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Air in Diesel


dmr

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Help and ideas needed.

Input from Tony and Biz etc very welcome.

 

For the last few months I have been having issues with air in my diesel causing rough running and speed changes. I am sure its air as whenever I have trouble I can get a lot of air out of the second filter bleed, but none when the engine is running well.

System is low down fuel tank, shut off valve, first filter, lift pump, second filter and Injection pump.

Second filter is standard CAV296, first filter is CAV style but spin on filter.

Odd thing is that if I replace the first filter the issue goes away for about 50 hours of running, and if I bypass the first filter the problem goes away, but there are no air leaks because Ive just fitted a vacuum gauge and done a leakage test.

I am starting to think that its dissolved air coming out of the fuel, but why has it started now after ten years of reliable running? Is it a recent bio diesel thing?

I believe BMC engines have a restricted bleed from the filter top? (so they must anticipate this problem) Do other engines do this? and how would I get the components? but I don't want to "paper over" some other problem.

Any ideas?

 

very interesting article:

https://www.parker.com/literature/Racor/Mobile_Air_Separation_In_Diesel_Fuel.pdf

 

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

 

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It does sound as though air is getting sucked in somewhere, between the tank and the lift pump. I have a similar setup, and never touch either the drain plug or the bleed point on the pre-filter - to ensure they remain airtight. 

Might you have a leak in the lift pipe inside the tank? - this might suck air in when the tank is nearing empty, and not when the tank is full. (You don't mention any correlation between the problem and how full the tank is, so this may be a red herring).  I am assuming that the fuel is sucked out of the tank by a pipe going down through the top of the tank and stopping an inch or so above the base?

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16 minutes ago, Scholar Gypsy said:

It does sound as though air is getting sucked in somewhere, between the tank and the lift pump. 

Minimal experience with diesel engines but that is exactly what I would suspect.  They leak on the pressure side and draw air on the suction side of the pump.  When you did the vacuum test I assume it was on the pressure side of the pump, right? 

 

I would start looking at the other side and would start with adding fuel to the tank. Doubtful but could be so low it is start to pull air with the diesel at the pickup. At least that is a easy one to eliminate and your going to use the fuel anyway so not wasting money.

Edited by Kudzucraft
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There is a possibility that the fuel pick up pipe in the tank which are usually steel is rusted and perforated and when the fuel level drops to a certain level air is drawn in, this gets worse as the perforations gradually creep up the pipe. I'd also go for the 296 filter unit, taking it to bits making sure all the 0 ring seals are good, and great care when reassembling, not forgetting the seal washer under the bolt head, also of course all fuel pipe connections at the tank, tap, filter and lift pump.

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For me the interesting comment is “if I bypass the first filter the problem goes away”.  Also if you take off and put on a first filter the problem goes away for 50 hours.  To me this sounds like an imperfect seal between the filter bowl and housing.  I would check the sealing face and sealing washer for damage or contamination.

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3 hours ago, dmr said:

Help and ideas needed.

Input from Tony and Biz etc very welcome.

 

For the last few months I have been having issues with air in my diesel causing rough running and speed changes. I am sure its air as whenever I have trouble I can get a lot of air out of the second filter bleed, but none when the engine is running well.

System is low down fuel tank, shut off valve, first filter, lift pump, second filter and Injection pump.

Second filter is standard CAV296, first filter is CAV style but spin on filter.

Odd thing is that if I replace the first filter the issue goes away for about 50 hours of running, and if I bypass the first filter the problem goes away, but there are no air leaks because Ive just fitted a vacuum gauge and done a leakage test.

I am starting to think that its dissolved air coming out of the fuel, but why has it started now after ten years of reliable running? Is it a recent bio diesel thing?

I believe BMC engines have a restricted bleed from the filter top? (so they must anticipate this problem) Do other engines do this? and how would I get the components? but I don't want to "paper over" some other problem.

Any ideas?

 

very interesting article:

https://www.parker.com/literature/Racor/Mobile_Air_Separation_In_Diesel_Fuel.pdf

 

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

 

All the above need checking!

 

Yes, its a 0.5mm hole in the side of a banjo bolt on top of the 296 filter assemble. If yours is a BMC 1.x then taaaake the bolt out and make sue the little hole is clear.

 

Most if not all modern engines pass far more fuel back to the tank to make them self bleeding so yes they do pass fuel and any air in the fuel back to the tank. Really old diesels like the Volvo MD2s seemed not to need a return to the tank, a return into the engine filter seemed fine.

 

Not knowing the engine and thus not knowing what injector pump you have makes it hard to say more but if its a DPx pump like the small BMCs then it has its own transfer pump that  is likely to suck fuel so that would make the situation worse if there is a poor joint, sealing washer or soft washer that is not soft any more. This would include lift pump parts or its diaphragm.

 

A poorly seating injector can push gasses into the fuel system that build up if there is no leak back to the tank.

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1 minute ago, Tony Brooks said:

Not knowing the engine and thus not knowing what injector pump you have

 

As I'm here, Dave has a Beta JD3 tug engine, with that weird and fragile Stanadyne injector pump IIRC.

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Just now, Mike the Boilerman said:

 

As I'm here, Dave has a Beta JD3 tug engine, with that weird and fragile Stanadyne injector pump IIRC.

 

Thanks Mike, I suspect it does have a transfer pump. I think Dave needs to trace all the fuel pipes to see if any join to go back to the tank and if there are any restrictors that might block in use.

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been out on the town, or should I say out on the village ?

