Air in fuel system, any help in diagnosing?

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pcatterall

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I have one of those mystery leaks and would like to know if there is some diagnosis that will help me locate it.
Engine starts and runs well at normal cruising speeds BUT repeatedly has failed after prolonged use on tickover ( like waiting in a French lock). Once failed it needs a complete bleed to get going again.

The system ( on a 4108) has A water seperator, A filter with a water bowl then after the pump the main engine filter. All CAV units.

I have gone through the entire system , thought it was ok then it failed again.

Any clues as to the sort of leak that causes this sort of failure which may help me focus my attention will be valued.

Yes I know that CAV is old hat and guess I dont need the seperate seperator etc but this is the system I have!!

many thanks
 
You can try fitting a priming pump like the ones used on outboard fuel tanks supply pipes to get fuel through initially. This can be useful to pressurise the inlet side of the system to look for leaks. Another thing to check is the fuel hose clips. Are you using jubilee clips or proper fuel clips? There is a difference. The jubilee clips don't compress the fuel line properly and can be a source of air leaks. As mentioned in another thread running at the moment, ditch the CAV filter set up and go for a spin on filter. You may find that undoing everything and simply putting back together carefully solves the problem, especially if you use proper fuel clips
 
I'm not sure if this helps, but a friend had a system like yours which would allow air into the system after periods of idling. He fitted an electric fuel pump in the hope of over coming the problem and discovered that there was a hair line crack in one of the CAV filter heads which would allow air in when the pressure dropped at tick over. The crack was so small that it did not show signs of leaking during normal running. The electric fuel pump created enough pressure to show up the leak.

Might I suggest that you try to pressurise the system in the hope of finding something similar?
 
I agree that pressurising the system may be revealing. Doubly so if you clean it all up and sprinkle with talcum to detect any leaks. There are a number of mechanisms for irritating leaks of the sort that stop the engine rather than slosh fuel into the bilge. Apart from the cracked components suggested above, I was thinking of a badly seated o-ring seal in the CAV units (many people say this is common, but I've never known it) or in my case old flexible fuel hoses which had softened under the pressure of the jubilee clip and solvent nature of the fuel. No fuel leaked out, but when running it could suck air in! This fault was only discovered because one of my crew - an experienced diesel mechanic - was utterly methodical in looking for the fault at every possible site along the line. Cutting off the mangled end and remaking the joint was a complete cure.

Rob.
 
Hi Peter, I'm really just responding so that this appears in my 'subscribed threads' list. A couple of thoughts as below.
I can't help thinking that, as this problem first began to occur after you changed filter elements, the cause must lie within the filter assemblies. Perhaps a cracked casting as someone has suggested or a crimped/misplaced O-ring. Would it be possible to narrow this down by by-passing the sedimenter and the primary filter and seeing if the air problem disappears? The engine would still be protected by the secondary filter.
I've read several reports of people having problems with CAV filter assemblies and recommending Racor spin on units but I had a CAV on my last boat and never had any problems. Plus they're widely used on Landrovers and tractors.
Hope it's not too hot in France and that your Aircon is keeping you comfortable!
 
ghostlymoron said:
Hi Peter, I'm really just responding so that this appears in my 'subscribed threads' list. A couple of thoughts as below.
I can't help thinking that, as this problem first began to occur after you changed filter elements, the cause must lie within the filter assemblies. Perhaps a cracked casting as someone has suggested or a crimped/misplaced O-ring. Would it be possible to narrow this down by by-passing the sedimenter and the primary filter and seeing if the air problem disappears? The engine would still be protected by the secondary filter.
I've read several reports of people having problems with CAV filter assemblies and recommending Racor spin on units but I had a CAV on my last boat and never had any problems. Plus they're widely used on Landrovers and tractors.
Hope it's not too hot in France and that your Aircon is keeping you comfortable!
Click to expand...

Sensible suggestion by-passing the filter, i may try that on mine. A cheaper version of spin on filter suggested at reply #16 on my thread http://www.ybw.com/forums/showthread.php?366749-Fitting-a-CAV-filter-assembly/page2
 
In my case after a year I found the bleed screw seat on the final filter before the injector pump had a scar on it. I thought that the pressure from the lift pump would cause any leaks to be fuel out, but this bleeder (language, Timothy!) was between the last filter element and the injector pump, so when a bit of crud accumulated it sucked air in. It was always OK on new filters, then trouble after a few days.
 
I've mentioned this many many times before. Put talcum powder on the bleed screw, all pipe fittings, around the bowl where it screws onto the fuel filter and around the water separator bowl and if they have bleed screws on the bottom around those too. Run the engine until the revs drop then wait for five minutes, you will easily see any leaks. Please let us know how you get on.
 
