What if......Fuel Filter failures.

[tom52]

I have a CAV filter and also a fine fuel filter, both of which I religiously change at every annual lift out.

Visually the ones I take out look just like the new ones, so a theoretical question.

What are the likely consequences of not bothering for a few years.
Assuming they are not suddenly blocked by an outbreak of diesel bug, and I waited for signs that the fuel filters needed changing what would the symptoms/signs be ?

Do old filters get so clogged that they starve the engine of fuel or do they disintegrate and start letting bits through to block the injectors ?

 

[savageseadog]

Yes, they can clog to the point of stopping the engine. You should have at least two filters so the failure of one doesn't do the injector pump/injectors damage

 

[Spyro]

It's an interesting question because like you I've changed mine and although there's been a bit of crud in the primary (cav type) the secondry (yanmar) has looked good as new.
Does the secondry have a finer filter than the primary?
Would there be symtoms of a blockage like not reaching full revs because some fuel is getting through before there was a complete blockage?

 

[barnaclephill]

Assuming they are not suddenly blocked by an outbreak of diesel bug, and I waited for signs that the fuel filters needed changing what would the symptoms/signs be ?

Do old filters get so clogged that they starve the engine of fuel or do they disintegrate and start letting bits through to block the injectors ?

Getting caught in a cold front stirred up tank sediment and that cut our speed from 4 to 2kts, just as we were approaching within 10nm of our anchorage. With the boat speed cut, plus tide, and then tacking against the westerly winds, it made it slow and precarious to enter the bay of our anchorage.

This was 2 filters in line of each other, so the second one blocked anything the first one let pass. (although same model).
I'd assume that bug blockages would be similar, though stickier. Symptoms of not changing regularly would be an imperceptible loss of speed. Letting bits through to the injectors would be the next stage of symptoms, resulting in cyllinder shut-down and cost.

 

[Fr J Hackett]

I would have thought having two filters of the same size was superfluous as the first would take out all the material that the second one would capture ie the first would block long before the second. However if the two filters get progressively finer as in the CAV primary / Yanmar fine secondary then it's perfectly possible for the second filter to block before the first as it takes out a different fraction of solids that can pass through the first coarser filter. Where two primary CAV filters come into their own is when they are mounted in parallel and when one blocks the supply can be switched to the other common in MOBOs. If you have filters mounted in series they should remove progressivly smaller / finer particles otherwise you are wasting your money.

 

[pvb]

As fuel filters gradually get clogged, the engine has to try harder to suck fuel through, and the vacuum in the fuel line increases. Racor sell a useful vacuum gauge, which can be remote mounted. I have one on my switch panel, and it indicates when the filter is getting close to needing changing.

 

[chippysmith]

On a charter boat some years ago, and on a particularly windless day, we found ourselves motoring from Yarmouth to Portsmouth, just off Cape Kicker the engine failed, after about 20 minutes of messing around with bleeding etc the engine started and we made on, we were abeam of the blockhouse making little way pushing tide when it failed again.. we were spat out of the harbour under no control whatsoever with many boats all around us, not a particularly nice experience that ended up us getting a tow from a friend. As we approached Port Solent She fired up again and I managed to get her tied up inside on the waiting pontoon... I finished my Charter there and went home reporting our troubles to the charterer.

I called him a week later to book the boat for a week away, on asking about the troubles he said that a Volvo engineer had checked the boat over and everything had been fine, and nothing found to cause my problems, he said most probably that I'd run her to low on fuel.. I took it on the chin but was annoyed as this was definitely not the case.

Our trip away was to Cherbourg and we sailed through the night for a noon'ish arrival at Cherbourg corresponding with slack tide on arrival, all went well until the wind died and and we had to motor for the last few hours. On entering the harbours Eastern entrance the donk died, luckily it was slack so it gave me precious moments to get her going again, filled the tank with fuel, it was already half full at least! bled the system but nothing, crew had instructions to notify me as soon as we started moving with the tide, it was all taking to long so I radioed in my concerns.. they would get back to me!! I carried on but I got the call that we were moving to the East, not good so reluctantly chucked the pick in and waited..

To cut a long story short (I can here a sigh of relief), we had a local engineer come to the pontoon we were put on and after just a minute or two diagnosed the pump as imploded.. I must have pumped that thing a thousand times, he pumped it less than ten before he realised something was amiss, he removed the lift pump and shook it, I guess the rattling he gesticulated about was a wrong thing!

It was the primary filter, all looked good through the glass but the flow of fuel, vibration etc picks up the sediment and coats the surface of the paper which in turn blew the pump. Not good.

All finished well with the charter guy admitting liability, refunded the charter, paid the bill and supplied a free charter as and when.. wasn't good to be stuck at anchor in the middle of the entrance, have nightmares about engine tone now.

