Mercury XR6 fuel loss

[colaynug]

Hello,
I appreciate any and all input ….. this issue has me stumped
2001 Mercury XR6 serial 0T303736
90% of the time after fishing when I go to start up and move to another location the fuel prime bulb has gone soft, if I don’t reprime the engine will start , run a very short time the die.
When this first started it was much worst in hot weather… now this winter it has gotten much worst than it has ever been in the past.
The odd thing is I can prime it at home , let it sit an hour and the bulb is still full primed and hard.
I don’t know if it is a coincidence or not but this problem started shortly after my tech removed the auto oil pump drive shaft and bypassed the oil lines so that I add oil manually.

I have rebuilt the fuel pump twice….. the last time with OEM parts
I have replace the prime bulb twice… again the last time with OEM parts.
The bulb is mounted at about a 45 degree angle. I can take it off prime it with water and it may leak off a tablespoon in a couple hours. Have done that test several times.
I have pressure and vacuum tested every line, joint and connection from the bulb to the bottom of the fuel tank and found nothing.

I have experimented by adding a high grade ball valve on the input side of the bulb and then later moved it to the line on the output side. If I shut this valve off when I finish running to a new spot and fish no matter how long I am there the bulb stays hard and fully primed.

Outside of this problem the engine runs like a dream
.
Thanks for your help.

 

[sam am I]

The bulb will go soft normally from being pumped up hard, it's on the vacuum side of the fuel pump.

When you pump it, you create a positive pressure (between the primer bulb check balls and the [closed] carb's float needle valves) in the line, when the motor runs, the line then goes to a negative pressure......Merc spec says the input line side of the fuel pump should run/remain around "-2.5 psi nominally" when the motor is idling.

It should however stay full'ish of fuel!

Wondering if your fuel tank is venting properly? Think if it were me and I was suspect of a fuel delivery issue, I'd be trying to eliminate the entire tank and any of it's feed path.....I just took a full'ish 2 gal can (plastic)sat it down in the motor well and poked the motors fuel line directly down into the can and ran down the lake..... Go from there based on results.

As it sounds, you probably should be eyeballing over that motor since someone tore into it to changed you over to adding oil, how does the bypass look? Anything maybe left loose? Did this guy follow the "standard method" of gutting it? ete, etc

 

[Dukedog]

removing tha oiler junk junk will not have any bearing on tha start and/or runnin' of tha motor at all.. did tha problem go away for a time then return after you did tha bulb and/or fuel pump fix?

 

[colaynug]

No sir....once it started it has never went away. I went thru the fuel cap venting system yesterday and it looked good. Went fishing.. problem still there. Going to try to find a portable fuel tank and try that to see what I can learn.

 

[Dukedog]

might check for a "new" plug right under tha starter.. that's where tha pluse for tha in boat oil tank was fed from.. it shouldn't effect anything with tha motor itself other than erratic idle and/or runnin' issue.. will make a mess on that side of cowl also.. think your on tha right track eliminating one thing at a time.. one thing you can do also is pick up a "cheap" electric fuel pump (car parts store, 4 ta 7 lbs pressure) to "by pass" tha bulb and mechanical pump.. if it runs as it should with electric ya down to one of two could-be's...

power tha 'lectric from "any purple" temporally...

 

[colaynug]

Thanks Dukedog...... I appreciate your input

One thing that REALLY puzzles me is why when I put the ball valve on either side of the bulb and shut it off each time I make a run and stop the bulb stays primed no matter how long I fish a spot . I was using the valve to troubleshoot the bulb but it works on both the input and output side of the bulb

 

[Dukedog]

that is real strange.. but does tha motor perform as it should with tha "ball valve" in place?.. easier ta start, run etc?.. I've heard, read on other sites that some of tha XR6's need tha bulb pumped every time it sits for any amount of time.. I don't know this as fact.. I've bought, built, run every version of tha 2.0, 2.5 and 2.5 V6 both high performance and production since 1976.. can't remember a time that was required (siting dead for 3 hrs or less) unless there was a problem somewhere in tha fuel delivery circuit.. but none tha less "they" say its not uncommon with some XR6's???

 

[sam am I]

My guess

Short of leaky fuel lines, if while sitting, engine off and having a primer bulb that is going significantly "soft"/empty of fuel after previously and recently being squeezed up to a positive pressure and full of fuel, the fuel is then draining out, but which way?

So from the get go, bulb squeezed, fuel fills in bulb and is under positive pressure(between bulb check valves and carb's needle valves), engine starts, fuel once under positive pressure changes over to fuel in a vacuum as the fuel pump draws/pulls fuel up from tank and in through the bulb toward pump's intake, engine stops,, negative line pressure goes essentially to zero'ish psi....

