Air getting in the high pressure side

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SimonA

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I have a BMC 1.5 diesel engine in my boat and recently started to get problems with air getting into the high pressure side. After half an hour or so of running the engine starts to rev up all by its self every so often, so I will be running at 2000 rpm and it will jump up to about 2400 rpm for a couple of seconds and then back to 2000 rpm. I understand this is caused by lack of fuel. Then when I stop the engine it won't start again unless I bleed number 4 and 3 injector pipes.

I had a slight leak on number 4 injector pipe so replaced this with a new one, this hasn't solved the issue. I have checked all the low pressure side and no bubbles, I have bled the nut on the side of the injection pump and no bubbles.

Any ideas on where to look for a leak? It's starting to get annoying now. I'm suprised air would get in to the high pressure side while it's running.
 
It won't be on the high pressure side. You'll have a leak between tank and injector pump and most likely between tank and lift pump.
 
Not sure this is caused by air.
Could be:
Lube oil getting into cylinder
Sticky governor

If it is either of those it would be prudent to sort it before it gets much worse.

Air will normally give a loss of revs followed by an over-reaction of increased revs, in my limited experience?
 
savageseadog said:
It won't be on the high pressure side. You'll have a leak between tank and injector pump and most likely between tank and lift pump.
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Would that not show up as bubbles when I bleed the fuel filter on the engine? No sign of any bubbles in the fuel anywhere before the high pressure side. I will double check the low pressure side though thank you.
 
lw395 said:
Not sure this is caused by air.
Could be:
Lube oil getting into cylinder
Sticky governor

If it is either of those it would be prudent to sort it before it gets much worse.

Air will normally give a loss of revs followed by an over-reaction of increased revs, in my limited experience?
Click to expand...

Not sure but the only way to get the engine going again and to stop it doing this is to bleed the injector pipes on 3 & 4 and then I find air bubbles. I thought on the CAV style governor/injector pump a loss of fuel results in higher revs? Hopefully it's nothing serious as I spent £3500 on having the engine rebuilt a couple of years ago and it hasn't seen more than 50 hours use since.
 
I'd vote with savagesd - unreservedly. Air being drawn into the system between the tank and the bit that's sucking - possibly exascerbated by a filter that's getting a bit clogged and adding to the work the pump has to do.
Over-tightening olive joints won't help, they need to be properly remade, I get the impression copper work-hardens like SS over the years, if it's an old installation it may be worth re-plumbing.
 
SimonA said:
Would that not show up as bubbles when I bleed the fuel filter on the engine? No sign of any bubbles in the fuel anywhere before the high pressure side. I will double check the low pressure side though thank you.
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You won't see bubbles. It can be very difficult to locate leaks as the leak may not let fuel out but will let air in. You can try pressurising with air and looking or fit a short clear length of pipe going into the injector pump. It's sometimes worth considering changing everything for new.
 
lw395 said:
Not sure this is caused by air.
Could be:
Lube oil getting into cylinder
Sticky governor

If it is either of those it would be prudent to sort it before it gets much worse.

Air will normally give a loss of revs followed by an over-reaction of increased revs, in my limited experience?
Click to expand...

I know that it is counter-intuitive, but an air leak can cause an engine to increase revolutions at intervals. In my case it only showed up after a period of slow running so I cruised happily all day but then experienced random engine speeds while trying to shoot a very low bridge!
 
> I will be running at 2000 rpm and it will jump up to about 2400 rpm for a couple of seconds and then back to 2000 rpm. I understand this is caused by lack of fuel.

The syptoms of an air in/diesel leak are the engine revs drop then pick up, eventually the engine will cut out. Your engine's syptoms are the opposite but I have never come across it or heard of it before.

From what you are saying your cruising revs are 2,000, is that the case?
 
KellysEye said:
> I will be running at 2000 rpm and it will jump up to about 2400 rpm for a couple of seconds and then back to 2000 rpm. I understand this is caused by lack of fuel.

The syptoms of an air in/diesel leak are the engine revs drop then pick up, eventually the engine will cut out. Your engine's syptoms are the opposite but I have never come across it or heard of it before.

From what you are saying your cruising revs are 2,000, is that the case?
Click to expand...

Yes cruising revs are 2000 rpm. just been reading Tony Brooks website, he runs courses on boat engine repairs and knows the BMC well. It does seem the BMC will rev up if it is starved of fuel rather than dropping revs, something to do with the CAV DPA pump.
 
SimonA said:
Yes cruising revs are 2000 rpm. just been reading Tony Brooks website, he runs courses on boat engine repairs and knows the BMC well. It does seem the BMC will rev up if it is starved of fuel rather than dropping revs, something to do with the CAV DPA pump.
Click to expand...

