OK - I give up ! And I thought I understood diesels !!!

superheat6k

superheat6k

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In some prior threads I have referred ad nauseam to the nightmare of an engine which will, without due reason, periodically refuse to start. In my case the starboard.

So today it decided this morning, yet again, it would start for ~ 2 seconds then stop and refuse to restart. However, I now have this routine sussed - proceed through a simple fuel bleeding exercise:

1. Vent the on engine fuel filter (no air),

2. Vent fuel pump lower bleed (a few tiny bubbles) and

3. Vent upper bleed (no air / bubbles)

4. Crack off the easiest four of the six injectors, spin over for ~5 seconds, re-tighten these.

Then after a good spin away she went. She proceeded to behave herself all day at subsequent restarts, including twice playing musical movements at a moderately busy and very understaffed Cowes Yacht Haven.

I have no leaks evident anywhere on the fuel system, yet the engine seems to acquire an air gulp leading to an air lock in the HP fuel injector pipes during the period of shut down. Fuel tanks are filled to ~ 3/4, and at approximately the level of the engine fuel pumps, so no negative pressure from a lower fuel level.

So any ideas where I look next ?


BTW I spotted a rather bewildered looking Colhel of this parish exiting Cowes Yacht Haven from my table in the Anchor at lunchtime, but he chose not to call by, hey ho.
 
Had this on a car we owned, ended up crack in fuel line, disconnected fuel line ,ran engine direct from separate fuel tank, no problems, then pressurised fuel line to find leak, crack was so small did not leak under normal load but drew in air.
 
Trev
Could try capping-off ends of any fuel lines on suction side of pump and use your fridge vac pump and gauges to check for tiny leaks, too small to leak fuel....? Just a thought if air is getting in somewhere.
 
Do the return lines into tank go to the bottom ? could be taking air in through there if not.
 
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I had similar symptoms and it turned out to be a spin on fuel filter not quite on tight enough. It was just loose enough to not leak fuel out but would let air in.
 
Thanks for all the positive comments ands pointers.

Ben - yes it is a CAV DPA type pump - when I had the problem at first I had the pump fully stripped and rebuilt and set up by PB Asher. I then found after many other red herrings in my quest to resolve the issue until I found an issue with the shut off solenoid return spring not properly positioned into its deeply recessed hole, instead it was sitting upon the shoulder the pintle normally moves against and this preventing the solenoid from always opening properly. I have since replaced the fuel solenoid, and thought that was the end of the problem.

I have also re-wired the entire 12 v distribution with decent quality battery switches, shorter cables,oing from the batteries and a separate take off for the engine ancillary supplies direct from the battery switches rather than the starter terminal, where the voltage drop on start up was at its highest.

But this is definitely an air entry problem. The engine initially starts then stops after say 2-3 seconds and, when it decides to play up, will not re-start unless I manually bleed from: the filter vent - normally free flow of fuel there; lower pump bleed - normally a few bubbles then free flow; upper pump bleed - normally free flow; then crack off the first four injector nuts and spin on the starter for say 5-10 seconds, until the pipes show a dampness of fuel, nip these up tight, then after a good thorough turn for ~ 10-15 seconds away she goes.

I ran the engine last weekend, then yesterday morning as I was about to go out, she played up, but after the bleed through was fine the rest of the day (4 further re-starts) and was fine again this morning.

I did notice a tiny drip of fuel on the fuel filter canister so I tried to tighten this, but that could have been residue from yesterday's bleed from the vent above the filter.

I will remove the lift pump and check the diaphragm and seals, but the stop seems too quick to be that, and certainly nothing further back than the lift pump. The engine oil level is constant dropping slowly with use - if it were a split diaphragm I would expect to see the oil level increasing.

The spill return is the original set-up on the boat. I did recently discover the tank has an inspection hatch so I can look inside if necessary, but the spill line goes up above the engine on its route back to the tank, so if it were just dribbling inside it would not form a siphon.

