Mercruiser 350 MAG MPI Hesitation...Serial # 0L009670

[will_v]

Hi guys, new to the forum. I have been working on this issue for too long now.

I have a 1983 ski nautique. The previous owner replaced the motor with the 1998 Mercruiser 350 MAG MPI. It starts and runs great except for quick accelerations. This is where it boggs down and stalls. Slow acceleration is fine. Being a ski boat, this is not too fun for the skier.
After reading another thread last week i checked the fuel pressure on the rail. It reads about 20-22psi. I pulled the vacume hose and it jumps to 30 psi and seems to fix it. Not sure if i should have 30 or 43 with the hose attached. I replaced the fuel pressure regulator. The pink marking. I also used another gas tank to see if something is wrong with my orignal (1983) tank, but got the same result. So i am now guessing one of the fuel lines (from separater to cool fuel) are restricted) or the fuel pump is the problem. How can i test the fuel pump to see if its performing like it should? Can i pull it out and take it some where?


with a bunch of other misdiagnositcs we replaced:
1. idle air controller
2. mapi sensor
3. fuel injectors
4. cap rotor
5. plugs and plug wires
6. coil
7. fuel pump regulator (pink marking) this might be wrong???????????
8. fuel line and anti syphon thing.
9. fuel filter/separator

Thanks for reading.
Will

 

[floater212]

my first thought is a fuel issue, fuel pump or the regulators. my boat has 2 regulators one on the pump itself and the other is on the rails by the injectors. the regulator on the pump keeps pressure at the set psi while the one on the rail varies to the engine's vacuum; low rpm needs less pressure so rail regulator lets more fuel back to tank, when you crack it wide open it needs all the pressure so the vacuum drops and closes the return and you have the full pressure to the injectors. you can look up to make sure you get the right regulator and pump at www.mercurymarine.com it will ask for serial numbers and you get the diagrams and part numbers you'll need.

 

[alldodge]

Hi guys, new to the forum. I have been working on this issue for too long now.

I have a 1983 ski nautique. The previous owner replaced the motor with the 1998 Mercruiser 350 MAG MPI. It starts and runs great except for quick accelerations. This is where it boggs down and stalls. Slow acceleration is fine. Being a ski boat, this is not too fun for the skier.
After reading another thread last week i checked the fuel pressure on the rail. It reads about 20-22psi. I pulled the vacume hose and it jumps to 30 psi and seems to fix it. Not sure if i should have 30 or 43 with the hose attached. I replaced the fuel pressure regulator. The pink marking. I also used another gas tank to see if something is wrong with my orignal (1983) tank, but got the same result. So i am now guessing one of the fuel lines (from separater to cool fuel) are restricted) or the fuel pump is the problem. How can i test the fuel pump to see if its performing like it should? Can i pull it out and take it some where?


with a bunch of other misdiagnositcs we replaced:
1. idle air controller
2. mapi sensor
3. fuel injectors
4. cap rotor
5. plugs and plug wires
6. coil
7. fuel pump regulator (pink marking) this might be wrong???????????
8. fuel line and anti syphon thing.
9. fuel filter/separator

Thanks for reading.
Will

:welcome: to iboats

Its starting sound like you need a fuel pump and if so your current fuel pump is 861156A1 and if it needs replacing the new number is 807949A1

Your motor does not appear to be the Black Scorpion just the 5.7 Mag inboard. The fuel pressure should be between 28 to 32psi. Testing procedure is shown below.

 

[will_v]

Thanks for the quick reply.
floater212: i just have the one regualator, its on with the fuel pump. i did replace the regulator last year. guess it could have got the wrong one. will check that site. (blocked at work)

AllDodge: love the diagram. guessing the same steps apply with the mpi model. will check fuel line this weekend and report back.

 

[alldodge]

Thanks for the quick reply.

AllDodge: love the diagram. guessing the same steps apply with the mpi model. will check fuel line this weekend and report back.

Was wondering why things was not adding up. Looked up your serial number and then the year of the boat and it didn't add up, should have read more closely your post. "New engine in 1998", will get back with shortly, have to look at another manual

My bad :facepalm:

 

[alldodge]

Checks are pretty much the same just slightly different. Your engine relies highly on the ECM setting codes for the OBD scanner to read. Without one of those troubleshooting gets harder.

 

[Tail_Gunner]

https://drive.google.com/file/d/0B3w...Z2s/edit?pli=1

start on page 22 yes your regulator is highly suspect.27 on spell's it out

 

[Fun Times]

The correct fuel pressure regulator is part number 807952A1 (Pink = 30 PSI.)

Service manual # 24, 90-861327 1 http://www.ebay.com/itm/1999-Mercur...327-/181704804683?hash=item2a4e73494b&vxp=mtr

You'll need to test to see if the fuel pump is capable of producing a lot more than 30 psi under load by restricting the fuel return line. You're looking for a solid 60 + psi.

Keep us updated please and good luck.

 

[will_v]

Tail_Gunner i read that link. now i am confused. It says to pull the vacuum when reading the fuel pressure. i did that and got 30 psi. So do i not have a fuel pressure problem?

 

[alldodge]

You should have 30 psi at 1800 rpm. If test 2 in post 6 passed then your injectors are not leaking. In step 4 and engine t idle, applying 10Hg vacuum (remove line from reg and connect to hand operated vacuum pump) is should drop by about 5psi, if it does not you need a regulator. If you were at idle and applied 10Hg and it did drop then fuel supply is restricted or low

 

[will_v]

Today i replaced the fuel filter and checked the lines from the filter to the pump. Nothing blocking the lines. After everything went back together no hesitation when i hit the gas. Filter was the only thing changed and it looked ok (from what i dumped into a ball jar) Still getting 22psi with the vacuum hose in and 30psi when unplugged. tried to purchase a piece to block the return line but it didn't fit. opps my bad.

will hit the water next weekend and see how it goes

on a side note, i get 2 alarm beeps when i turn the key to the on position. I cant find anything in google land about this. The beeps do not continue a few mins later. Not sure if its trying to tell me something or if this is the norm. From the mercruiser alarm pdf looks like i should get one beep.

should i start a new thread for this? Again new to forums.

 

[alldodge]

Today i replaced the fuel filter and checked the lines from the filter to the pump. Nothing blocking the lines. After everything went back together no hesitation when i hit the gas. Filter was the only thing changed and it looked ok (from what i dumped into a ball jar) Still getting 22psi with the vacuum hose in and 30psi when unplugged. tried to purchase a piece to block the return line but it didn't fit. opps my bad.

will hit the water next weekend and see how it goes

on a side note, i get 2 alarm beeps when i turn the key to the on position. I cant find anything in google land about this. The beeps do not continue a few mins later. Not sure if its trying to tell me something or if this is the norm. From the mercruiser alarm pdf looks like i should get one beep.

should i start a new thread for this? Again new to forums.

The two beeps are normal, tells you the computer has been initialized.