sucking air ?

[ms1]

Ivan to answer your original question.
yes you can buy jointing compound which help to seal such joints .. Like Hylomar Universal Blue (and others) which does not set hard. As other have said there are, of course, many possible causes of leaks in joints or even stop cock. Filters are often the culprits. Its a process of elemination usually. I had an extreme situ where I actually bought a length of copper tube and a handful of olives and nuts etc and made myself a by pass line in order to identify location of air ingress. Bit of a pain, but it found the problem.

 

[oldharry]

Sorry PVB, but I stand by what I have said. When a fast running imnjector pump is operating it is most definitely not 'sucking gently'. It is grabbing a precise quantity of fuel, compressing it, and sending it to the injectors. In spite of the low positive pressure of the lift pump, there is almost always a period when the injection pump opens for the next fuel charge when the whole system drops well below atmospheric pressure. At that point any leak in the system will admit air to the fuel. The rest of the time fuel will leak out from the positive pressure in the line.

Air would not be drawn in through a leak if the line was pressurised, as the escaping fuel would prevent it.

The suction line is subjected to a series of hydraulic shocks as the injector pump takes each charge for compression. This results in flow BOTH ways through a leak - air in, followed by fuel out as the pressure varies from above to below atmospheric pressure.

Of course, the new Common rail injection system solves this particular problem, as the injection pump only has to create pressure in the rail, and is not involved with metering and distributing the individual charges to the injectors.

But believe me: if air can get in, traces of diesel will get out, and the only exception would be where the diesel tank is a long way below the engine requiring serious suction to lift it. That would be a serious hazard at sea!

 

[pvb]

Oh, really...

[ QUOTE ]
In spite of the low positive pressure of the lift pump, there is almost always a period when the injection pump opens for the next fuel charge when the whole system drops well below atmospheric pressure. At that point any leak in the system will admit air to the fuel. The rest of the time fuel will leak out from the positive pressure in the line.

[/ QUOTE ]Oh, really, that is such a load of rubbish. You're now apparently confining your theory to the small section of pipe between the lift pump and the injection pump. I've never fully subscribed to the belief that the PBO forum largely consists of people who don't know what on earth they're on about, but posts like yours do make me wonder if I need to review my position.

 

[ipw]

Correct we are engine specialists not fuel line diagnostic experts ie diesel fitters ,ie fitters / mechanics remove engines then we fix em .This thread has had up to now given me and others good advice especially the shaving foam one .By the way are trolls usually newbys and not regulars ?

 

[oldharry]

How does air get sucked in to a line that according to you has positive pressure? /forums/images/graemlins/laugh.gif

 

[brianrunyard]

Re: Oh, really...

I agree with pvb
If the suction side has a positive pressure there's something wrong with the non return valves in the lift pump.
I've certainly known cases where air has been getting in but there has been no evidence of fuel leaking out, often on the suction side of the lift pump.

 

[oldharry]

Re: Oh, really...

Well I dunno! Bloke has a problem, so I pass on advice given me by several professionally qualified mechanics and marine engineers, and which always proved good while I was responsible for running a fleet of passenger launches....

... and get flamed for it!

The coloured tissue idea for tracing leaks is certainly not mine - but it works! I am told talc does the same, but more messy!

 

[brianrunyard]

Re: Oh, really...

I like the coloured tissue idea, I've used talc, but as you say it's messy, although it does make everything smell nice!!!

I wasn't flaming you, just stating that I've known situations where air has got in but fuel hasn't got out.

 

[oldharry]

Re: Oh, really...

Sorry Brian - that wasnt aimed at you, (the problem of the 'quick reply' button). I reckon in technical debates the rule of thumb needs to be 'dont flame - explain' and so far all I had from one certain contributor is flaming - not helpful to the original poster (unless he is indeed trolling).

I have PM'd him with a more detailed account of the process as it was explained to me when I was 'in the trade' managing a fleet of class V/VI passenger vessels, and it will be interesting to see if he can explain his point of view without the smokescreen!