Fuel Pump Prime

[first230sl]

We were just leaving the marina on the way back from an 8-day trip when after 10 minutes of running at idle, the stbd engine quit. Was able to make it run (at idle) if I kept the fuel prime pump running.

After some diagnosis I decided it was the fuel pump (8V71TI by the way). Got a new pump and installed it. When I ran the engine with the new pump, it would not pump fuel from the tank (I tested this by putting the return line into a bucket). But when I tried to have it suck fuel from a bucket instead of from the tanks, it pumped fine. Put the hoses all back together and everything seems fine now.

So I have two questions:
1) These pumps (according to this experience) are clearly not able to self-prime from the tank, but my prime pump bypasses the fuel pump and only fills the filters and some of the lines (doesn't fill the hose leading to the pump). What is the correct procedure to prime the fuel pump itself if you lose prime? Disconnect the input line and put it in a bucket of fuel like I did? I coould see losing prime again once I take the line out of the bucket to reconnect it. Or is there some other way?

2) There is a small little nagging voice in the back of my head that asks whether the original pump (that I repaced) was actually OK, but had just, for some unknown reason, lost prime. I doubt it because there was no reason for it to lose prime. Tanks full, I was not fiddling with any hoses, etc. I also checked it by putting the intake into a bucket and it would not draw fuel. But how finickey are they in pulling air? Could it have just been an air-lock that caused it to lose prime and I replaced a perfectly good (but very old) pump? There were no visibly obvious defects with it. The releif valve plunger had a couple shiny spots one the sides - but certainly nothing obviously broken with the pump.

BTW - I was so impressed that the fuel pump for this beast was only $107 Canadian - about $80 US. I know they are not the most efficient engines, but they are simple, parts are cheap, and DD was open until 10:00pm on a Saturday night for after-hours parts! Gotta love that!

Best regards - Murray

P.S. - Had a great trip and looking forward to the rest of the summer.

 

[Pascal]

i'm sure our resident expert, Karl, will have some good advice for you (and the rest of us)

fwiw, i don't see why your priming pump is set up not to prime the pump. are you saying it's bypassing the DD pump and directly pulling fuel from the tank, thru the primary and directly into the secondary and onto the injectors?

i thought it was better to set it up between the primary and the fuel pump, to pull fuel into the primary and pushing it to the pump.

i've had occasional priming issues on the port engine, always at or immediately after start up though never while running, and have always reprimed by forcing fuel from the primary to the DD pump. either with a hand pump, electric pump, or even once when i didn't have any of the above, by feeding the DD thru the genset electric pum and primary.

there as been a few thread on this, including the one in the FAQ about Karl's set up.

 

[first230sl]

Yes - the prime pump is indeed set up to draw from the primary and push into the secondary *bypassing* the fuel lift pump. So the prime pump fills all the filters, but misses the lift pump and about 4 feet of intake hose leading up to the lift pump.

I asked my mechanic about it and he said that it would damage the lift pump to push fuel through it (using a prime pump) if the lift pump (engine) is not running - so I was not to change the prime pump setup.


Best regards - Murray

i'm sure our resident expert, Karl, will have some good advice for you (and the rest of us)

fwiw, i don't see why your priming pump is set up not to prime the pump. are you saying it's bypassing the DD pump and directly pulling fuel from the tank, thru the primary and directly into the secondary and onto the injectors?

i thought it was better to set it up between the primary and the fuel pump, to pull fuel into the primary and pushing it to the pump.

i've had occasional priming issues on the port engine, always at or immediately after start up though never while running, and have always reprimed by forcing fuel from the primary to the DD pump. either with a hand pump, electric pump, or even once when i didn't have any of the above, by feeding the DD thru the genset electric pum and primary.

there as been a few thread on this, including the one in the FAQ about Karl's set up.

 

[Genesis]

DD fuel pumps IN GOOD CONDITION absolutely will suck fuel from a tank.

They are gear pumps though, and rely on VERY tight tolerances between the gears and housing to operate properly. They also have a pressure regulator that can stick open in them, and cause them to either refuse to suck a prime or not pump at all.

In any event, unless you are prepared to work on a high-precision device the best advice is to replace one that is acting up. They're not expensive, as you discovered.

Note that a NEW pump should not be run dry to prime itself, as it has nothing in it to lubricate the mechanism.

Beware that very small AIR leaks on the suction side will cause a pump to lose prime and/or refuse to re-prime.

 

[first230sl]

Interesting. When I put the new one in, it would not suck from the tank (at least there was absolutely no output from the return line - none at all). It primed fine when I put the intake line into a bucket though and worked fine after that. And this was a new pump.

Also - fortunately it seemed to come out of the box with a good coating of oil in the gears.

Maybe on my old pump the pressure regulator plunger stuck in some position to cause it to fail. Who knows. If the new one keeps working after some time I'll rest easier.

Take care.

Murray

DD fuel pumps IN GOOD CONDITION absolutely will suck fuel from a tank.

Note that a NEW pump should not be run dry to prime itself, as it has nothing in it to lubricate the mechanism.

Beware that very small AIR leaks on the suction side will cause a pump to lose prime and/or refuse to re-prime.

 

[Nonchalant1]

Murray,

DD mechanical fuel pumps are very slow when they are pumping a lot of air. Maybe the hose length and size from the tank would have taken a LONG time but a short hose from the bucket showed results?

Oh well.....it's fixed now.

Doug

 

[Genesis]

Interesting. When I put the new one in, it would not suck from the tank (at least there was absolutely no output from the return line - none at all). It primed fine when I put the intake line into a bucket though and worked fine after that. And this was a new pump.

