RE: LOW FUEL PRESSURE?

[helix757]

Good evening!

I have a 6.2L MX MPI (Serial #0M099522). While crusing this weekend, over a sudden it felt like someone pulled the throttle back by 50%. I discovered, that now going over about 3200 RPMs causes this to happen. The engine does not stall, but bogs down.

I tested the fuel pressure, when you turn the key on, the gauge reads 25psi. Once you start the engine, reads about 22-23psi. When I did throttle only in neutral, it dropped to 20. I will test the pressure under load, but I am almost certain pressure is the culprit.

According to what I read online, since I have square fuel rails, it should be a 25psi system.

I already changed the fuel filter/water separator and same thing.

What is next on the list? Anti Siphon? Also, should I replace the fuel pressure SENSOR on the rail? I assume this electronically tells the regulator what to do?

 

[Fun Times]

Good evening!

I have a 6.2L MX MPI (Serial #0M099522). While crusing this weekend, over a sudden it felt like someone pulled the throttle back by 50%. I discovered, that now going over about 3200 RPMs causes this to happen. The engine does not stall, but bogs down.

I tested the fuel pressure, when you turn the key on, the gauge reads 25psi. Once you start the engine, reads about 22-23psi. When I did throttle only in neutral, it dropped to 20. I will test the pressure under load, but I am almost certain pressure is the culprit.

According to what I read online, since I have square fuel rails, it should be a 25psi system.

I already changed the fuel filter/water separator and same thing.

What is next on the list? Anti Siphon? Also, should I replace the fuel pressure SENSOR on the rail? I assume this electronically tells the regulator what to do?

Hi, there should be no need to replace the fuel pressure sensor at the fuel rail as it's designed to sound a warning horn when pressure gets way to low (I want to say 12psi but don't recall exactly at the moment) and maybe show psi on the scan tool...

Looks like your fuel pressure should be at 30 psi being your regulator is Merc part number 807952A1 which is rated at 30psi and a pink mark... Try removing the vacuum line off the regulator to see if the psi goes up roughly 5 psi which may help suggest the regulator is functional to certain degrees.

Could try bypassing the fuel tank and tank hose by bringing along an external fuel source to see if you're chasing a boat issue or an engine issue.

The following is of your fuel components suggested by your engine serail number, https://www.mercruiserparts.com/bam/subassemblydetail/36117940/28188320

Here's your complete parts catalog,

Genuine Mercury Marine parts, large inventory, fast shipping. MX 6.2L MPI BRAVO 0L680003 THRU 0M299999

MX 6.2L MPI BRAVO 0L680003 THRU 0M299999

www.mercruiserparts.com

While your engine doesn't appear to have come with a low-pressure fuel pump from stock, sometimes a low psi pump might be added on afterwards which would be mounted between the fuel tank and engine mounted fuel filter, that or a secondary inline fuel filter in between the tank and engine is possible too so you'd have to double check if something is there...If there is then sometimes the screens on the low psi pump could become blocked with debris so you'd have to pull it apart and check.

Did you see anything inside the fuel when you dumped it out in a clear container?

 

[helix757]

Hi, there should be no need to replace the fuel pressure sensor at the fuel rail as it's designed to sound a warning horn when pressure gets way to low (I want to say 12psi but don't recall exactly at the moment) and maybe show psi on the scan tool...

Looks like your fuel pressure should be at 30 psi being your regulator is Merc part number 807952A1 which is rated at 30psi and a pink mark... Try removing the vacuum line off the regulator to see if the psi goes up roughly 5 psi which may help suggest the regulator is functional to certain degrees.

Could try bypassing the fuel tank and tank hose by bringing along an external fuel source to see if you're chasing a boat issue or an engine issue.

The following is of your fuel components suggested by your engine serail number, https://www.mercruiserparts.com/bam/subassemblydetail/36117940/28188320

Here's your complete parts catalog,

Genuine Mercury Marine parts, large inventory, fast shipping. MX 6.2L MPI BRAVO 0L680003 THRU 0M299999

MX 6.2L MPI BRAVO 0L680003 THRU 0M299999

www.mercruiserparts.com

While your engine doesn't appear to have come with a low-pressure fuel pump from stock, sometimes a low psi pump might be added on afterwards which would be mounted between the fuel tank and engine mounted fuel filter, that or a secondary inline fuel filter in between the tank and engine is possible too so you'd have to double check if something is there...If there is then sometimes the screens on the low psi pump could become blocked with debris so you'd have to pull it apart and check.

Did you see anything inside the fuel when you dumped it out in a clear container?

Thanks for the reply.

No secondary fuel filter or low pressure fuel pump.

The cool fuel module is hard to get to without removing the port manifold and elbow. Rather than remove the vacuum line, would briefly pinching the return fuel line and watching the fuel pressure also be a way to verify the regulator?

I am going to verify a fuel pressure drop when the RPMs drop this week. I will also try running on an external tank to isolate boat vs engine.

Also, nothing really remarkable in the fuel. It was yellow (from last season), a few black specs, no visible water.

 

[alldodge]

The cool fuel module is hard to get to without removing the port manifold and elbow. Rather than remove the vacuum line, would briefly pinching the return fuel line and watching the fuel pressure also be a way to verify the regulator?

Remove the vacuum line at the intake manifold
Pinching the fuel return line should not be done, the pressure might increase enough to damage the regulator

Also, nothing really remarkable in the fuel. It was yellow (from last season), a few black specs, no visible water.