Last edited:
Thanks guys. Plenty of good tips for me to try. Talc powder being my favourite!!.
I will see if I can simplify the system as well. Possibly remove the water sedimeter ( as there is one in the filter)
and I may just send the return fuel directly back to the tankas it currently goes to the top of the engine filter then back to the tank. This means there is no real bleed screw on the engine filter I have to use the screw which holds the 2 banjo connectors for the return fuel. There was a bit of a debate about this return fuel route last year but it was inconclusive.
Thanks again.
 
pcatterall said:
Thanks guys. Plenty of good tips for me to try. Talc powder being my favourite!!.
I will see if I can simplify the system as well. Possibly remove the water sedimeter ( as there is one in the filter)
and I may just send the return fuel directly back to the tankas it currently goes to the top of the engine filter then back to the tank. This means there is no real bleed screw on the engine filter I have to use the screw which holds the 2 banjo connectors for the return fuel. There was a bit of a debate about this return fuel route last year but it was inconclusive.
Thanks again.
Click to expand...

Every boat I've been on with that arrangement has been changed to direct back to the tank.
 
I just commissioned an installation which earned a "Please Re-do! Love from Teacher!" the first time I went to check it.
What happened was they had a kinked supply line and the lift pump wasn't man enough to overcome the resistance, yet when the engine was run for a bit then allowed to stand, the starvation was reduced. No air in the system, just a whole lot of vacuum!
It took about twenty minutes to starve the pump, but only sea trialling proved the problem wasn't an air leak!
 
fisherman said:
Every boat I've been on with that arrangement has been changed to direct back to the tank.
Click to expand...

I now realise that there are two returns. The top one is from the injectors and there is another one from the injector pump area. The top one looks easy to 'bypass' and would a) provide a proper bleed screw and b) give me easy access to the filter securing bolt whose head is currently obscured under the said return pipe.
Thanks for the help
 
>Trouble with talc is, it will show fuel leaking out but, not necessarily, air leaking in. but worth a try.

Diesel can only leak out if there is an air leak, diesel can pass through a microscopic hole, so the talc shows where both the air and diesel leaks are. Fixing it is stopping the air leak and thereby the diesel leak.
 
KellysEye said:
>Trouble with talc is, it will show fuel leaking out but, not necessarily, air leaking in. but worth a try.

Diesel can only leak out if there is an air leak, diesel can pass through a microscopic hole, so the talc shows where both the air and diesel leaks are. Fixing it is stopping the air leak and thereby the diesel leak.
Click to expand...

Yerbut, if the leak is at the top, ie under no pressure but subject to vacuum, you only get air in and no diesel out, so the first assertion applies.
 
Don't ignore the possibility of debris of some kind in the tank, just been through this situation and found lumps of a gel type black material which caused ours. It was blocking the tank outlet and a stripped thread on the lift pump union and a stripped bleed screw on the fuel filter (not done by us) let the air in. It's possibly diesel bug but I'm not sure. The
 
savageseadog said:
Don't ignore the possibility of debris of some kind in the tank, just been through this situation and found lumps of a gel type black material which caused ours. It was blocking the tank outlet and a stripped thread on the lift pump union and a stripped bleed screw on the fuel filter (not done by us) let the air in. It's possibly diesel bug but I'm not sure. The
Click to expand...

The OP said "Engine starts and runs well at normal cruising speeds", so a blocked pickup pipe isn't a possibility.
 
What I don't understand is why air is drawn into the fuel system when ticking over when the suction should be lower as opposed to when running at higher revs with a higher rate of fuel flow and a higher suction in the fuel system.

You say it is repeatable, but is it perhaps a function of the length of time the engine has been running rather than the revs at the point the engine actually stops?

In my case the problem was fuel pipe partially blocked with crud which led to the lift pumps creating much higher suction at the inlet. Wasn't obvious and I was able to fix the air leak, but things weren't totally right. Only finally worked it out after the crud completely blocked the fuel pipe.
 
Ah, yes, fails only at tickover... Well, my first thought on that is that perhaps the lift pump is inefficient (leaking diaphragm?) so whilst OK at cruising revs it may be insufficient flow at low revs. I've often wondered about my own lift pump but as the tank is mostly above the engine gravity feed is probably sufficient until the tank gets pretty low - and I don't use enough fuel in a season to have found out! I did find that when renewing fuel lines, the fuel level needs to be above the filter so that gravity allows me to bleed up to that point and then a few bursts on the starter allows me to bleed the remainder of the plumbing.

Rob.
 
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