Could fuel leak/drain off toward carb's? not likely, it's sort of uphill and with no pressure and gravity, doubt it, . Must be draining out back downhill into/towards the tank?

But how?

Internally to the primer bulb, both check valve balls have small push springs that assist in pressing these check balls into their respective seats, they should stay seated and prevent fuel from draining/siphoning back towards the tank when the engine is stopped.

This then should, in theory, keep the bulb and associated fuel lines **above the last check valve balls (tank side) full of fuel and effectually "hard"/full of fuel if you were to squeeze it.

Unless....

The springs are not strong enough and are allowing the balls to not seat firmly enough......Might be happening, I've heard tale of wimpy springs these days.

If the manu's makes the springs too strong and only having -2.5 psi nominally combined with having to accommodate a varying head height probably anywhere from 1' to 5' from boat to boat, they have to make these springs vary light else they risk reducing forward fuel full too much from engine to engine, boat to boat........Shake the primer bulb, you should hear them rattling around, tells you their not to tough and if the bulb it tipped downward they might be cracking open just enough.

Gravity pulling the fuel back towards the tank should seat the balls even better but, if there's very little of that(not much head height), then there is no real pressure assist really that exists to firmly seat the balls, the springs seating the balls are the only thing prevent the fuel from leaking/siphoning back towards the tank.

What orientation is the primer bulb as it naturally rests in your boat? Is the bulb pointing downwards? I leave mine horizontal in the motor well.Ya might try tipping the primer bulb to where is rests vertical with arrow toward sky, gravity will assist the springs to seat the balls better.



**Realize that below the last check valve ball (tank side), the fuel can in fact drain back toward the tank, there isn't typically anything to prevent this

 

[racerone]

Perhaps fuel is NOT leaking out of the carburetors.-------Too much heat may be the issue here.----Replaced water pump impeller and maintained cooling system ??

 

[QBhoy]

Do you have a booster or electric pump in line before the bulb ? I had a v6 merc that did. Rightly or wrongly.

 

[colaynug]

My guess

Short of leaky fuel lines, if while sitting, engine off and having a primer bulb that is going significantly "soft"/empty of fuel after previously and recently being squeezed up to a positive pressure and full of fuel, the fuel is then draining out, but which way?

So from the get go, bulb squeezed, fuel fills in bulb and is under positive pressure(between bulb check valves and carb's needle valves), engine starts, fuel once under positive pressure changes over to fuel in a vacuum as the fuel pump draws/pulls fuel up from tank and in through the bulb toward pump's intake, engine stops,, negative line pressure goes essentially to zero'ish psi....

Could fuel leak/drain off toward carb's? not likely, it's sort of uphill and with no pressure and gravity, doubt it, . Must be draining out back downhill into/towards the tank?

But how?

Internally to the primer bulb, both check valve balls have small push springs that assist in pressing these check balls into their respective seats, they should stay seated and prevent fuel from draining/siphoning back towards the tank when the engine is stopped.

This then should, in theory, keep the bulb and associated fuel lines **above the last check valve balls (tank side) full of fuel and effectually "hard"/full of fuel if you were to squeeze it.

Unless....

The springs are not strong enough and are allowing the balls to not seat firmly enough......Might be happening, I've heard tale of wimpy springs these days.

If the manu's makes the springs too strong and only having -2.5 psi nominally combined with having to accommodate a varying head height probably anywhere from 1' to 5' from boat to boat, they have to make these springs vary light else they risk reducing forward fuel full too much from engine to engine, boat to boat........Shake the primer bulb, you should hear them rattling around, tells you their not to tough and if the bulb it tipped downward they might be cracking open just enough.

Gravity pulling the fuel back towards the tank should seat the balls even better but, if there's very little of that(not much head height), then there is no real pressure assist really that exists to firmly seat the balls, the springs seating the balls are the only thing prevent the fuel from leaking/siphoning back towards the tank.

What orientation is the primer bulb as it naturally rests in your boat? Is the bulb pointing downwards? I leave mine horizontal in the motor well.Ya might try tipping the primer bulb to where is rests vertical with arrow toward sky, gravity will assist the springs to seat the balls better.



