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Constant priming required


Mikexx

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My BMC will stop once in a while after running during the warm up period. It has the classic speed-up symptom before failing.

 

This will happen every so many hours of running and without warning. It's a pain, especially when I'm not on the boat.

 

We religiously turn off the fuel tap after every run, I'm ever fearful of the tank emptying itself into the bilge for one. Since diesel probably has a higher thermal expansion coefficient than most metals I am wondering if the fluid contracts and pulls in air from somewhere not obvious during the cool-down There are no leaks apparent in the fuel system.

 

Typically when I get to the boat I can undo the top screw of the fuel filter and there are some bubbles. However, given the outlet is effectively a dip-tube I believe the air should be above this point. I might see a small amount of air from the side bleed screw on the pump. It then takes a couple of goes of opening and closing the injector unions (plus the ones I mentioned) to get the engine running.

 

The fuel system is not a full recirculation type. Instead the fuel is returned to the agglomeration filter. Without adding taps and holes in the fuel tank making this into the ideal, a full recirculation system is not a trivial thing to do.

 

I shall be visiting the boat tomorrow and going to check on the other fuel unions to make sure they're tight, and also the tower screws as the gasket was changed a few years ago. I believe the fuel tank is sufficiently full that the fuel level is able the level of any pipework on the engine.

 

Is this common feature? Is there a solution to this? I will post again tomorrow when I've made a more comprehensive check.

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10 minutes ago, Mikexx said:

My BMC will stop once in a while after running during the warm up period. It has the classic speed-up symptom before failing.

 

This will happen every so many hours of running and without warning. It's a pain, especially when I'm not on the boat.

 

We religiously turn off the fuel tap after every run, I'm ever fearful of the tank emptying itself into the bilge for one. Since diesel probably has a higher thermal expansion coefficient than most metals I am wondering if the fluid contracts and pulls in air from somewhere not obvious during the cool-down There are no leaks apparent in the fuel system.

 

Typically when I get to the boat I can undo the top screw of the fuel filter and there are some bubbles. However, given the outlet is effectively a dip-tube I believe the air should be above this point. I might see a small amount of air from the side bleed screw on the pump. It then takes a couple of goes of opening and closing the injector unions (plus the ones I mentioned) to get the engine running.

 

The fuel system is not a full recirculation type. Instead the fuel is returned to the agglomeration filter. Without adding taps and holes in the fuel tank making this into the ideal, a full recirculation system is not a trivial thing to do.

 

I shall be visiting the boat tomorrow and going to check on the other fuel unions to make sure they're tight, and also the tower screws as the gasket was changed a few years ago. I believe the fuel tank is sufficiently full that the fuel level is able the level of any pipework on the engine.

 

Is this common feature? Is there a solution to this? I will post again tomorrow when I've made a more comprehensive check.

 

But as your system is not as advised by CAV do you not think that may be the reason for your problem? It may not be trivial, although I don't see anything particularly difficult because it would normally return to the top or right at the top of the side of the tank, it's the surest way of solving the problem. You could spend days looking for the one connection or tap that is letting a small amount of air into the system and even then it would not solve the problems caused by any air dissolved in the fuel. At present there is no way of any air trapped in your system to be got rid of so it builds up in the top of the engine filter until it is passed to the injection pump and stops the engine.

 

I suspect that either you are not telling us exactly what you do to bleed your fuel system or your technique is not the normal one.

 

EDITED TO ADD - I have assumed that this is a BMC 1.X or 2.X with a DPA injection pump.

Edited by Tony Brooks
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2 minutes ago, Tracy D'arth said:

Is the tiny air bleed hole in the banjo union on the top of the filter head clear?

 

It won't make any difference if it is as the OP seems not to be returning the excess fuel and a bit of air back to the tank: -

1 hour ago, Mikexx said:

The fuel system is not a full recirculation type. Instead the fuel is returned to the agglomeration filter. Without adding taps and holes in the fuel tank making this into the ideal, a full recirculation system is not a trivial thing to do.

 

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14 minutes ago, Tracy D'arth said:

Is the tiny air bleed hole in the banjo union on the top of the filter head clear?

 

There is a bolt, and if I slacken this off I do get some air bubbles, but not enough to suggest the top of the filter housing is full of air. This is after manual priming of the fuel lift pump.

