Bleeding fuel filters!

[richardf]

It was annual service day yesterday and I was joined by Dick Goble who always helps out my Kelvin P4. We were both dreading changing the fuel filters as Dick says that it is always a problem with my engine. I have what I now know to be a Simms dual filter set up - that has a common top for both filters. We initially dropped out the downstream (furthest from tank) filter and bled that successfully (our hope that by leaving fuel in the pipework to the pump that we might make restarting easier) - fuel control being done by using the main fuel valve near the top of the tank. The fuel soon flowed without any bubbles - so we set about changing the primary one nearest the tank. We were unable to get any fuel through without loads of air bubbles. Initially Dick thought it might be sucking in air somewhere, but I cannot see why, it was only running through by gravity and hence I would assume that if air can leak in, the fuel would run out. Dick has taken the filter assemply away to test on a gravity tank - in the hope that he can find a crack or something that might explain it. I have a feeling that this is not the case, but we were both left wondering what was going on. Maybe we were not running it through fast enough to completely fill the supply pipe - it was a very hot and frustrating day! Somehow bleeding fuel systems has always defeated me! Has anyone got any ideas what we might be doing wrong?

Thanks

[Iain_S]

(snip) We initially dropped out the downstream (furthest from tank) filter and bled that successfully (our hope that by leaving fuel in the pipework to the pump that we might make restarting easier) - fuel control being done by using the main fuel valve near the top of the tank. The fuel soon flowed without any bubbles - so we set about changing the primary one nearest the tank. We were unable to get any fuel through without loads of air bubbles. Initially Dick thought it might be sucking in air somewhere, but I cannot see why, it was only running through by gravity and hence I would assume that if air can leak in, the fuel would run out. (snip)

 

Is the fuel take off from the tank near the top, with an internal tube dipping into the tank? If so, this will probably have filled with air when the filter was removed, or when the fuel cock was opened after filter replacement.

 

However, if that was the case, I don't see how any fuel would flow through at all.

 

Iain

[richardf]

Is the fuel take off from the tank near the top, with an internal tube dipping into the tank? If so, this will probably have filled with air when the filter was removed, or when the fuel cock was opened after filter replacement.

 

However, if that was the case, I don't see how any fuel would flow through at all.

 

Iain

 

Yes, the valve is tucked up just below the counter

[WotEver]

Is the fuel take off from the tank near the top, with an internal tube dipping into the tank? If so, this will probably have filled with air when the filter was removed, or when the fuel cock was opened after filter replacement.

 

However, if that was the case, I don't see how any fuel would flow through at all.

 

Iain

Maybe fuel ISN'T flowing - just bubbles.

 

Tony

[RLWP]

Maybe fuel ISN'T flowing - just bubbles.

 

Tony

 

Good point. Is the fuel tank empty?

 

Richard

[richardf]

Good point. Is the fuel tank empty?

 

Richard

 

Nope - full to the brim, well less a gallon so so that was wasted yesterday

[WotEver]

Nope - full to the brim, well less a gallon so so that was wasted yesterday

Are you getting lots of diesel out of the filter with some bubbles, or are you just getting bubbles?

 

Tony

[richardf]

Are you getting lots of diesel out of the filter with some bubbles, or are you just getting bubbles?

 

Tony

 

Lots of diesel with bubbles....

[WotEver]

Lots of diesel with bubbles....

In that case I'd think that Dick Goble's diagnosis is probably close to the truth.

 

Tony

[colin stone]

My filter is a twin unit with changeover valve. Fuel take off is through top side of tank, with tube to bottom inside, and filter tops are above tank top level.

I had very similar problems with air in fuel to the extent on our maiden trip the engine shut down on the Bergse Maas and we sailed across the river before emergency anchoring. Took me a while to work out what the issue was, but transpired that the filters liked to be on a pressure side rather than suction side. I made a detachable bleed rig which I could supply fuel from above the filters and fill up the filter housings and pipework. Once bled supplying fuel from above, had no problems in 2 years. I also make sure that the filter bowls are as full as possible before refitting.

[richardf]

My filter is a twin unit with changeover valve. Fuel take off is through top side of tank, with tube to bottom inside, and filter tops are above tank top level.

I had very similar problems with air in fuel to the extent on our maiden trip the engine shut down on the Bergse Maas and we sailed across the river before emergency anchoring. Took me a while to work out what the issue was, but transpired that the filters liked to be on a pressure side rather than suction side. I made a detachable bleed rig which I could supply fuel from above the filters and fill up the filter housings and pipework. Once bled supplying fuel from above, had no problems in 2 years. I also make sure that the filter bowls are as full as possible before refitting.

