VP AQAD41 just died, where would I find an air leak?

[Cleopatra]

Hubby and I are a pair of novices who recently bought a 40 year old motor cruiser with a pair of Volvo aqad41-a engines from 1984/85ish. They were fully serviced in October and running sweetly (if smokily) for the 8 hours we have used her. Then on Thursday the Port engine died, we didn't see any weirdness on the dials. Temp had been sitting at its usual 80. The Starboard engine was running fine and managed to limp us for the hour it took to get back to the marina.

Both engines run from the same fuel tank, so in my head that rules out water - or am I wrong?

This morning we bled the port engine in stages and there was definite air coming out the injectors, but she nearly picked after no 2 was bled although we continued to bleed them all...finally started and seemed fine, but she will only run for up to about 3 mins before stuttering and dying and we have to start the whole cycle again.

Am I right in thinking air leak - dirt cause this? If it is a leak where would you next expect to look (for this needle in a haystack!) There does not appear to be any visible fuel leaking on or around the engine or in the bay. The lines run under the aft cabin into the salon where the tanks (two massive steel type things with interconnector) live under the floor. There is no smell of fuel anywhere... the mechanic is hopefully going down on Tuesday to check, but i'd prefer if I can suss it myself...(bearing in mind neither of us are mechanically minded, just stubborn!)

Any suggestions appreciated.
Regards, Di

 

[Momac]

You say both engines run off one tank.
Then you appear to describe two tanks.
It is usual for the flow and return for each engine to be connected to its own tank . There may be valves to alter this set up. I wonder if the valves are wrongly set causing one tank to run empty . Or possibly you have simply run out of fuel.

 

[spannerman]

Finding airleaks can be tedious, assuming the obvious such as filters are ok then a quick way of tracing an airleak is to fit a foot of clear hose just before the lift pump on the engine and bleed out and start up. If there are air bubbles visible then the leak is external and between the clear hose and tank. If there air no bubbles then the airleak is between the lift pump and injection pump, although this is pressurised by the lift pump and any leak should be evidenced by diesel, I have experienced airleaks on the engine mounted filter and internally on the lift pump.

 

[kashurst]

Although you have had the boat serviced I would change all the fuel filters again as a precaution. If the boat hasn't been used much and now you are out and about,potentially you have 40 years of gunk swilling about in the fuel tanks that you have now stirred up. The symptom you describe sounds to me more like a blocked fuel supply than an air leak.

 

[omega2]

We found the supply pipe from tank to lift pump had turned into a pan pipe, due to rust.

 

[Cleopatra]

Thank you all for your prompt replies - the tanks were retrofitted in about '94 and the survey reported them of good condition - but I'm not sure he looked inside for gunk! He also said the copper fuel lines running aft were all in good condition but I shall check their full length now. I've never worked on boats or fuel systems before so I am a little unclear on return lines - why does it return fuel? Old BSA Bantams or Yamaha 2 strokes are the only thing's I have worked on before (under supervision).

The two tanks I spoke of sit kindof like this (apologies for crap not to scale diagram)
[/I]

My first thought was that we had run out of fuel on one tank, but husband said that was impossible with the set up (and the tank was filled to brimming before we first used her, she is now still at well over 3 quarters full...my second thought was dirt and it sounds like I shouldn't rule it out despite fuel coming through to the injectors. Can I rule out water now?

I doubt the mechanic will actually make it down there tomorrow so I've probably got loads of time for more checks...although I'm not confident on fitting a clear hose myself (worried that I might damage something or not tighten something enough) but the thought of paying someone for hours of searching should frighten Himself into action... thank you spannerman!

Hearing all your experiences is very useful, thank you again.

 

[Cleopatra]

Sorry for the delay in replying to people - it is taking 12 hour for my posts to be approved....I presume that is because I'm new here...

Di

 

[Freebee]

diesels usually have a return line flowing back to tank , i think this is to do with using fuel to cool the injectors

 

[scottie]

diesels usually have a return line flowing back to tank , i think this is to do with using fuel to cool the injectors

Basically the injection pump draws a constant flow of fuel which is more than required to power the engine the excess is returned to the tank.
This actually results in heating the fuel.
With Two tanks and or engines both fuel returns may go back to one tank but this is not best practice.
Keeping both engine with their own independent fuel systems is preferred but the ability to switch supplies is an obvious plus.

 

[scottie]

Hubby and I are a pair of novices who recently bought a 40 year old motor cruiser with a pair of Volvo aqad41-a engines from 1984/85ish. They were fully serviced in October and running sweetly (if smokily) for the 8 hours we have used her. Then on Thursday the Port engine died, we didn't see any weirdness on the dials. Temp had been sitting at its usual 80. The Starboard engine was running fine and managed to limp us for the hour it took to get back to the marina.

Both engines run from the same fuel tank, so in my head that rules out water - or am I wrong?

