Low Fuel Pressure 2005 Mercruiser 4.3L MPI

[Tristan1985]

Hi all,
We have a Mercruiser 4.3L MPi 2005 model and we have just undertaken a full engine rebuild and we are now having persistent Low fuel Pressure issues.

Serial number - OW324384

The pressure is consistently around 33 PSI when running at 1800 rpm. When we turn the key a few times without starting engine the pressure can climb to around 40 but then as soon as the engine runs it drops to 33 and the pressure gauge needle does jump around a bit.

We have replaced with new fuel pump at time of rebuild, we then replaced the pressure regulator
when the flow pressure issue was discovered. However this didn’t fix the problem.

Some notes-

When you squeeze the return line from the pressure regulator to the fuel filter and you squeeze it right we can get the pressure to sit at 43, this is what made us think it was the regulator as the pump is capable of pumping to required pressure. But replacing the regulator didn’t fix the problem.
We have placed a second tank connected to the fuel filter to see if it was an issue with the tank/lines. No change.
We have even today rigged up a bike pump to pressurise the fuel line to the fuel filter to check for leaks between fuel filter and fuel rail. We originally found a leak where the fuel line attaches to the pump side of the fuel filter and thought this has to be it. We removed the nuts and retightened, no leak but problem still exists.

When the pump runs it sounds like it’s got air in it but I’m really not sure.
We are at a loss now as to what it is???
Thinking remains issues could be power related or faulty new pump, maybe collapsing fuel lines or air getting in somewhere still?

Any help please and thankyou.

 

[Fun Times]

What fuel regulator did you change, the one up on top of the engine at the fuel rail or the one down by the fuel pump? Hopefully the one down by the pump.

If you fully were/are able to be sure to block off the return line heading to the filter base housing assembly, then the pump should be able to easily build 60++psi...If it only went to up to 43, even though that (43) is the specification you need to reach under normal conditions, it would be to low and usually interacting a weak fuel pump.

That mentioned, though I get the sentiment of the testing for something quick and on the fly, please bear in mind that squeezing the line really isn't a foolproof way to get a proper testing to know for sure as some fuel may still happen to leak by and not allowing the pump to get to say 60 to 80ish+ psi that it can do when working right. It needs to be fully blocked off (deadheaded) to not leak by.

One can't really find/get the right shut off tool from Merc anymore but it would be Merc part number 91-805918A1 and look like this,

https://www.2040-parts.com/_content/items/images/97/1570397/001.jpg

Sell 805918A1, 91-805918A1 Fuel Shut-Off Valve Tool, Mercruiser in Rancho Cordova, California, United States, for US $53.00

Sell 805918A1, 91-805918A1 Fuel Shut-Off Valve Tool, Mercruiser in Rancho Cordova, California, United States, for US $53.00

www.2040-parts.com

Sounds like a weak pump to be honest.... If you changed the regulator on the fuel rail, then you didn't change the actual pressure.

4.3L MPI ALPHA/BRAVO - 0W310000 THRU 0W649999

4.3L MPI ALPHA/BRAVO - 0W310000 THRU 0W649999

www.mercruiserparts.com

Where did you buy the new pump? OEM or aftermarket?

GEN II COOL FUEL PUMP REPLACED BY PUMP / COOLER KITS | Service Bulletin | Crowley Marine

14-SEP-2004 - Manufacturer Service Bulletin: GEN II COOL FUEL PUMP REPLACED BY PUMP / COOLER KITS

www.crowleymarine.com

ELECTRIC FUEL PUMP TROUBLESHOOTING | Service Bulletin | Crowley Marine

30-JAN-2004 - Manufacturer Service Bulletin: ELECTRIC FUEL PUMP TROUBLESHOOTING

www.crowleymarine.com

 

[Tristan1985]

Thanks.
The regulator next to the pump was the one replaced.
Both the pump and the regulator were OEM.
With regards to the squeezing of the return line- if I squeeze it hard and block off fully. the pressure will reach 60+.
If I squeeze at just the right amount i can get it to sit at around 43.

More info- when the pump is running upon turning the key it is not a consistent noise, the pump seems to skip a bit, kind of like it has air in it and the pressure fluctuates when it makes this noise. I’m not sure it’s air just a guess.

Also, later today, when the engine was running, we played with some of the wires on top of the engine where the 2 relays are located and the pump stopped working and engine shut off. So now thinking maybe bad connection somewhere there? I removed both relays and put back on, engine started fine with same low pressure.. Will check the actual wire connections to those relay connection next time at boat.

I’m still at a loss.

