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Best way find a tiny air leak in diesel engine?


canalboat77

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Only because I've never had to do this before I thought I would ask for best practices on here. In short: after speaking to a local mechanic he believes I may have a tiny air leak on my boat. After describing the symptoms to the mechanic he thought it would be a good idea to start there first. 

 

To start I've gone around a tightened all of the connectors mainly on the diesel line going into the engine. Would next step be to start switching pipes? 

 

Wondered if there was a simple trick I am missing? 

 

Thanks

 

 

  • Greenie 1
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Try cleaning all the joints on your fuel system with WD40 or similar,then wrap kitchen roll around every joint,run your engine and check the kitchen roll for diesel stains.

Not a foolproof method because sometimes air can still get into the fuel lines without fuel coming out,but you will know if there is a major breach.

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30 minutes ago, John Hartley said:

May I ask what the symptoms were

 

Sure. It happens at different times. Last time, I warmed up the boat for about 25 minutes before pulling away, as I was pulling away (going forward) you could hear the engine start to struggle and almost die. I increased the revs - sometimes this works, sometimes it gets worse when I will just cut the engine. 

 

 

 

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What is your supply pipework from the tank? steel or copper?

Where do you draw from the tank? Top or bottom?

You have checked all your screw connections, but one source that I encountered was the flexi between the rigid pipe and the lift pump. Disconnect the flexi, and if you can turn the connector in the rubber, replace your flexi.

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From your symptoms I infer it is not a BMC 1.x or an older Perkins so what is the make & model.  It is known that Betas have had problems with porous filter housings and faulty priming pump diaphragms.

 

There are many things it could be like a load of muck floating about in the tank, excess water in the tank, broken pick up pipe, lack of fuel (empty could be as much as 6" above the bottom of the tank), faulty lift pump, and blocked return to the tank. The likelihood of each will depend upon the engine.

 

Edited to add:

 

I would suggest that as long as the engine has a decent return to the tank, and especially if it is supposed to be self bleeding that a tiny air leak is unlikely to give the symptoms you describe. Once its running most modern engines would simply bleed the air back into the tank.

Edited by Tony Brooks
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1 hour ago, canalboat77 said:

Sure. It happens at different times. Last time, I warmed up the boat for about 25 minutes before pulling away, as I was pulling away (going forward) you could hear the engine start to struggle and almost die. I increased the revs - sometimes this works, sometimes it gets worse when I will just cut the engine. 

 

 

 

And does it start again without you bleeding the fuel system?

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Is this boat new to you or has it been ok for many years and just developed this problem?  Is there a high spot where any trapped air can collect  and where you would bleed the air out? and if so is there any air in there?

 

Could be a blocked filter, when did you last change the filters?

 

..............Dave

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45 minutes ago, Tony Brooks said:

Isn't it interesting that several on here think a fuel problem more likely but the "engineer" feels its an air leak - just musing.

A boat mechanic can be anything from a boat expert with huge knowledge and experience (like your good self) to a lad who helps out with the turn arounds on a Saturday, or maybe just another boater who said he knows some stuff ?

 

.............Dave

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3 hours ago, Tony Brooks said:

 

 

I would suggest that as long as the engine has a decent return to the tank, and especially if it is supposed to be self bleeding that a tiny air leak is unlikely to give the symptoms you describe. Once its running most modern engines would simply bleed the air back into the tank.

The similar problem I had was a leak on the flexi clamp on the inlet fitting to the lift pump. The pump sucked in a small amount of air which made its way up the pipe to the agglomerator, and created an air lock.

I've also seen similar on an old boat with a steel pipe from the tank to the lift pump. This would run happily on tickover, but as soon as you accelerated, it would stutter and cut out. The perforations allowed air in, but there was very little evidence of a fuel leak even though the pipe was gravity fed.

edit to add: I see from the first post that the 'engineer's ' advice was after the symptoms were described. I mistakenly thought he had attended.

Edited by Ex Brummie
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9 minutes ago, Ex Brummie said:

The similar problem I had was a leak on the flexi clamp on the inlet fitting to the lift pump. The pump sucked in a small amount of air which made its way up the pipe to the agglomerator, and created an air lock.

I've also seen similar on an old boat with a steel pipe from the tank to the lift pump. This would run happily on tickover, but as soon as you accelerated, it would stutter and cut out. The perforations allowed air in, but there was very little evidence of a fuel leak even though the pipe was gravity fed.

edit to add: I see from the first post that the 'engineer's ' advice was after the symptoms were described. I mistakenly thought he had attended.

I could well understand that on something like a BMC 1.5 with only a 0.5mm hole to the leak back or on something like the old Penta MD range with no leak back from the filter. However on a Beta ,and I assume Nanni, or a Vetus with plenty of leak back to the tank I can't see it.  Note the OP has yet to tell us what engine he has.

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B***** air leaks. I have had a couple of occasions when the filter needed changing, Engine would tick over but not speed up. Also an occasion when crud and slime blocked the filter shortly after changing it. Therefore I would change the filter first.  Last year the engine became reluctant to start, that turned out to be a loose union but it took a devil of a long time to find, no leak visible as it was sucking  a tiny bit of air. Dripping fuel is not too hard to find but v. small leaks can be a bother.

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Again, assuming it is an air leak causing the problem, I once found one by listening at each joint. Engine off, opened one of the joints on the engine to bleed air out worked the mechanical lift pump to draw fuel and drop the pressure in the intake pipe, I could hear air being sucked in at a joint between the tank and the lift pump. There was no drip of fuel, or even any dampness, but enough of a leak to draw air in when the lift pump was working. An electric lift pump would be noisy enough to drown out the noise though.

Jen

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So there seems to be no leak off from the main filter so air could build up in it until it delivers a large slug to the injector pump(s) but if it is self bleeding I would expect the engine to stutter as most of the air is  returned to the tank.  The area looks new enough to make a pick up pipe in the tank problem less likely. However as that filter is on the suction side a leak back cant be fitted to it.

 

I note that the OP has not answered Brian's question about does he have to bleed it to restart the engine or confirmed he has checked/changed both filters.

 

If someone has recently changed the primary filter I would be looking at the little O ring that is supposed to seal the centre bolt. Some  filter manufacturers have been known to supply an undersized O ring. Also worth loosening the centre bolt (turn the fuel off first) and make sure all the parts are properly located as the bolt is re-tightened.

 

I would also dismantle and have a very close look at the olive on the compression joint where the supply connected to the  steel/alloy banjo on the main filter inlet.. Along with the advice about hoses etc. 

 

Edited to add - if the primary filter was changed just before this started and/or you can't find any air leaks I would suggest that you fit a genuine CAV filter and not a pattern one. That way you stand the best chance of ensuring the bolt o ring is correct.

Edited by Tony Brooks
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So there seems to be no leak off from the main filter so air could build up in it until it delivers a large slug to the injector pump(s) but if it is self bleeding I would expect the engine to stutter as most of the air is  returned to the tank.  The area looks new enough to make a pick up pipe in the tank problem less likely. However as that filter is on the suction side a leak back cant be fitted to it.

 

Just for information. On this engine there is no leak off from the filter, but instead from the injection pump. The engine is therefore self bleed. Also engine will be fitted with an electric fuel pump.

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