Don't know how a secondary filter helps but a Parallel system with changeover valves would be sensible although my pump popped so I had no second chance. Ideally the gauge idea above would be my choice, sensing the vacuum in the system is a good idea, points you to the issue directly, even with a system of methodically changing filters you can get caught out by the dreaded bug

 

[KenMcCulloch]

Interesting thread. I have certainly had an engine failure due to the filter (and upstream hose) clogging with bacterial slime.

Regarding consequential failures, can anyone explain the mechanism by which a clogged filter causes a lift pump to fail? Do filters eventually break down rather than just clogging up??

 

[chippysmith]

The lift pump draws in the fuel by way of a vacuum, the more resistance the bigger the vacuum, as it's mechanical it won't stall so carries on pumping regardless until the inevitable happens, and the pump imploded, mine was a shattered mess. The reason I got it started when in the Solent and when the Volvo engineer never found a fault was due to the detritus falling back to the bottom of the bowl once the fuel has stopped flowing and vibration settled

 

[nimbusgb]

A mates boat had a bug induced failure a couple of weeks ago.

Inlet pipe blocked by lump of bug rather than the filter element itself getting blocked. His engine ( Thornycroft ) has an electric lift pump setup. I found the problem for him by getting him to try to bleed the system at a banjo on the injector pump. No fuel just a few bubbles. Working backward we loosened the inlet to the primary fuel filter/separator to test for fuel there. Sucking on the pipe ( with a long piece of transparent hose to isolate me from the pipe ) provided nothing at all. Blowing back cleared the blockage and the sub subsequent inspection of the tank found the bug.

The point of the tale is that once blocked the lift pump continues to try to suck up fuel. Given enough resistance, in this case a total blockage, the pump cavitates and allows air from the upstream side to get back into the line. Either that or it builds up an internal vacuum and a relief vent may open. These pumps are designed to operate with little or no flow resistance and their lift is usually restricted to a few feet at maximum. Most are designed to not be run dry for any length of time.

Once blocked you have a lift pump operating without lubrication and it can destroy itself. Consider that with the steadily declining fuel feed and then the fuel remaining in the secondary filter, lines and injector pump body the engine may run for several hours under less than ideal conditions.

 

[prv]

The fuel lift pump is lubricated by engine oil from the camshaft and oil mist from the crankcase, as long as the engine turns it will be lubricated.

The post you're replying to referred to an electric fuel pump.

Pete

 

[cmedsailor]

Where two primary CAV filters come into their own is when they are mounted in parallel and when one blocks the supply can be switched to the other .

Anyone know how this can be done? Yes, I can imagine by using a second pipe for the filter in parallel and a 3-way valve before and after the filter, but my question is, how do you make sure that there is no air in the system? Unless switching from one to the other requires to stop the engine, do the switch, bleed the system etc, etc
Thanks

 

[NormanS]

You bleed the air out of both filters then you are able to change from one to another without stopping the engine, you are NOT able to change a filter though, unless you stop the engine to bleed the air out.

You can, if you have the good old fashioned way of a gravity feed from the tank.

 

[KellysEye]

One thought fuel filters can be difficult to get off which could put you in trouble. We had two pre-filters in series but I changed them to in parallel like Mobos have, for obvious reasons. If one blocks just switch them over. Make sure to bleed both after fitting.

 

[Stu Jackson]

Whilst some might think heretical, we have a 2 micron primary and a 10 micron secondary (engine mounted). I'd rather have to change the easy accessible primary than the hard to get at secondary any time.

Change filters yearly, why wait for three years to have one clog up, usually, always, at the worst of times?

 

[pvb]

Change filters yearly, why wait for three years to have one clog up, usually, always, at the worst of times?

With a vacuum gauge, you can just wait until the vacuum starts to increase, then change the filter. I haven't changed a fuel filter for years.

 

[Stemar]

After a traumatic entry into Portsmouth Harbour under in F7 due to the crud in the nearly empty tank being stirred up by all the bouncing around, I got a couple of matching cartridge-type fuel filters from a car breakers. 3-way valves were silly money, so I have a valve each side of each filter. Both are bled, ready to go, so if I get another blockage, it's a few seconds work to switch. Because it's so easy to do, I haven't bothered with a routine change for a few years, though I think it's probably about time to do it.

It's also a lot easier if I have to change a filter at sea than the CAV one with all the bits and different sized O rings.

I got lucky, My filters came from Volvo V40s or the equivalent Renault, and take the same cartridge as the secondary filter on my Volvo 2003.

 

[Pleiades]

Twin primary filters

Set up for two primary fliters looks like this:-

Two primary filters at the bottom of pickie with two fuel taps at top and a few 'T' pieces. Run with one filter in line and the other closed off. This set up ensures that if the primary filter gets clogged up with crud from the tank - first warning you will get is probably engine missing or spluttering as the crud blocks the primary and cuts off supply to the fine particulate secondary filter on the engine - you then should have time to flick the taps and switch to the second primary. You can then change the first primary filter at leisure without having to stop the engine when you might need it most. (But clean out the tank of course to get rid of the crud.)

Robin
Pleiades of Birdham
MXWQ5