This morning we bled the port engine in stages and there was definite air coming out the injectors, but she nearly picked after no 2 was bled although we continued to bleed them all...finally started and seemed fine, but she will only run for up to about 3 mins before stuttering and dying and we have to start the whole cycle again.

Am I right in thinking air leak - dirt cause this? If it is a leak where would you next expect to look (for this needle in a haystack!) There does not appear to be any visible fuel leaking on or around the engine or in the bay. The lines run under the aft cabin into the salon where the tanks (two massive steel type things with interconnector) live under the floor. There is no smell of fuel anywhere... the mechanic is hopefully going down on Tuesday to check, but i'd prefer if I can suss it myself...(bearing in mind neither of us are mechanically minded, just stubborn!)

Any suggestions appreciated.
Regards, Di

Try and be at the boat when your professional is there and you will find that he will talk you through what he is doing.

 

[Cleopatra]

Unfortunately he went while I was at work, however "after a few hours" (eeek £££) he found the leak which was near the fuel filter. He's replaced the filter, fixed the air leak, bled and pressure tested the engine, so the port side is all running fine now...but he does mention while testing he noticed the Sbd side was excessively smoky and he wonders if we might be a cylinder down... :-( I have noticed a fair amount of blue tinged smoke as we are going along, so I'm hoping he's not right.

It is frustrating that we probably could have found the leak if we'd had a bit more patience and weren't so nervous that we were barking up the wrong tree. Thank you all for the helpful suggestions which have been noted for future (and I'm guessing with engines this old there will be future need!) I'm also going to delve into the depths and make a proper map of the fuel system.

Thank you.

 

[scottie]

Simple test is if the engine reaches approx 3800 rpm under load it will be ok also If you slightly loosen the injector pipes in sequence (cover the connection with a rag ) you should notice the drop in revs but if one cylinder is not working then that cyl will not give you the same drop

 

[Cleopatra]

Basically the injection pump draws a constant flow of fuel which is more than required to power the engine the excess is returned to the tank.
This actually results in heating the fuel.
With Two tanks and or engines both fuel returns may go back to one tank but this is not best practice.
Keeping both engine with their own independent fuel systems is preferred but the ability to switch supplies is an obvious plus.

Wouldn't this result in condensation in the tank as the returned fuel would be warm?!

 

[Cleopatra]

Simple test is if the engine reaches approx 3800 rpm under load it will be ok also If you slightly loosen the injector pipes in sequence (cover the connection with a rag ) you should notice the drop in revs but if one cylinder is not working then that cyl will not give you the same drop

Thank you, that sounds simple enough for me! Being afraid of damaging something so expensive, I didn't realise you could do tests like this without misaligning stuff etc. I forget these things are pretty much built like tanks!

 

[rafiki_]

Wouldn't this result in condensation in the tank as the returned fuel would be warm?!

The returned fuel warms the bulk fuel slowly, so condensation not really an issue. Actually, with the age of your engines, if you have a fuel return, the fuel temp will not be high as they will not be high pressure common rail or eui engines.

 

[scottie]

Thank you, that sounds simple enough for me! Being afraid of damaging something so expensive, I didn't realise you could do tests like this without misaligning stuff etc. I forget these things are pretty much built like tanks!

Reading this over you only loosen the nuts at idle not full revs
DO NOT take the pipes of whilst doing this
You may well find that on an engine of this age the copper and or alloy fuel washers are hardened
Copper ones can be annealed but best to get a small bag of them Volvo used to do a kit
Well worth going on a DIY engine maintenance course
As you have found already engines do not always have problems in places and at times to suit you but have perverse sense of humour it would seem.

 

[Cleopatra]

Reading this over you only loosen the nuts at idle not full revs
DO NOT take the pipes of whilst doing this
You may well find that on an engine of this age the copper and or alloy fuel washers are hardened
Copper ones can be annealed but best to get a small bag of them Volvo used to do a kit
Well worth going on a DIY engine maintenance course
As you have found already engines do not always have problems in places and at times to suit you but have perverse sense of humour it would seem.

Thank you - the warning was timely! I never thought of a course, but I'm currently checking Amazon for a decent book on marine diesels (recommendations welcome).

When we bought her I warned the Other Half that we were buying a big bag of trouble due to her age, but heart overruled head - and the survey report came back better than expected...I'm just thinking about keeping a set of oars onboard just in case

 

[NormanB]

This one - link. But in this case do not get the kindle version.

Of course a book of words specific to your engine including parts list is an invaluable reference too.

 

[Cleopatra]

This one - link. But in this case do not get the kindle version.

Of course a book of words specific to your engine including parts list is an invaluable reference too.

The hardback version is now winging its way to me now, thank you. I do have the owners & workshop manuals for the engine with clearances etc...but they were a little bit light on useful help - both pointed to fuel system trouble but nothing further...