 

 

That's a good tip and also Baldock has suggested pressurising the tank to force diesel down to the filters. Mine are not above the tank though - would estimate that the tops are roughly level with the base of the tank. I'm still confused as to how any fuel actually runs down to the filters when the engine is not running!

[NB Esk]

On this type of filter there is a small "O" ring under the head of the bolt that sandwiches all the components together. It can easily get left out, worth checking...

 

The order should be- bolt head, close fitting washer, "O" ring, filter housing. Hope this helps...

[Iain_S]

That's a good tip and also Baldock has suggested pressurising the tank to force diesel down to the filters. Mine are not above the tank though - would estimate that the tops are roughly level with the base of the tank. I'm still confused as to how any fuel actually runs down to the filters when the engine is not running!

 

Our canal society boat has a fuel filter fitted on the "suction" side of the fuel pump. I bleed this by running the engine until it stops, and then bleeding the engine mounted filter, where all the air has ended up.

 

One problem we've found with this arrangement is that if the filter gets blocked, which has happened in the past due to water in the fuel, air tends to get pulled in to the fuel line.

 

Iain

[Alan Saunders]

As I understand it, this is a 'gravity' fed fuel system a bit like this:

There is no 'lift pump' and the injection pump does not 'suck'?

 

You replace 'filter 2' and bleed it - it will mostly be filled with fuel from 'filter 1'.

 

You replace 'filter 1' and now have to purge air from the relatively long pipe to the tank.

Air will be trapped at the high point and a considerable flow will be required to purge it.

 

On a diesel gen-set with gravity feed I failed to bleed the system with a half full fuel tank and that was with a bottom feed .

 

If the suply pipe was emptied you would need to pressurise the tank to re-establish the syphon loop.

One way to do this is to temporarily extend the filler tube by a couple of feet. Make sure the breather is higher or block it off.

 

HTH, Alan.

[richardf]

As I understand it, this is a 'gravity' fed fuel system a bit like this:

There is no 'lift pump' and the injection pump does not 'suck'?

 

You replace 'filter 2' and bleed it - it will mostly be filled with fuel from 'filter 1'.

 

You replace 'filter 1' and now have to purge air from the relatively long pipe to the tank.

Air will be trapped at the high point and a considerable flow will be required to purge it.

 

On a diesel gen-set with gravity feed I failed to bleed the system with a half full fuel tank and that was with a bottom feed .

 

If the suply pipe was emptied you would need to pressurise the tank to re-establish the syphon loop.

One way to do this is to temporarily extend the filler tube by a couple of feet. Make sure the breather is higher or block it off.

 

HTH, Alan.

 

Thanks Alan,

 

That's exactly my set up - I think we just need to purge the pipe with more diesel rather than just trickling it through!

[colin stone]

As I understand it, this is a 'gravity' fed fuel system a bit like this:

There is no 'lift pump' and the injection pump does not 'suck'?

 

You replace 'filter 2' and bleed it - it will mostly be filled with fuel from 'filter 1'.

 

You replace 'filter 1' and now have to purge air from the relatively long pipe to the tank.

Air will be trapped at the high point and a considerable flow will be required to purge it.

 

On a diesel gen-set with gravity feed I failed to bleed the system with a half full fuel tank and that was with a bottom feed .

 

If the suply pipe was emptied you would need to pressurise the tank to re-establish the syphon loop.

One way to do this is to temporarily extend the filler tube by a couple of feet. Make sure the breather is higher or block it off.

 

HTH, Alan.

 

I would consider fitting a junction on the filter side of the tank valve and attach a diesel line to that from a small container.

 

 

On the right hand side of this pic is the "T" with a valve and connection for my diesel container.

[nb Innisfree]

I installed a fuel shut off valve below the bottom of the tank, when valve is turned off fuel pipe is left full, change filter then open valve so then you still have a head of fuel.

[richardf]

Thanks for all the people who offered solutions - the filter housing was air tested and proved to be sound. I met Dick at the boat this morning and filled the diesel tank right to the very brim (I had a 2'' upstand ready to screw onto the filler point, but didn't need it in the end) - this additional head seemed enough to dislodge the air in the pipe from tank to filters and the engine fired up first time. I will know in future, but it's a good tip to fill the tank up completely before trying to bleed the system when you have arrangement detailed above.