 

[alldodge]

Double check the part number of the regulator, should be 861126A1

 

[Tristan1985]

Double check the part number of the regulator, should be 861126A1

Yes it’s definitely that number.

 

[Fun Times]

Pull the fuel regulator vacuum line off while the engine is running and see if the psi goes up roughly 5 psi also make sure there’s no fuel in that vacuum line/coming out of the regulator.

Plus may as well check for full battery voltage at the fuel pump wiring harness so you know for sure.

If you’re completely sure the pump can create 60+ then back to the regular…. Though the correct part number, possibly the wrong regulator was packaged.

Was the feel pressure low that made you have to change all this in the first place?

Also be sure the water separating fill filter isn’t double O-ringed for a possible source of air.

O-rings are good at the pump connections points?

 

[alldodge]

The Reg should have a Blue ring on the vacuum port

Try and test it.
Remove Reg and take to the bench. Use a air compressor and air regulator. Can use a air nozzle with a rubber tip. Place rubber tip on Reg port and slowly increase air until air starts to escape on the fuel line port. It should go to 43 but my guess like FT mentioned, someone put the wrong Reg in the box

 

[Tristan1985]

Pull the fuel regulator vacuum line off while the engine is running and see if the psi goes up roughly 5 psi also make sure there’s no fuel in that vacuum line/coming out of the regulator.

Plus may as well check for full battery voltage at the fuel pump wiring harness so you know for sure.

If you’re completely sure the pump can create 60+ then back to the regular…. Though the correct part number, possibly the wrong regulator was packaged.

Was the feel pressure low that made you have to change all this in the first place?

Also be sure the water separating fill filter isn’t double O-ringed for a possible source of air.

O-rings are good at the pump connections points?

The pump originally wasn’t working along with engine loss of compression which required us to rebuild engine and replace new pump at same time. It was only after doing that work where we realised the pressure wasn’t right, assumed it was the reg so replaced that, and now here we are and issue not resolved .

I will check voltage today, what’s best way to do this? Was also thinking about running power directly to pump and bypassing relay to test?

With regards vacuum line from reg, is this the return line from the reg back to fuel filter? I didn’t realise that works in vacuum, can you please explain how this works? Where would I disconnect this line to rest as you suggested?

Also is there a low pressure pump in the tank?

Not sure about o rings at pump , as can’t get to it anymore. But I pressure tester the whole system from fuel inlet to the fuel filter and there is no leaks.

When you say water separating fuel filter is that just the fuel filter? See photo

Thankyou.

 

[Tristan1985]

The pump originally wasn’t working along with engine loss of compression which required us to rebuild engine and replace new pump at same time. It was only after doing that work where we realised the pressure wasn’t right, assumed it was the reg so replaced that, and now here we are and issue not resolved .

I will check voltage today, what’s best way to do this? Was also thinking about running power directly to pump and bypassing relay to test?

With regards vacuum line from reg, is this the return line from the reg back to fuel filter? I didn’t realise that works in vacuum, can you please explain how this works? Where would I disconnect this line to rest as you suggested?

Also is there a low pressure pump in the tank?

Not sure about o rings at pump , as can’t get to it anymore. But I pressure tester the whole system from fuel inlet to the fuel filter and there is no leaks.

When you say water separating fuel filter is that just the fuel filter? See photo

Thankyou.

The Reg should have a Blue ring on the vacuum port

Try and test it.
Remove Reg and take to the bench. Use an air compressor and air regulator. Can use an air nozzle with a rubber tip. Place rubber tip on Reg port and slowly increase air until air starts to escape on the fuel line port. It should go to 43 but my guess like FT mentioned, someone put the wrong Reg in the box

I don’t remember what colour it was. But this is the second reg we have had on now, with same low pressure issue, so I don’t think it’s the reg.

It’s so much effort to get the regulator out to test, do really want to avoid that.last resort
Thankyou

 

[Fun Times]

The regulator hose his rubber…, item number 19 to 27 and runs up to the top of the engine.

https://www.mercruiserparts.com/bam/subassembly/31501/11705/60

 

[alldodge]

Check voltage to fuel pump at the fuel pump relay or fuel pump.

You have 2 relays on top the motor, one is main power relay (MPR) and other if FP. Turn relays over and which ever one the Red/Purple stripe wire, that's the MPR.

