Black Scorpion Mag Mpi fuel pressure

[Dremmick1985]

According the 24 service manual I am supposed to be getting 34-38 psi. Is that at idle with regulator vacuum connected? I'm getting 26psi at idle with regulator vacuum hooked up. If i disconnect the vacuum should it increase? I'm to determine if I have a fuel or regulator issue. After a few minutes of >3,000rpm rpm's will drop to 1,000rpm out of no where and try to recover.

 

[Fun Times]

According the 24 service manual I am supposed to be getting 34-38 psi. Is that at idle with regulator vacuum connected? I'm getting 26psi at idle with regulator vacuum hooked up. If i disconnect the vacuum should it increase? I'm to determine if I have a fuel or regulator issue. After a few minutes of >3,000rpm rpm's will drop to 1,000rpm out of no where and try to recover.

At idle is fine for testing too while some service manual recommends checking the psi at 1800 rpm under load too <in which is the better thing to do depending on issue type.

When all is working normal the fuel pressure should go up to the regulator specification when you remove the vacuum line at idle and then drop back down when reconnected.

See the document on the left side of the following post number 7,

What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

My boat is at the shop after experiencing loss of power and WOT of only 3000 RPM. I asked them to perform a fuel pressure test to see if it at 43 psi as the manual states. They are claiming that the only way to get an accutare fuel pressure reading it to take it to the lake and run it at WOT...

forums.iboats.com

What is your engine serial number and engine/boat year if original?

 

[alldodge]

Agree, need a serial number, the Black Scorpion had several changes but only ones that list 43 psi is 17 and 31

Manual 17 from 1993 to 1997 Scorpion 43 psi
Manual 24 All 30 psi
Manual 24 All 30 psi
Manual 31 for 2001 and up All 43 psi

 

[Dremmick1985]

At idle is fine for testing too while some service manual recommends checking the psi at 1800 rpm under load too <in which is the better thing to do depending on issue type.

When all is working normal the fuel pressure should go up to the regulator specification when you remove the vacuum line at idle and then drop back down when reconnected.

See the document on the left side of the following post number 7,

What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

My boat is at the shop after experiencing loss of power and WOT of only 3000 RPM. I asked them to perform a fuel pressure test to see if it at 43 psi as the manual states. They are claiming that the only way to get an accutare fuel pressure reading it to take it to the lake and run it at WOT...

forums.iboats.com

What is your engine serial number and engine/boat year if original?

2004 scorpion mag mpi SN 0M393328. Is the regulator spec the 34-38 or 30psi what is show on a bracket over the regulator?

 

[alldodge]

Reg part number is 807952A1 or Mallory 9-33205 (30 psi)
Item 13
https://www.mercruiserparts.com/bam/subassembly/31440/10108/50

 

[Dremmick1985]

Is there a way to diagnose if it the pump or regulator?

 

[Fun Times]

According the 24 service manual I am supposed to be getting 34-38 psi. Is that at idle with regulator vacuum connected? I'm getting 26psi at idle with regulator vacuum hooked up. If i disconnect the vacuum should it increase? I'm to determine if I have a fuel or regulator issue. After a few minutes of >3,000rpm rpm's will drop to 1,000rpm out of no where and try to recover.

Lets go back, What is/does the PSI do when you remove the vacuum line at idle? You're at 26 right now so it ought to jump up to 30ish psi spec when you remove the line...Need to recheck that test.

Next this is where you'd need to be in the water with the fuel tester gauge connected to see if the fuel psi is dropping when the engine drops RPM.

Bear in mind that your engine system seems to have a low fuel pressure booster pump inline as well,

Fuel Filter And Boost Pump

Fuel Filter And Boost Pump

www.mercruiserparts.com

BLACK SCORPION 350 MAG MPI - 0M391600 THRU 0W689999

BLACK SCORPION 350 MAG MPI - 0M391600 THRU 0W689999

www.mercruiserparts.com

 

[Dremmick1985]

Lets go back, What is/does the PSI do when you remove the vacuum line at idle? You're at 26 right now so it ought to jump up to 30ish psi spec when you remove the line...Need to recheck that test.

Next this is where you'd need to be in the water with the fuel tester gauge connected to see if the fuel psi is dropping when the engine drops RPM.