 

There has to be quite a bit of air here trapped in the filter housing before it flows into the injection pump. Therefore I feel air is getting in post filter.

 

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1 hour ago, Tony Brooks said:

 

But as your system is not as advised by CAV do you not think that may be the reason for your problem? It may not be trivial, although I don't see anything particularly difficult because it would normally return to the top or right at the top of the side of the tank, it's the surest way of solving the problem. You could spend days looking for the one connection or tap that is letting a small amount of air into the system and even then it would not solve the problems caused by any air dissolved in the fuel. At present there is no way of any air trapped in your system to be got rid of so it builds up in the top of the engine filter until it is passed to the injection pump and stops the engine.

 

I suspect that either you are not telling us exactly what you do to bleed your fuel system or your technique is not the normal one.

 

EDITED TO ADD - I have assumed that this is a BMC 1.X or 2.X with a DPA injection pump.

 

Many thanks, yes, this is a 1.5

 

The system circulation is largely unchanged for the past 35 years as delivered to AngloWelsh with a minor exception, namely that the bleed off from the injectors flows back to the agglomeration filter and not to the main fuel filter. This mod was a decade or more ago and before my ownership. This issue has only appeared in the past 6 months or so.

 

I am aware of the ideal fuel system, and yes air can accumulate, but would have thought this would accumulate in the fuel filter and I find very little there.

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2 minutes ago, Mikexx said:

 

There is a bolt, and if I slacken this off I do get some air bubbles, but not enough to suggest the top of the filter housing is full of air. This is after manual priming of the fuel lift pump.

 

There has to be quite a bit of air here trapped in the filter housing before it flows into the injection pump. Therefore I feel air is getting in post filter.

 

There are toe "bolts" on the top of the standard engine filter for the small BMCs. The small one holds the whole thing together so I doubt you are undoing that one. The other, larger one, should be a hollow banjo bolt with a 0.5mm hole through the shank to bleed a little fuel and any air bubbles out of the filter head and back to the tank. That is what CAV, the designers of the system, say should happen. Now to be blunt you seem to be ignoring CAV and going your own way so no wonder you have hit problems.

 

Of course there is  a lot of air trapped in the filter head. It's made up of thousands of tiny bubbles and dissolved air that has not been passed back into the top of the tank as per the original design. If your injector leak off pipe runs back t that banjo bolts it might even originate from a slightly poor seat in an injector.

 

I will stick my neck out and say ALL MODERN diesels used in boats pass a degree of fuel back into the tank and in that  include ant with distributor type pumps. They do this to make them self bleeding (but bot for the DPAs) but also to avoid the problem you seem to be having.

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3 minutes ago, Mikexx said:

 

Many thanks, yes, this is a 1.5

 

The system circulation is largely unchanged for the past 35 years as delivered to AngloWelsh with a minor exception, namely that the bleed off from the injectors flows back to the agglomeration filter and not to the main fuel filter. This mod was a decade or more ago and before my ownership. This issue has only appeared in the past 6 months or so.

 

I am aware of the ideal fuel system, and yes air can accumulate, but would have thought this would accumulate in the fuel filter and I find very little there.

 

So are you saying the injector leak off is back to a water trap (agglomerator you said - not the best for that position) in the line between tank and lift pump but is the air is trapped in the top of the engine filter or water trap? I now suspect it may well be an injector problem but not bad enough to affect running or smoking until the air/gas builds up enough to be passed through to the pump etc. However, your system is wrong and will always be prone to such problems.

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51 minutes ago, Mikexx said:

This issue has only appeared in the past 6 months or so.

Probably because an air leak has appeared in the 35 year old pipework in the past 6 months or so.

If you have a fuel leak off pipe which returns to the tank, then the engine will cope with small amounts of air leakage without any obvious problems. Deny that air the opportunity to get out of the system and the consequence is inevitable. You may not want to put a tank return pipe in, but it is the only long term answer.

Edited by David Mack
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11 minutes ago, David Mack said:

Probably because an air leak has appeared in the 35 year old pipework in the past 6 months or so.

If you have a fuel leak off pipe which returns to the tank, then the engine will cope with small amounts of air leakage without any obvious problems. Deny that air the opportunity to get out of the system and the consequence is inevitable. You may not want to put a tank return pipe in, but it is the only long term answer.

