Mercruisier 7.4 litre gas pressure

[raynald]

Hi I have a 2001 bayliner ciera 2855 with a 7.4 litres moi engine. My gas pressure is 35 lbs at idle and drops to 32 lbs at 2000 rpm. I have removed fuel cooler cleaned it out replaced fuel pump and regulator. Also changed gas filtrer. Gas lines changed connector to fuel tank and tried with seperate gas tank. Still not getting the good pressure which should be at least 45 lbs. Need your help. Thankyou

 

[alldodge]

Welcome
What is the motor serial number?
What is the Regulator part number that was replaced?

 

[raynald]

My serial number is is 0m009670
And regulator number p/n 710-861126a1 quicksilver

 

[alldodge]

Thank you

The S/N and Reg part numbers are correct and you should have 43 psi Reg

Think of a Reg as a check valve set to a specific setting, in this case it's 43 psi
The good fuel pump can pump fuel above the Reg pressure which in most cases is at least 60 psi. Anything above 43 psi is released back to the fuel filter.

The inability of the fuel pressure to reach 43 psi means either the fuel pump is defective or the Reg is

The Reg is easiest to test with air pressure and a compressor and air regulator. Remove Reg and place on bench. Attach air to base of Reg and slowly increase pressure until Reg opens and expels excess thru Reg, note when this happens

The fuel pump will need to be bench tested attaching gauge to output of pump. Bleed air out then run pump noting if pump is able to produce pressure at least 60 psi, should be more

 

[raynald]

Wé just found out that the new fuel pump we installed is a jobber ( ciera ) just ordered a new one but a Quicksilver it is one that says low fuel pressure with fuel boost. This pump number is 861155a3 sold by Land N Sae . Lets hope it works and my next choice is a kit they came out with vas à new pump and comes with the fuel cooler. Oh yeah also changing the régulateur again in case it is defective.

 

[alldodge]

Fuel pump 861155a3 is a boost pump, meaning it is installed between fuel filter and cool fuel pump. The pump was a add on pump to help stop vapor lock on motors. This pump does not produce the 43 psi your looking for

The HP pump you need is 861156A1 which is NLA from Merc but can be found on line

 

[raynald]

Wé just found out that the new fuel pump we installed is a jobber ( ciera ) just ordered a new one but a Quicksilver it is one that says low fuel pressure with fuel boost. This pump number is 861155a3 sold by Land N Sae . Lets hope it works and my next choice is a kit they came out with vas à new pump and comes with the fuel cooler. Oh yeah also changing the régulateur again in case it is defective.

861156A1
Not in stock and is no longer made - we do not have any
Manufacturer: Mercury Marine

 

[alldodge]

861156A1
Not in stock and is no longer made - we do not have any
Manufacturer: Mercury Marine

It's still being made but not supplied by Merc
Merc never made the pump to begin with, it can be found just by searching the number

 

[Scott Danforth]

Carter made the fuel pumps. Airtex owns Carter

Stay away from chineseum brands you can not pronounce

 

[raynald]

Fuel pump 861155a3 is a boost pump, meaning it is installed between fuel filter and cool fuel pump. The pump was a add on pump to help stop vapor lock on motors. This pump does not produce the 43 psi your looking for

The HP pump you need is 861156A1 which is NLA from Merc but can be found on line

We have now changed the fuel pump again also the fuel cooler. This is the kit that has a bulletin as the replacement part. Now when we test it we get 30 lbs gas pressure and when we unplug the vacuum hose from the regulator we get 40 lbs of presseur. We went to try it and with the vacuum hose unpluged but cap off we get our 4400 rpm and a top speed of 60.8 kmh. Ifind this great when we reconnected the hose we get a max of 4100 rpm at 59.6 kmh. We followed the vacuum hose it brings us to the rear of the engine near the distributor cap and the vacuum hose has a T joint one hose goes into the engine the other into another valve that looks like a regulator. We are unable so far to find out what is this piece is this my problem to maintain my 40 lbs of pressure. I will try to attach some pictures if anyone can help identify this part. Thankyou

 

[alldodge]

If it's connected to the rear of the fuel rail then it's item 5 and is called a fuel Reg. The unit is more a damper then a Reg but that's what it's called
https://www.mercruiserparts.com/bam/subassembly/31918/2705/130

The Boost reg in your first pic I don't know what that would do. The Reg on the cool fuel should keep it at 43 psi

The difference between no vacuum line connected and with line connected is no much. It does concern me that your only getting 30 psi at idle

What cool fuel regulator was installed (part number)?

 

[dubs283]

That is a regulator that is crimped/stamped in place at the aft end of the fuel rail. Mercruiser calls it a damper which lessens the feedback pressure in the fuel system from the injectors pulsing and is not used for pressure regulation

It's a GM part that is non serviceable and AFAIK is NLA. You'd have to source a complete fuel rail, most likely used to replace it

The only time I've had a failure related to the damper is an extremely rich running condition at idle due to the fuel passing through it into the vacuum line and then the manifold

If there is no fuel present at the vacuum line connection with the engine running it most likely is okay.

 

[raynald]

If it's connected to the rear of the fuel rail then it's item 5 and is called a fuel Reg. The unit is more a damper then a Reg but that's what it's called
https://www.mercruiserparts.com/bam/subassembly/31918/2705/130

The Boost reg in your first pic I don't know what that would do. The Reg on the cool fuel should keep it at 43 psi

The difference between no vacuum line connected and with line connected is no much. It does concern me that your only getting 30 psi at idle

What cool fuel regulator was installed (part number)?

This is the Fuel regulator that I have just installed,

 

[alldodge]

That's the correct Reg

Things don't add up and I would look at bench testing the Reg.

Apply air pressure of bottom of Reg with adjustable air Reg. Slowly bring air pressure up to see it should open at 43 psi.

Reduce air and then with a hand operated vacuum pump apply vac to port and bring it up to 12-14 in Hg. Then bring air pressure back up to see what the changes are

 

[Bt Doctur]

Just to add to the mix but has anyone checked the anti-syphon or the electric fuel shut off solenoid? Might try putting a vac gage in the fuel line to see if there is excessive vac present indicating a restriction in the fuel system.

 

[alldodge]

Been thinking about this pic

This is a block diagram pic to make the system more clear of how it works.
Example: the fuel pump is behind the plate, so Reg is also shown separately

 

[raynald]

Been thinking about this pic

This is a block diagram pic to make the system more clear of how it works.
Example: the fuel pump is behind the plate, so Reg is also shown separately

In this picture where you see the metal plate with the two hoses behind the plate you have a fuel pump and the regulator the other item in the circle seems to be the damper valve. But where the plate is and regulator thats where we unpluged the vacum hose and get 40 psi. I have ordered another regulator to find out if this one is not defective. The test run we did with vacum disconnected got 4400rpm and 61.8 kmh and when it is pluged max 4000 rpm and 54 to 55 kmh.

 

[alldodge]

The damper Reg is attached to the fuel rail

 

[raynald]

The damper Reg is attached to the fuel rail

Yes the damper seems to be attached to the fuel rail. I was told that if no gas comes out of the vacum to the damper then the damper is good.

 

[alldodge]

Agree