**Realize that below the last check valve ball (tank side), the fuel can in fact drain back toward the tank, there isn't typically anything to prevent this

Sam,
I have the bulb at about 45 degree with the output side pointing up. Thinking that would help seat the check valves.
I have tried 2 new bulbs beside the one that was on it when the problem started. I even take the OEM bulb off and primed up with water in the house and would let it sit for a couple hours. It would leak back maybe 2 tablespoons at most. Nothing like what I am needing when I have to squeeze it 3 times or so to make sure the engine doesn't die after it starts.
Thanks for your help

 

[colaynug]

Do you have a booster or electric pump in line before the bulb ? I had a v6 merc that did. Rightly or wrongly.

I do not but have thought about doing that. Just that I am stubborn trying to find the problem. It use to work fine

 

[colaynug]

Perhaps fuel is NOT leaking out of the carburetors.-------Too much heat may be the issue here.----Replaced water pump impeller and maintained cooling system ??

Yes sir, have kept the cooling system in good order. When this problem started is was much worst in the hear of summer, now this winter it has gotten worst than its ever been .. I use to think it was heat related and maybe some carb evaparation ?

 

[dingbat]

You didn’t by chance remove the anti-siphon valve from the fuel tank? Could be stuck open as well

 

[racerone]

Find some of those stickers that you put on the motor block.----They are temperature sensitive.------Your motor is black and you might be surprised by what goes on with motor in the sun.-------I know I have some black wrenches that need to be cooled off before I can use them after time in the sun.----Stickers have 4 dots that turn black depending on temperature seen.----Available in many ranges of course.

 

[sam am I]

You didn’t by chance remove the anti-siphon valve from the fuel tank? Could be stuck open as well

Ding makes a good point and If it has an anti-siphon valve, although i think not required by CG as long as the fuel line leaving the tank always stays above the tank...IO or OB, therefor not all boat have them, but some do, I guess I haven't looked to know ever either way.....Not totally clear to me why, never really checked too deep into this, perhaps someone else knows

At any rate, they can ALSO go bad by sticking closed intermittently I guess as legend has it, have drove many a sane men mad with fuel related issues!....Hmmmmm(insert mysterious music)

If this were the case....with the engine running/pump drawing on a plugged line, that would tend to draw the bulb down soft eh?

Were you able to test with that suggested 2 gal plastic tank yet? Or use a pressure/vac gauge inline? Merc spec states, a negative pressure level in fuel line (pump intake side) lower(more negative) than -2.5psi at idle is to much.....And to start looking for restriction down stream (tank wise) of pump intake as an issue in their trouble shooting for fuel related issues.



After I rebuilt my engine, I set this up on my engine few years back, should be close to yours. The "Oil Pump Out" reading is also the fuel line intake side fuel pump. The "Fuel" reading is at the carbs after the pump

 

[dingbat]

Ding makes a good point and If it has an anti-siphon valve, although i think not required by CG as long as the fuel line leaving the tank always stays above the tank...IO or OB, therefor not all boat have them, but some do, I guess I haven't looked to know ever either way.

Most OEM install anti-siphons on permanently installed tanks as a precautionary measure. Every boat I’ve own had one.

Without a functioning anti-siphon, the fuel in the line will drain back into the tank if you have a leak in the fuel line between the tank and the fuel pump.

Been there, done that....

 

[sam am I]

Without a functioning anti-siphon, the fuel in the line will drain back into the tank if you have a leak in the fuel line between the tank and the fuel pump.

Been there, done that....

Got that part, wasn't clear to me as to why apparently the anti-siphon valve isn't CG required (OB or I/O) if the fuel line from the tank to the motor stays above the tank its entire run (no dips, etc)......It's a tangent and off topic.

https://www.greatgrady.com/threads/a...-or-foe.25603/

I actually run a "Y" type so my kicker and OB (both share same tank feed) don't pull and suck air from one or another if one or the other float's drop and their needles open...Another tangent but PERHAPS "on topic" if OP was running a kicker but, he's not?

Anyway, gotta check that feed line by using that stand in gas can or put a gauge on it.

 

[Dukedog]

tha only things I had problems (very few) with tha wmv carbs after eliminating bulb and/or mech. fuel pump were two things.. thermal valve, messed up idle and/or starting, especially starting.. other was float seats working loose which caused to little fuel in tha bowl.. messed up higher rpm and caused motor ta run terrible and/or die off idle…

bite tha bullet and put tha 'lectric on it.. if it works use it that way while ya still lookin' for original problem....

 

[colaynug]

You didn’t by chance remove the anti-siphon valve from the fuel tank? Could be stuck open as well

I haven't removed it... but I don't think it was there when i bought the boat. Would it be in the 90 steel elbow that screws into the top of the fuel tank? If so it is hollow. Line runs from bulb to this elbow and the elbow has a line and screen running to bottom of the gas tank