Also - fortunately it seemed to come out of the box with a good coating of oil in the gears.

Maybe on my old pump the pressure regulator plunger stuck in some position to cause it to fail. Who knows. If the new one keeps working after some time I'll rest easier.

Take care.

Murray

It would have reprimed, but it takes a while. The secondary cannister is full of AIR and that has to be pushed through. If there is air in the primaries too (especially if they're racors, which are huge volume-wise) then it will take a LONG time.

A stuck-open regulator (or a failed spring in the regulator well) will cause the pump to appear to fail - the pump itself is fine, but the stuck-open valve prevents the pump from developing pressure.

These pumps are VERY reliable; most of the time when they fail it is due to a seal failure which causes a fuel leak out the weep hole. Failure to pump is nearly always caused by a fault in the pressure regulator.

 

[first230sl]

In my case the primaries and secondaries were full because my prime pump filled them.

Also, I don't think it was pumping at all because nothing, not even air was coming out of the return line. In fact, with the engine running and the return line in a bucket, the fuel movement, if any, was *backward* into the return line toward the engine. I imagine it was partly feeding the running engine (which subsequently stopped, of course).

And yes - my weep holes were dry before replacing the pump, the shaft was turning, and there were no obvious pump problems with the original pump (except that the engine did not run).

So - my guess is that it was either a stuck releif valve, or loss of prime issue. If it never comes back, then I'll assume it was a stuck releif valve, and I won't feel bad about replacing a good pump (which I don't mind that much anyway - it is now new, did not cost much, and was an excellent learning experience).

Take care and thanks for all the feedback - Murray

It would have reprimed, but it takes a while. The secondary cannister is full of AIR and that has to be pushed through. If there is air in the primaries too (especially if they're racors, which are huge volume-wise) then it will take a LONG time.

These pumps are VERY reliable; most of the time when they fail it is due to a seal failure which causes a fuel leak out the weep hole. Failure to pump is nearly always caused by a fault in the pressure regulator.

 

[Genesis]

If the SECONDARY was full that's beyond the pump (its on the pressure side) and the engine should have been running.

Many people don't realize that the secondary "last chance" filter on the engine is on the PRESSURE side of the pump. If its full of air due to loss of prime then you must push fuel through IT before fuel gets to the rail. This takes quite a while!

If the relief valve does not seal COMPLETELY, air in the fuel pump will bypass at the valve and the result is no re-prime. A new pump often has a valve that is SLIGHTLY open due to assembly lube.

However, a properly-working pump absolutely will prime the engine on its own. I've done it myself using nothing other than the starter. Its hard on the starter, but it does work. I now have a prime/polish setup, but didn't until this year..... and HAVE had to use the "prime the hard way" method.

 

[first230sl]

Hey Karl - yes - I realize it (the secondary) is on the pressure side, and indeed it was full because the prime pump pulls from the primaries, and pushes into the secondaries, bypassing the fuel pump. So about 5 feet of suction-side hose and the fuel pump itself are out of the prime loop.

However, the fuel pump *was not* pushing the fuel that was in the secondaries through into the engine.

That leads me to believe (and hope) that the issue was the valve. Time will tell. But it still does not explain why the new pump would not initially move fuel at all until I put the intake hose in a bucket of fuel to make the initial pump draw easier. I can live with the mystery as along as everything continues to work fine from now on. If not - I'll be back!

Thanks again!

Murray

If the SECONDARY was full that's beyond the pump (its on the pressure side) and the engine should have been running.

Many people don't realize that the secondary "last chance" filter on the engine is on the PRESSURE side of the pump. If its full of air due to loss of prime then you must push fuel through IT before fuel gets to the rail. This takes quite a while!

If the relief valve does not seal COMPLETELY, air in the fuel pump will bypass at the valve and the result is no re-prime. A new pump often has a valve that is SLIGHTLY open due to assembly lube.

However, a properly-working pump absolutely will prime the engine on its own. I've done it myself using nothing other than the starter. Its hard on the starter, but it does work. I now have a prime/polish setup, but didn't until this year..... and HAVE had to use the "prime the hard way" method.

 

[Genesis]

I can believe it on a new pump is the relief valve was cracked open as a consequence of assembly lube..... it takes VERY LITTLE opening for the pump to pull and recirculate air instead of fuel in a priming situation.

 

[Pascal]

curious... how did you get that 3rd bolt out to change the pump? i looked at it once, when i had a prime problem, and couldn't figure out how to get it out? what's the trick?

 

[first230sl]

Yes - when I picked up the pump I asked if they had the special wrench (obstruction or governor wrench) that is designed to do the job. They, of course, did not.

So first I tried bending a really cheap wrench I had. It did bend without breaking, but I could not get it into the needed shape.

So then I though I might be able to get at the bolt by going *under* the fuel pump (instead of beside it). To get the needed space and clearance I had to remove the stop solenoid and linkage, as well as the crossover fuel-return line (and 90 degree fitting that it connects with) that ties the two heads together.

I then took a normal 1/2" wrench, tucked it under the pump (from the outboard side) and there was enough access and swing clearance to remove and replace the bolt (and even tighten it with confidence on reassembly). Remember this was my starboard engine. When I told my mechanic he was really impressed as he thought the custom wrench was an absolute necessity!

Take care - Murray

curious... how did you get that 3rd bolt out to change the pump? i looked at it once, when i had a prime problem, and couldn't figure out how to get it out? what's the trick?