Gas is clear, seeing yellow means your gas is starting to phase separate

 

[Chris1956]

Yellow fuel is not always bad. Some gas used to be dyed. Anyone remember blue SUNOCO?

Put an oz of 2 cycle oil into the gasoline. It will color the gasoline and alcohol, but not the water. If it is separating, you should see it. Check the bottom of the fuel tank for water.

 

[helix757]

Remove the vacuum line at the intake manifold
Pinching the fuel return line should not be done, the pressure might increase enough to damage the regulator


Gas is clear, seeing yellow means your gas is starting to phase separate

I will try removing the vacuum like from the intake today and report back.

 

[helix757]

So I very briefly took the boat out today as it was very choppy and windy.

Firstly, removing the vacuum line did increase the pressure by about 5PSI.

During idle and underway fuel PSI was about 24-25PSI. Getting on plane and cruising was 30PSI — which I think is good because the regulator is made for 30psi?

However, the engine didn’t bog down this time. Not sure if changing the fuel filter did it, or its an intermittent problem. Granted, when this happened initially, it was after about 20 mins of cruising— maybe it only happens after some run time?

Next nice day I will run it again and see if it re-appears.

However, and I am not sure if this is me over analyzing because I never ran the boat under load without the engine hatch, it sounded like there was a squeal/whine. Not sure if its the fuel pump or maybe a pulley?

If its a fuel pump, is that indicative of a pump starting to fail? Or possibly an anti siphon restricting flow so it has to work harder?

Video in mid RPM range (not on plane)

Video at hot idle after coming back

 

[alldodge]

Don't hear anything that bad, the whine might come from the power steering pump

 

[helix757]

Don't hear anything that bad, the whine might come from the power steering pump

I will check the fluid level, it could be low.

What are your thoughts on the problem no longer presenting itself? Could it really be the fuel filter/water separator was clogged?

I am going to fill the boat up with fresh fuel and run her again for a while to see if it presents itself again, but just curious on your thoughts.

 

[DeepCMark58A]

My fuel water seperator was past due this spring boat still started and ran well enough to pull a skier but would not hit WOT. Changed out the filters good to go.

 

[helix757]

My fuel water seperator was past due this spring boat still started and ran well enough to pull a skier but would not hit WOT. Changed out the filters good to go.

Good to hear -- I really hope that is all it was!

Curious, did you pour out the contents of the old one? See anything remarkable?

 

[alldodge]

I will check the fluid level, it could be low.

What are your thoughts on the problem no longer presenting itself? Could it really be the fuel filter/water separator was clogged?

I am going to fill the boat up with fresh fuel and run her again for a while to see if it presents itself again, but just curious on your thoughts.

Could be but just guessing
In any case your pressure is back where it should be

 

[Chris1956]

This is a bit off topic, but I had a fuel pickup on my SeaRay sterndrive where the fuel pickup in the fuel tank had a filter under the pickup tube. It partially clogged one day when I took my son fishing. The clog would allow minimal planning, and got worst as the day progressed. We eventually came home at fast idle. Easy fix if you know what is wrong. Pull the fuel pickup tube, unscrew the fuel pickup tube over the filter, clean the fuel filter, and reinstall.

 

[DeepCMark58A]

Good to hear -- I really hope that is all it was!

Curious, did you pour out the contents of the old one? See anything remarkable?

I did not find any sign of water in the housing. My son was coming up to ski Friday night and I didn't have the new filter so I took a can of carb cleaner and blasted the filter ended up finding fine rust colored sediment in the bottom of the bowl. I picked up the new filter today a cross referenced WIX counter guy said boy they are proud of this filter $69 for a customer walking in off the street, he gave me the account holder pricing $29. Guess who has all my business?

 

[helix757]

This is a bit off topic, but I had a fuel pickup on my SeaRay sterndrive where the fuel pickup in the fuel tank had a filter under the pickup tube. It partially clogged one day when I took my son fishing. The clog would allow minimal planning, and got worst as the day progressed. We eventually came home at fast idle. Easy fix if you know what is wrong. Pull the fuel pickup tube, unscrew the fuel pickup tube over the filter, clean the fuel filter, and reinstall.

Thanks for the reply.

I don’t think this is too off topic at all.

I was considering replacing my picking and anti-siphon anyway.

Here is what my pickup looks like. I assume they welded the threads to the tank so I don’t have to worry about unscrewing that while doing the pickup?

Thoughts?

 

[helix757]

I did not find any sign of water in the housing. My son was coming up to ski Friday night and I didn't have the new filter so I took a can of carb cleaner and blasted the filter ended up finding fine rust colored sediment in the bottom of the bowl. I picked up the new filter today a cross referenced WIX counter guy said boy they are proud of this filter $69 for a customer walking in off the street, he gave me the account holder pricing $29. Guess who has all my business?

So just so I understand correctly, when you poured out the contents, you didn’t find anything unusual. It was only when you attempted to clean the filter with carb cleaner that you discovered the sediment it ended up holding — correct?

 

[alldodge]

Here is what my pickup looks like. I assume they welded the threads to the tank so I don’t have to worry about unscrewing that while doing the pickup?

No
They weld a fitting into the tank so the pickup tube gets screwed into it

 

[helix757]

No
They weld a fitting into the tank so the pickup tube gets screwed into it

Right. So I should be able to just unscrew it out — correct?

Im scared to death of it snapping off.

 

[alldodge]

Right. So I should be able to just unscrew it out — correct?

Im scared to death of it snapping off.

Yes, just don't force, I know that's a abstract answer, but that is all we have to work with