Bear in mind that your engine system seems to have a low fuel pressure booster pump inline as well,

Fuel Filter And Boost Pump

Fuel Filter And Boost Pump

www.mercruiserparts.com

BLACK SCORPION 350 MAG MPI - 0M391600 THRU 0W689999

BLACK SCORPION 350 MAG MPI - 0M391600 THRU 0W689999

www.mercruiserparts.com

26 psi with vacuum attached and 32 with vacuum line disconnected

 

[Fun Times]

26 psi with vacuum attached and 32 with vacuum line disconnected

You're in spec for now... If possible, go run the boat with the fuel gauge connected ... Also suggest bringing an external fuel tank and fuel hose to bypass the boat tank and see if it runs normal...Also open the boat fuel filler cap to vent the boat tank to see if it helps any...50/50% chance there.

Let us know, good luck.

 

[Dremmick1985]

You're in spec for now... If possible, go run the boat with the fuel gauge connected ... Also suggest bringing an external fuel tank and fuel hose to bypass the boat tank and see if it runs normal...Also open the boat fuel

Its a fuel related issue, my pressure would slowly creep down after a couple minutes and started to run lean around 15psi. Opening the gas cap did not help. I ran off the an auxiliary tank plumbed into the low pressure fuel pump and did not give me an issues until I ran out of fuel. It was only a one gallon fuel can. At 2500rpm im still only at 27psi which is lower than the 34-38psi spec. I need to inpsect fuel pickup in tank and anti siphon valve.

 

[alldodge]

Antisiphon valve
It's the barbed fitting the gas line is attached to

 

[QBhoy]

Have you checked the engine oil for signs of lots of fuel in there ? Just in case you have a leaking injector somewhere ? Could test the injectors perhaps ? Sorry if that’s been covered already.
Other thing you could do…is using the shrader valve on the rail…key on and off the key to get the pump running…check there is no sustained signs of air from the rail. This could mean that you are drawing air from the LP side of the pump. By means of the filter or a compromised fuel line perhaps. The pressure can only drop (in test situations) by means of air induction or a leaking injector, most commonly perhaps. Often just the sound of the fuel pump can tell a story.
You’ll also maybe already know…but on some of these type engines, of a certain era..there was known issues with the internal coating or paintwork finish within the fuel pump and regulator components. Caused flakes of it to block or restrict the lines. Usually meant a lean popping condition when running up the rpms.

 

[Dremmick1985]

Antisiphon valve
It's the barbed fitting the gas line is attached to

Can this be tested? Or do I just replace it?

 

[Dremmick1985]

Have you checked the engine oil for signs of lots of fuel in there ? Just in case you have a leaking injector somewhere ? Could test the injectors perhaps ? Sorry if that’s been covered already.
Other thing you could do…is using the shrader valve on the rail…key on and off the key to get the pump running…check there is no sustained signs of air from the rail. This could mean that you are drawing air from the LP side of the pump. By means of the filter or a compromised fuel line perhaps. The pressure can only drop (in test situations) by means of air induction or a leaking injector, most commonly perhaps. Often just the sound of the fuel pump can tell a story.
You’ll also maybe already know…but on some of these type engines, of a certain era..there was known issues with the internal coating or paintwork finish within the fuel pump and regulator components. Caused flakes of it to block or restrict the lines. Usually meant a lean popping condition when running up the rpms.

No fuel in oil or leaks. I should remove the hp pump and regulator to take a look.

 

[QBhoy]

No fuel in oil or leaks. I should remove the hp pump and regulator to take a look.

Yeah. Good idea maybe. Can’t remember quite the exact details about that. But look up mercruiser mpi fuel pump paint or something similar

 

[QBhoy]

Found one of the reports on here. Loads online. Is it the cool fuel you have ?

Low fuel pressure on 350 MAG MPI

Hi guys. My Mercruiser 350 MAG MPI w/186 hours on it cannot get on plane, its maxes out at around 3000 RPMs. To fix I've been throwing parts at it since the local mechanics can't look at it for 3-4 weeks. It has a Cool Fuel II 2 system without the paint flaking issues. The engine reads 30 PSI...

forums.iboats.com

 

[alldodge]

Can this be tested? Or do I just replace it?

Kind of, you should be able to blow thru it with very little pressure from your mouth, don't blow hard. You should not be able to blow thru it with more pressure going back toward the tank

 

[Dremmick1985]

Well I found the culprit. Suprised it even ran. Hard chunk of of something plastic/glue and varnish

 

[alldodge]

Good deal

 

[nola mike]

Well I found the culprit. Suprised it even ran. Hard chunk of of something plastic/glue and varnish

Wow, where does that go?