How long running engine before i know my fuel filter is not leaking air?

Burnham Bob

Burnham Bob

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I've been changing and bleeding fuel filters for longer than I care to remember but this boat has a CAV type filter and previous attempts to change the filter have been fiddly to say the least. Lack of boating vouchers mean I can't afford a spin on conversion. Previously I've worried for a while that the engine might stop and I'd have to go back in the cockpit locker. I currently have the luxury of a marina berth so changing the filter now seems a good idea. If I run on the mooring how long do you think it will be before i can be sure the filter isn't leaking? The engine is a beta 14 and the filter is about a metre away by fuel tube from the lift pump. And does anyone have any suggestions how many hours a CAV filter element is good for? As far as I can tell there has never been any water or crud in the tank. I also dose all new diesel with Marine 16 so diesel bug doesn't worry me.
 
Good question. I run my Beta 25 for about 40- 45 mins. to reassure myself, but that time is not derived very scientifically, except that once after a filter change, I forgot to turn on the fuel tank valve, and I was surprised at length of time the engine ran for, by which time I had cast off and was well on my way through the marina.
Also I run in forward gear to load the engine, and I don't like running my TMC 'box for too long in astern at more than low throttle settings.
 
If the filter points vertically down, they normally do, then a length of toilet paper round it held on by a lacky band will soon show if it is leaking (fuel not air).

PS the how many hours Q causes lots of debate. If you liken it to a car, then say annual miles of 10,000 at ave 40mph would give you 250 hrs.
I doubt that you motor anywhere near that, but it gives you a starting point!
 
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LadyInBed said:
If the filter points vertically down, they normally do, then a length of toilet paper round it held on by a lacky band will soon show if it is leaking (fuel not air).
Click to expand...

But if it's on the suction side, as a CAV typically is, then it would leak air in and not fuel out.

Pete
 
Burnham Bob said:
I currently have the luxury of a marina berth so changing the filter now seems a good idea. If I run on the mooring how long do you think it will be before i can be sure the filter isn't leaking?
Click to expand...

I can't see why a mooring versus a pontoon makes any difference to the issue?

I guess if you wanted to test the engine under load you'd have to run in reverse :)

Pete
 
A few years ago i had a tiny air leak which would stop the engine after about 6 hrs running. Hell of a job to find it and i lost trust in the engine that year.
 
I should have thought air would make itself known very quickly, since the take-off is from the top of the CAV and from the top of the secondary filter too.
I've had CAV's on all my boats and while they are certainly a right messy fiddle to change, I have never heard of them being prone to air leaks. After all there are bazillions in use world-wide.
 
Maybe an hour at sea will tell you it's OK, but a bit of air can cause poor starting after leaving the boat a while too. A tiny leak will likely give warning - the engine doesn't just stop dead, it'll get rough or weaken first?
 
I have had an air leak on a CAV filter but the engine would run normally for hours. However, after a period of slow running it would cause the engine revolutions to increase for a few seconds. As the slow running in question was to shoot a very tight bridge with twin engines the resulting change of course made life interesting!
 
Having spent my apprenticeship years working on engines when the Lucas CAV Filter was the only filter system available, although yes they did leak you could stop it by tightening the nut, I certainly don't remember having difficulties like I am doing recently with my present CAV filter set up.
And I believe I know the problem. To prove a point, I deliberately bought an old (but unused) Filter off EBay which had been lying around in someones garage for perhaps 40+ Years, purely to have a look at the rubber sealing ring which accompanies the filter, and I was right, it is thicker, much thicker, and better.
Perhaps in the 60's - 1980's the rubber sealing ring would have been rubber or butyl, whatever, the point is that they were thicker than the ones which accompany the CAV type filters which you buy today, consequently when you tighten them up today they just dont make a proper seal. One of my filters had the original CAV casting head and in an attempt to stop the drip I even tried two filter rings, which you would never have been able to do with the original.
I appreciate that you dont want to buy the Spin On Conversion and I was put off also by the cost as the ordinary ones cost peanuts and can be bought anywhere.
So I decided to buy a new (copy CAV) Double Unit, The first filter has the Glass Bowl and the second is the secondary filter. and the casing should be machined to accept the thinner (composite rubber) sealing rings.

When I bought my present boat it had a fuel supply problem which I hope will be eradicated by the fitting of my new filters.
In order to check to see if they are going to leak (or suck in air) I have bought a cheap 12v Electric Fuel Pump which I will connect to the fuel pipe and pressurize the system and if something is not sealed properly it should spurt out.
Thats my solution anyway.
 
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Just my findings here..................We have an old BMC thumping away on the Grumpy Uncle and she is armed with a total of 3 cav type filters.......a twin filter unit near the tank(primary) and a single mounted on the engine(secondary). We are lucky in that they are both easily accessable so changing filters, although a bit messy with the dripping fuel, it is a simple procedure. We have found that if the filters are indeed sucking in air, all be it a very small amount, they will also slowly let fuel out when the engine is stopped.......the paper tissue idea is a good way to find a leak as mentioned earlier. Careful re-fitting should not cause air leaks though, you just have to be certain the O rings are fitted snug and correctly. We have also fitted a 12v facet type pump before the first filter. This is about the best 20 odd quid I have spent! It pushes fuel through and makes bleeding the system an absolute doddle, plus its there if ever the mechanical lift pump fails.....indeed, we found a very very slight drip on a hose connection just this weekend that we didn't know we had just by running the pump and presurising the system for 10 minutes.
As for how long to run the engine, we give it about 30 mins or so when on the hard, or a 20 minute blast around the marina.
 
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