The FP relay has a Pink/Green and a Pink/Yellow wires. Pink/Green is power coming in to relay and pink/yellow is going out to FP. You can remove the relay and place a jumper across the 2 contacts. When key is turned ON the MPR will send power to the pump. With jumper in place and key ON measure voltage at the jumper

Yes water separating filter is the one in your pic

 

[alldodge]

BTW are you buying the Regulators from a dealer or ebay?
Dealer/Merc cost is $183

 

[Tristan1985]

The regulator hose his rubber…, item number 19 to 27 and runs up to the top of the engine.

https://www.mercruiserparts.com/bam/subassembly/31501/11705/60

Thanks, so that is the vacuum line you were rwfrrring to? Do I disconnect that on the fuel rail to do the test?

 

[Tristan1985]

Check voltage to fuel pump at the fuel pump relay or fuel pump.

You have 2 relays on top the motor, one is main power relay (MPR) and other if FP. Turn relays over and which ever one the Red/Purple stripe wire, that's the MPR.

The FP relay has a Pink/Green and a Pink/Yellow wires. Pink/Green is power coming in to relay and pink/yellow is going out to FP. You can remove the relay and place a jumper across the 2 contacts. When key is turned ON the MPR will send power to the pump. With jumper in place and key ON measure voltage at the jumper

Yes water separating filter is the one in your pic

Thanks will try and report back

 

[Tristan1985]

BTW are you buying the Regulators from a dealer or ebay?
Dealer/Merc cost is $183

Bought from Mercruiser parts supplier. Reg cost $235 Au

 

[alldodge]

Bought from Mercruiser parts supplier. Reg cost $235 Au

ok, try this; take the one you have replaced and do the test I mentioned above. I'm I would bet a lion light it's a 30 psi reg

 

[Fun Times]

In seeing your photo, this may sound slightly silly but you may also want to try a new Mercruiser fuel filter vs aftermarket Sierra Marine filter mostly because the OEM filter is 25 microns and the Sierra Marine one is 10 microns… Doing a quick search also suggests that microns can be restrictive and being that your fuel pump is more of an pull through verse push through, the filter might be a contributing factor as there is no pump in the tank ‘though some engine models do offer a low pressure pump located before the filter and I don’t believe your engine application has that option.

35-802893Q01 Fuel/Water Separating Filter
25 micron
Used in both outboard and MerCruiser applications (except 1992 MerCruiser 4.3 L Gen II engines equipped with an in‑line fuel filter & 1.6L Vazer and 3.0L MPI engines).
Interchangeable with 35-802893T

https://www.mercruiserparts.com/802893q01-filter-fuel

https://www.mercruiserparts.com/bam/subassembly/31501/11705/70

 

[vroom ZOOM]

Ugh, you need to do some proper diagnostics to find out what is going on. Don't be a parts cannon (otherwise your dealer parts guy will love you). Here is a little procedure you can follow, I hope it helps:

Disconnect the dump line from the cool fuel module at the filter housing. Throw a chunk of clear hose onto the line and stick the end of the clear hose into a suitable container. Empty oil jugs work great for this. Turn the key and look if there are air bubbles. Do you have lots of air in the fuel? If so, then the problem is most likely NOT the HP pump. Dump out the fuel you just collected into a glass jar, let it sit for 30 seconds, and make sure you don't have WATER in the fuel. This can also cause the pump to sound like its sucking in air.

With a fuel pressure gauge hooked up, turn the key and note the fuel pressure as soon as the pumps turn off. After the pumps turn off, does the fuel pressure stay (even if its too low) or drop slowly, or does it drop to 0 quickly? If the fuel pressure does not immediately (within 5 seconds) drop to 0, and your fuel pressure is not too high (which it is not in your case), the fuel pressure regulator is GOOD. They fail either stuck open (rail pressure drops really fast) or more often, stuck CLOSED (fuel pressure has reached the moon and beyond)

I would like to mention that the ENTIRE system is pressurized after the low pressure pump. The fuel filter housing and line feeding the HP pump are pressurized to about 6 psi AFAIK. Therefore, it is IMPOSSIBLE for you to be sucking in air from any component downstream of the LP pump. The only Orings are on the inlet of the HP pump, the fuel filter, and the fuel pressure regulator. If any were bad, they would be leaking fuel. You can sniff around them to see if they are leaking.

The only place air can get in is the tank pickup, or the line running to the LP pump. Take your clear hose, and attach between the LP pump and the line from the tank. Turn the key and watch for air bubbles. Also watch closely to see how fast the pump sucks up the fuel from the tank, it should be pretty much instant. If it takes more than 10 seconds the LP pump is defective. Also put in a gauge after the LP pump to test it. You could try to unplug the HP pump for diagnostic purposes.

Do a proper diagnosis before chucking parts at it.

If I had to bet, I would say its the LP pump or the plumbing to the tank causing issues, but PLEASE, don't throw parts at it until you have done some testing!