 

Despite my reluctance I am considering this and the path it will have to take. I need to check fuel tank levels wrt to decking and where it will enter the tank. The pipe would pass through a gas locker which is not ideal. I will make a survey tomorrow.

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Not familiar with this set up but as a temporary measure could the return be channelled into a catch tank as seen on many older engines? At least it would pin down if that were the problem. 

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10 minutes ago, BWM said:

Not familiar with this set up but as a temporary measure could the return be channelled into a catch tank as seen on many older engines? At least it would pin down if that were the problem. 

Yes, providing it were large enough. All the time the engine is running it will have to cope with any leak off from the injectors plus what flows through a small hole in the banjo bolt in the filter top. I suspect it would be a BSS failure as well but it's a good test.

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I had a vaguely similar problem a few years ago with my 1.8, intermittently needing bleeding, after checking all unions, filters etc. I eventually tracked it down to a crack in the lift pump body. There was no sign of fuel leaking from the lift pump but it was managing to suck air in.

 

springy

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Leaving aside where the leak off pipe goes I once had very similar issues with air in my BMC 1.8 fuel system some years ago. Absolutely no trace of a diesel leak in any of the pipework. Eventually I pressurised the pipework using a tyre pump. Instantly bubbles appeared from a fitting that had appeared tight and shown no signs of a leak. A quick change of fitting (I wasn't messing about) , Problem solved ? 

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16 hours ago, springy said:

I had a vaguely similar problem a few years ago with my 1.8, intermittently needing bleeding, after checking all unions, filters etc. I eventually tracked it down to a crack in the lift pump body. There was no sign of fuel leaking from the lift pump but it was managing to suck air in.

 

springy

I had a similar problem with my engine, I think it might have been the brass fitting inside the fuel tank where the suction pipe passed through the top, hard to tell as I fiddled with so many bits of pipe but it was a sod to find and fix.

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I've had a similar problem with the engine in my boys boat, after working through a number of issues that effected a temporary improvement the root cause was unknown crap in the tank occasionally becoming stuck on the pick up - how this led to air entering the system i don't know but i assume the suction developed by the blockage...?

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I had a look today and bled the system and is now running.

 

There was very little air in the system.

There was no air in the fuel filter, once I had pressurised the LP side by pumping all that came out here was fresh diesel.

A miniscule amount of air came from the bled on the side of the pump.

I could tighten one screw 1/2 turn on the governor tower, and the other by a flat or so. I recall the gasket is a cork one and assume it has compressed a little.

I had a go at tightening other unions etc but they were already tight and didn't move.

 

I've got some research to do, I understand the filters take 1/2" UNF fittings.

 

Many thanks for those who have replied with their knowledge and experience.

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I had similar and eventually tracked it down to an air leak in the flexi inlet pipe to the lift pump. It was only after I managed to lift the deck board immediately it stopped that I noticed the tiniest trace of damp on the rubber held by the crimp fitting. If you can turn the spigot in the end of a flexi it needs replacing.

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  • 6 months later...

If you think there is air in the system but can't find the source, first of all check the fuel filter seals are good, wiggle the primer and try again, assuming there is in fact fuel in the tank rather than volts effect on the fuel gauge. If that does not sort it, find a small piece of glass tube and insert it into the fuel tank feed and then other pipes, to follow the air bubbles around to help figure out where they start entering the fuel syste., I did that once using 2 pipes cos the air in the return line was going round in circles basck to the feed. I finally found some expert marine fuel system plumber who had ran out of time installing the tank had a T piece too many, so he had in fact joined the return line to the fuel tank feed pipes just before the tank itself. It's also possible the chump had run out of fuel pipe, as there was a seperate fuel return line fitting only 6 inches from the T joiner. I just pulled the pipe off the T piece and then used a joiner to a one foot fuel feed line and all was sorted as requested.

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As we are reviving a very old thread....

I have a totally different engine which does circulate fuel back to the tank, but over the last couple of years have had increasing issues with air in the diesel.

After extensive testing I concluded that my system was air-tight but was actually sucking dissolved air out of the fuel.

Hence I also had a system that had worked fine for many years then slowly went wrong., I wonder if the increased % of FAME changes the dissolved air characteristic of diesel?????

 

After reading posts from Tony I made my own version of the BMC system to bleed a little fuel off the top of the second filter right back to the tank via an orifice, and its worked.

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1 hour ago, dmr said:

As we are reviving a very old thread....