Yes, its one of those fragile Stanadyne pumps so has an internal vane pump. Any air goes up into the governor housing then up to the spill rail and so back to the tank. I suspect a bit of air is ok but a big gulp upsets the governor. I've replaced or blown through all the fuel lines, replaced the lift pump and made a really good job of doing all the unions. In desperation Ive fitted a vacuum gauge to the first filter which reads 0...but I can close the shut off valve and use the lift pump primer to generate a big vacuum. This stays in overnight so I know the system is leak free. Something has changed in the last few months and I cant figure it out. Fitting a BMC style leak off banjo on the second filter is looking like a good option, but I want to understand what is going on.

 

This is quite urgent because if Carona virus kills me I will never know the answer ?

 

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

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The lift pump has two valves in it and when you create the vacuum in the first filter (water trap?) that vacuum will be sucking the lift pump's inlet valve hard onto its seat. Although I am surprised the valve seals well enough to sustain the vacuum overnight it clearly does. This means the filter and pipe work from shut off valve to lift pump is leak free but what about the rest of the lift pump and the system to the transfer pump in the injector pump.

 

Beta have been known to fit porous filter tops or filter tops that crack. If the Standyne pump is anything like the DPxs then it may have a strainer inside the inlet port.

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I had a very similar problem when I first launched Harnser. I had mounted the first filter with water separator (can't remember the proper name) in the engine room higher than the diesel tank. This was about a 20 foot pipe run from the diesel tank itself. After about 50 hours I would suck air into the system, change the filter and all was fine for another 50 hrs. Ran out of filters so punches a hole through the filter media, and it was OK. From that I deduced that I was pulling too much vacuum on the filter head with the restriction of dragging the diesel all the way up from the tank. I moved the filter to within 5 foot of the tank at low level and not had a problem since. I don't think it was poor fitting of the seals as the access was very good and with the new location its crap and I don't have a problem

3 hours ago, Tony Brooks said:

 

 

Beta have been known to fit porous filter tops or filter tops that crack. If the Standyne pump is anything like the DPxs then it may have a strainer inside the inlet port.

I have know a couple of cases of that personally, one tried to bodge it but in the end both had new filter tops supplied by Beta

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3 hours ago, Tony Brooks said:

The lift pump has two valves in it and when you create the vacuum in the first filter (water trap?) that vacuum will be sucking the lift pump's inlet valve hard onto its seat. Although I am surprised the valve seals well enough to sustain the vacuum overnight it clearly does. This means the filter and pipe work from shut off valve to lift pump is leak free but what about the rest of the lift pump and the system to the transfer pump in the injector pump.

 

Beta have been known to fit porous filter tops or filter tops that crack. If the Standyne pump is anything like the DPxs then it may have a strainer inside the inlet port.

I was also surprised at how long the system held a vacuum. I am assuming any air must get in somewhere between the tank and lift pump as thats all at slight negative pressure. Lift pump to IP via second filter will be at positive pressure (a couple of psi?) so I would expect diesel to leak out rather than air get in, and I find the air in the second filter head, its a short simple pipe run from the lift pump to the second filter.

I am aware of the filter head hairline crack possibility, a friend had a huge problem with that on a Transit camper. I fitted the first filter myself (I replaced a 296 with a spin on 496 as changing the 296 was a bit fiddly) but it was a very cheap filter head. I have a spare and was just about to do a swap but I reckon the vacuum test has pretty much confirmed that the filter is good. Getting 50 to 100 hours of air free running whenever I change the first filter must be a key fact but I just don't understand it.

 

Ditchcrawler having a similar issue is really interesting, this is turning into one of those things that I really want to understand.

 

Its really bad right now, get a load of air after two hours running, and no need to move the boat for a little while due to broken canal so I need to do some serious investigation.

 

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

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1 hour ago, ditchcrawler said:

I had a very similar problem when I first launched Harnser. I had mounted the first filter with water separator (can't remember the proper name) in the engine room higher than the diesel tank. This was about a 20 foot pipe run from the diesel tank itself. After about 50 hours I would suck air into the system, change the filter and all was fine for another 50 hrs. Ran out of filters so punches a hole through the filter media, and it was OK. From that I deduced that I was pulling too much vacuum on the filter head with the restriction of dragging the diesel all the way up from the tank. I moved the filter to within 5 foot of the tank at low level and not had a problem since. I don't think it was poor fitting of the seals as the access was very good and with the new location its crap and I don't have a problem

I have know a couple of cases of that personally, one tried to bodge it but in the end both had new filter tops supplied by Beta

That is really interesting. In my case its only about 18" from tank to first filter, and the filter is only a couple of inches above the tank height. The 50 to 100 hours is the really puzzling bit. I assumed it was a vacuum increase due to partial filter clogging but I cant see any reading on my vacuum gauge so can only be a tiny vacuum, and I have done three or four filter changes now and its a repeatable behaviour.   In the summer I did a sort of DIY fuel polish so could have put some fine particles into suspension but this should have long since sorted itself out.

 

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

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I had similar which eventually was solved by replacing the flexi hose into the lift pump. The rubber had perished, but was not obvious from the outside, so the pump was pulling air into the feed pipe, which eventually rose to the high point in the supply pipework. If the problem disappears after you bypassed the first filter, then you may have a similar problem in that area. A suction leak never shows any dampness.

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I am another for the pick up/tank, we experienced problems with air on my sons boat after being craned out and transported. I changed pretty much everything from tank to injector pump and still had a similar problem to the one you describe. I haven't had a chance to get into the tank yet but suspect the pick up or debris interfering with it (the previous owners had been into the tank to clean it sometime before we took it).

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