsing fuel line, air entering fuel system, or even the pump.

Take a small gas can and length of gas line and go from can to fuel filter. Prime a few times and bleed air out of fuel rail from the new hose and see if it still does it

 

[QBhoy]

Check what comes out the shrader valve on the fuel rail. A spurious varying tone from the fuel pump usually means air. Check you don’t have a means of air being able to get educated into the fuel line. A one off gulp of air from a filter change can be dealt with on its own, but a constant intake will cause issues. Check your fittings on the entire supply side perhaps.
It could also mean a restriction of course. Run it off a fuel can of known fresh fuel if so.
Probably not the issue, but these things start rich on a cold start until they warm up some. Determined by the temperature of the engine as far as I know. Is the temperature sensor reading correctly and working ok ?
If that’s not relevant. I’d finally ask if the engine was warm or cold when you took the fuel pressure ? And when accelerating from slow speed to planing speed, can you hear an audible high pitch noise from the high pressure fuel pump doing its thing.

 

[jwilson_46143]

Check what comes out the shrader valve on the fuel rail. A spurious varying tone from the fuel pump usually means air. Check you don’t have a means of air being able to get educated into the fuel line. A one off gulp of air from a filter change can be dealt with on its own, but a constant intake will cause issues. Check your fittings on the entire supply side perhaps.
It could also mean a restriction of course. Run it off a fuel can of known fresh fuel if so.
Probably not the issue, but these things start rich on a cold start until they warm up some. Determined by the temperature of the engine as far as I know. Is the temperature sensor reading correctly and working ok ?
If that’s not relevant. I’d finally ask if the engine was warm or cold when you took the fuel pressure ? And when accelerating from slow speed to planing speed, can you hear an audible high pitch noise from the high pressure fuel pump doing its thing.

Hi QBhoy,
I can't be sure if the temp sensor is working correctly - the temp gauge at the dash is not currently working. Could be gauge or sensor (sending unit?). Should I test/replace the sensor to be safe?
The engine has been cold when testing the fuel pressure.

Thanks!

 

[Fun Times]

If the engine doesn’t seem to be getting up to temperature then it may be a stuck open thermostat as that can make the gauge read low too.