I'm guessing the engine is lightly loaded and a small amount of air causes it to miss a beat then over-correct?
I'll take your word for it, it's new to me.

But, as other have said, air will only leak in where the pressure inside the line is below atmospheric. That means not beyond the lift pump, unless the lift pump is faulty and not pressurizing the line to the injector pump.

One one boat, I traced a leak by applying suction to the line with a big syringe while the fuel was off at the tank valve.
But some leaks only happen under vibration. Make sure joints are not stressed when the engine moves.
Olives and copper pipe are cheap.
Filter and lift pump are suspects too.
 
We have the same engine in the Grumpy Uncle as yours.
Last year we brought her over from Devon to Milford Haven without a single beat missed on a two day voyage. On our 2nd fishing trip we heared the engine revs drop a little then go up higher for a second then back down to normal. It done this 4 or 5 times over a few minutes then the engine died completely. Bleeding injectors got us going again but not enough to get us back to the moorings so we had the tow of shame from a mate!

When we investigated, the problem was the pick up pipe in the tank had a very small whole in it, and as the 2 day stint was done on a brim full tank at all times, the short fishing trip was done on a half tank and the level was low enough for the pin hole to be above the fuel, thus sucking in air from the tank.

Although we had clear fuel pipe...a bit faded but still ok....we had a real job to see any bubbles in the line. We DID see them, but they were very very hard to see.

Hope this helps mate......
Jeff.
 
> It does seem the BMC will rev up if it is starved of fuel rather than dropping revs, something to do with the CAV DPA pump.

Wow, that's a surprise. Sounds like it is a leak and to find it put talcum powder on all joints, the bleed screw, the join between the filter and bowl if you have one and anything else you think may be leaking and letting air in. Leave it for 12 hours. Although sometimes the diesel in the powder is obvious fairly quickly.
 
Damn thats a neat idea KellsEye!!!

IIRC, our engine certainly died down then picked back up, but not to the extent of about 400 extra rpm, but most definately an increase slightly. In our case this was only for a split second, as if you had just "blipped" the throttle on a motor bike for example,then quickly back down to normal running.
 
> Damn thats a neat idea KellsEye!!!

Thank you, praise is rare these days.

One other thought, which I didn't have to do, is if you the leave the talc for a while and there is no sign of diesel: run the engine uder load until the revs increase, then check the talc. If there is no diesel in the talc the leak is probably elsewhere, just a matter of figuring out where it might be.
 
KellysEye said:
> Damn thats a neat idea KellsEye!!!

Thank you, praise is rare these days.

One other thought, which I didn't have to do, is if you the leave the talc for a while and there is no sign of diesel: run the engine uder load until the revs increase, then check the talc. If there is no diesel in the talc the leak is probably elsewhere, just a matter of figuring out where it might be.
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Super, thank you. I will give that a go.
 
That's all good advise but the symptom, as several have suggested and I agree, is caused by air getting in on the low pressure end of the fuel system not by diesel leaking out of the high pressure end. Sure use talc to trace a diesel leak out but that is of no use in tracing an air leak in.
 
theoldsalt said:
That's all good advise but the symptom, as several have suggested and I agree, is caused by air getting in on the low pressure end of the fuel system not by diesel leaking out of the high pressure end. Sure use talc to trace a diesel leak out but that is of no use in tracing an air leak in.
Click to expand...

Don't go there! Logic plays no part in some people's recommendations. See this thread for more details.
 
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theoldsalt said:
That's all good advise but the symptom, as several have suggested and I agree, is caused by air getting in on the low pressure end of the fuel system not by diesel leaking out of the high pressure end. Sure use talc to trace a diesel leak out but that is of no use in tracing an air leak in.
Click to expand...

I know what you are saying, but the method is worth a try, as where air is leaking in by suction, a trace of disease-all will often leak out by capillary action.

And of course, the suction is greater when the fuel is being pumped, a leak may show when it's not.
It's another weapon in your arsenal and will make a change from stripping, re-assembling and hoping.

When we had issues with a friend's boat, we temporarily installed a transparent filter after the lift pump to catch the air. We never found the source of the air, it may have all come from a horizontal run in the pipe, or it may have been the CAV filter. Or maybe the 1/4 turn valve on the tank which we replaced....


Good Luck!
Persistance prevailed in the end though.
 
I had this on my 4108. Fitted a day tank filled by the lift pump from the main tank to gravity feed the injection pump and it was much better but bugs in the day tank outlet caused fuel starvation and back came the symptoms you refer to. Cleaned that and all was ok... for now!! Good luck
 
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