I have a totally different engine which does circulate fuel back to the tank, but over the last couple of years have had increasing issues with air in the diesel.

After extensive testing I concluded that my system was air-tight but was actually sucking dissolved air out of the fuel.

Hence I also had a system that had worked fine for many years then slowly went wrong., I wonder if the increased % of FAME changes the dissolved air characteristic of diesel?????

 

After reading posts from Tony I made my own version of the BMC system to bleed a little fuel off the top of the second filter right back to the tank via an orifice, and its worked.

 

Well done, but in reality you have only hidden the basic problem, long may it continue. I can't offer a solution but I doubt it is the FAME. More likely gradual deterioration of a union, seal or component.

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52 minutes ago, Tony Brooks said:

 

Well done, but in reality you have only hidden the basic problem, long may it continue. I can't offer a solution but I doubt it is the FAME. More likely gradual deterioration of a union, seal or component.

 

I am 99.9% sure its sucking dissolved air out of the diesel rather than an air through a leak, just don't understand why it worked ok (ish) for so many years.

The fuel filters also play a part, fine for 50-100 hours with a new filter (tried two different manufacturers) then it failed (consistently) so the slight increase in filter resistance must also be a factor. Renewed just about every union and pipe whilst trying to track down this issue.

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13 minutes ago, dmr said:

 

I am 99.9% sure its sucking dissolved air out of the diesel rather than an air through a leak, just don't understand why it worked ok (ish) for so many years.

The fuel filters also play a part, fine for 50-100 hours with a new filter (tried two different manufacturers) then it failed (consistently) so the slight increase in filter resistance must also be a factor. Renewed just about every union and pipe whilst trying to track down this issue.

 

If the injector pump uses a transfer pump (in addition to a lift pump) then there might be a problem with the lift pump. If you have a filter element in the water trap (first filter) make sure you use a pocket filter in it, that is a Swiss roll type and not a pleated element. Often second line makers often supply a pleated filter for CAV 296 pocket ones. Pocket filters should take longer to clog. Depending upon how it was assembled the problem could even be with the dip tube inside the tan, especially if it seems linked to the tank fuel level. However if it is then  for ease I would keep using your modification.

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6 hours ago, dmr said:

 

I am 99.9% sure its sucking dissolved air out of the diesel rather than an air through a leak, just don't understand why it worked ok (ish) for so many years.

The fuel filters also play a part, fine for 50-100 hours with a new filter (tried two different manufacturers) then it failed (consistently) so the slight increase in filter resistance must also be a factor. Renewed just about every union and pipe whilst trying to track down this issue.

 

I'm 100% sure that you are defintely not sucking air out of diesel. There are a number of possible air related issues, but the first things first, have you tried my trick with the glass tubes to see where the air enters the system, or which component is defective ??

 

CAV 296 fuel filters are cheap junk designed for cheap trucks. Buy a conversion kit so you can use spin on fuel filters. They cost about 30 quid, according to which filter the kit maker supplies. A real top quality fuel filter should cost nearly a tenner including VAT and postage.

 Next issue is what happens if the lift pump is weak, or even blocked. That issue I've had with an owner who used a CAV 296 and a well know weak fuel lift pump made by Vetus, results in the same sysptoms as air in the fuel, if or rather when the secondary filters gets even a tiny bit dirty. The only solution is to fit a full flow primary, like A Racor 500FG with a coarse filter, rather than a fine one. In some cases with long fuel pipes lifting fuel up to the engine, it might need an electric fuel feed pump like some cars have, BUT buy a real marine one if you want it to last. 

 

Ouside chance but it is possible to get some very odd symptoms from a damaged HP fuel injection pump, but that is kind of rare, and it's much more likely to be a damage fuel feed pipe on the engine near that pump causing the issue. Make sure you have a good set of Copper washers, Copper fuel pipe and all the various made to measure fuel system fittings, as they can crack on high time diesels that vibrate too much. The fuel return fittings seem to cause more trouble than the actual injection system high pressure pipes. tends to result in fuel leaks rather than air.

 

Just had another look at the original post and I would start by routing the fuel return line direct to a fitting on the fuel tank. That must be done as it allows air in the system to vent down the return line and into the tank. If it is not routed correctly or the fuel return line is restricted by a bent pipe or similar, the engine will do some kind of mental act with the RPM all over the place, but no smoke. 

Edited by TNLI
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