I do have a position where I intend to affix a suction gauge after the pre filters so I could attach an adapter there to my fridge gauges. I am tempted to put a positive pressure on the supply line, but guess I should disconnect the lift pump first as this would not like say 1-2 bar being pushed into it. I guess I could bridge the lift pump, purge the lines of any remaining fuel and apply say 0.5 - 2 Bar test pressure of air from a simple pump to the rest of the system.

I have just noticed Julians comment. I think this will be the very next check, with a good smear of grease on the Seal ring, and a new filter.
 
You know there is anecdotal talk that with the CAV DPA that where the return line goes back into the tank that the end of it needs to be submerged. I always thought it sounded like a load of bollocks but I'm told its true. Mine are not submerged and seems to be fine.

I have the same engines, once quite inexplicably I have had to bleed them after them doing exactly the same as yours, run for a few seconds, then stop. It was both engines and only once.

As long as you are certain its not V drop at the pump solenoid. Assume like mine yours are energise to run.
 
It may be a long shot but is the fuel pick up pipe pin holed above the tank level? its something which happens on industrial engines and on lots of older cars.
 
In trying to diagnose an air leak into my fuel system I temporarily fitted a length of clear hose from the tank to engine and could then monitor small bubbles being drawn along. Might help narrow down which end of the installation you should be concentrating on.
 
I had an identical problem on my stbd engine,
It was a very small hole in the fuel feed line, letting no fuel out but air in.
I only discovered it when bleeding the fuel system for the millionth time when I could hear a hissing sound, this was air being pulled in.
When running it was fine, I guess it was self bleeding enough air to keep running.
Hope this helps
Terry
 
burgundyben said:
You know there is anecdotal talk that with the CAV DPA that where the return line goes back into the tank that the end of it needs to be submerged. I always thought it sounded like a load of bollocks but I'm told its true. Mine are not submerged and seems to be fine.

I have the same engines, once quite inexplicably I have had to bleed them after them doing exactly the same as yours, run for a few seconds, then stop. It was both engines and only once.

As long as you are certain its not V drop at the pump solenoid. Assume like mine yours are energise to run.
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Thanks Ben - I have extensively proven it is not the solenoid ! Now the problem consistently resolves after a bleed at the pump itself, plus the stop is so quick after the initial brief start, it is like there is a bubble waiting to enter the central chamber of the distributor. The lower bleed then shows a few tiny bubbles.

I have had private advice of possibly a leaky O ring on the pump assembly, possibly due to pattern parts or attack by bio fuels, but I am not sure if I can strip it to O ring level without upsetting the fine tuning settings. I don't much fancy another £400 to Asher's !

However, I think I will have to speak with PB Asher on this, after all the engine has only run ~ 10 hours since the pump was rebuilt, albeit 18 months ago. I know getting the thing off and back on again was a right PITA.

I do wonder if I thoroughly smothered the pump joints with a heavy grease if that might stop the tiny amount of air that is entering. However, for air to enter then either the pump must be at a slight negative pressure during shut down, or else the fuel that was filling the space has to move somewhere, and both the fill and spill lines are above the pump level, although I guess it could siphon away through the spill line if the end were immersed, which could cause a slight negative pressure at the pump.

Any idea what the safe max pressure I could squeeze the pump to with air to see if I can find a leak out - a leak out under pressure = a leak in under light vacuum. I reckon it should put up with 0.5 Bar, which I could easily achieve with a cycle tyre pump !
 
Had it with cav 296 pre filters, the ones with the glass bowl, and the need for three rubbers, took ages to find but in the end it was the "SILLY" plastic drain screw on the bottom of the unit, replaced with a steel screw and problem solved. You do not mention what type of pre filters you have, hope this is relevant.
 
omega2 said:
Had it with cav 296 pre filters, the ones with the glass bowl, and the need for three rubbers, took ages to find but in the end it was the "SILLY" plastic drain screw on the bottom of the unit, replaced with a steel screw and problem solved. You do not mention what type of pre filters you have, hope this is relevant.
Click to expand...
The pre filters are Racor 500, but several meters from the lift pump. This air seems to already be in the pump when I start the engine after a prolonged shut down, say ~ one week or longer. The Racors are brimmed whenever the cap is removed, e.g. to change an element. I also have full fuel no air at the bleed point above the finial filter at the engine.
 
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