Why do I struggle to bleed my Perkins 4108

[CalicoJack]

Why does it take me four attempts to get the darn thing started after changing the fuel filters? The system consists of two fuel filters, one with a glass water trap. I bleed each filter (Lucas CAV) until fuel runs clear using the lift pump. Then I slacken the 8mm bolt on the rotary pump and bleed that, using the lift pump, until that runs clear. Then I slacken the union on the injectors and crank the engine over until that runs clear. At each bleed point, where I use the lift pump I keep pumping until I get clear fuel, then pump ten more times, just to be sure. Four attempts yesterday before it burst into life. Clearly there must be air still in the system, but why can't I get it out first time?

Any advice would be most welcome

 

[Billjratt]

Does the spillback go all the way to the tank, or just to one of the filters? I have one CAV filter/sedimentbowl then the lift pump, then a secondary filter. my 4108 doesn't seem to need the injectors cracked because there was nothing wrong with that part of the fuel circuit to start with _ I just bleed the filters. The spillback line catches any bubbles I miss at the secondary filter before they get to the high pressure pump .
Maybe there is a problem - like gunge in the tank outlet - that is causing low fuel pressure/air suction. Does the lift pump feel normal when you're bleeding manually - can you hear fuel going back to the tank? (I can judge the health of my filters by listening to the flow rate)

If your spillback looks like mine, you shouldn't need to bleed past it unless you have had an engine stop due to lack of fuel.

 

[Roberto]

If you are simply changing the filters, as Billjratt says there is no need to bleed the injection pump: just bleed the two fuel filters through their vent bolt by pumping on the lift pump (I use an outboard squeeze pump but the effect is the same), then the fuel goes from the engine filter through the return pipe into the tank.

Should you need to bleed the high pressure side (because you ran out of fuel, injection pump servicing, etc), there are three points: 1.a bolt on the body of the pump; 2.a bolt on the regulator of the pump; 3.the fuel inlet pipe.
Then a couple of injectors, full throttle and tighten the injectors back as the engine starts.

 

[contessaman]

I must be doing something wrong with mine then. I replaced both filters and bled the filters until the fuel ran clear of bubbles at the lift pump (my tank is higher than the engine so thats easy) cranked the engine which started and ran for about 10 seconds then stopped, I then had the same ordeal as the OP having to carry out several cycles of bleeding at the HP pump and the atomisers before she would run.

 

[Ruffles]

I used to follow the manual and bleed the system as you describe. Don't bother now. Just close the tap on on the tank to stop the pipe emptying and prime both new filters with clean diesel as I assemble. This is a bit messy with the CAV filter but the bleed screw above the fine filter on the VP 2030 is the high point; pour clean diesel in the hole and all air is eliminated. Works for me anyway.

The only time I have ever had to bleed the injectors was when I ran out of fuel!

 

[Roberto]

I then had the same ordeal as the OP having to carry out several cycles of bleeding at the HP pump and the atomisers before she would run.

My setup is as follows: tank (higher than the engine) ->squeeze bulb outboard pump -> primary filter -> lift pump -> secondary filter (on engine block)

After having replaced the two cartridges, I pump with the bulb until I have clear diesel, without bubbles, from the bleeding bolts (lower one first, engine filter second). After having closed the bleeding bolts, I keep on pumping until I usually hear a bubbling noise inside the tank, not sure if it is air or fuel drops coming from the return pipe.

If I replace the primary filter only, I only bleed the primary filter, without touching the secondary/engine filter, giving a few more strokes with the squeeze bulb after having closed the venting bolt.

I have run about 1200-1300 engine hours, changing the primary filter every 100 hours, basically the only time I had to bleed the injection pump was after it was taken off for rebuilding..

 

[contessaman]

Hmm... I dont get the noise of diesel return back into tank happening no matter how long I prime. This is making me even more suspicious of my lift pump. I was suspicious a month ago - I changed the oil and seemed to take a lot more out than I needed to refil it to the full on the dipstick -as if the engine were making oil- still monitoring levels at the moment as I havent put many hours on the engine since. Maybe my lift pump is shot and thats making priming difficult and also leaking diesel into the lube oil. thats the problem with gravity feed the engine will still run even if the lift pump is shot...

I seem to have the same setup as everyone else here re: 2 filter housings etc. an an aside, I dont know if its worse on my engine as im direct cooling, but its a swine to get to all the bleed points on the HP pump with the exhaust manifold in the way. I could get maybe a tenth of a turn at a time with a spanner. wouldnt want to have to do that at sea in an emergency...

 

[Roberto]

Hmm... I dont get the noise of diesel return back into tank happening no matter how long I prime.

This may be due to the final section of the return pipe inside the tank being deep, always immersed into the fuel; in this case one could only hear air bubbles -if any-, though with a weakish lift pump it might be difficult to push air that deep

I tried the lift pump while detached from the engine block: I reckon one squirt is about a few ml (a squeeze of the outboard rubber bulb is over half a glass), so the lift pump should be used for quite a long time...

 

[Billjratt]

sounds like your lift pump diaphragm has ruptured contessaman. That means it won't work properly, and your lube oil is being augmented by fuel.
You could have a big problem if the rings let a fuel mist past and the engine goes into hyperdrive.
Get it all fixed ASAP!

 

[contessaman]

sounds like your lift pump diaphragm has ruptured contessaman. That means it won't work properly, and your lube oil is being augmented by fuel.
You could have a big problem if the rings let a fuel mist past and the engine goes into hyperdrive.
Get it all fixed ASAP!

yeah I think that might be the case, its the only way I can account for the amount taken out versus what I put in. Since the pumps are so cheap im going to replace it soon anyway. I just need to check whether I have a 2 or 4 bolt pump next time im at the boat (4 hour round trip from my house ) then I'll order one. In the meanwhile I'll use the motor with caution keeping a close watch on the level. if its not the diaphragm then I understand next suspect is the HP pump or piston rings. lets hope its neither of these its a pity becuase she starts and runs beautifully.

 

[ianj99]

yeah I think that might be the case, its the only way I can account for the amount taken out versus what I put in. Since the pumps are so cheap im going to replace it soon anyway. I just need to check whether I have a 2 or 4 bolt pump next time im at the boat (4 hour round trip from my house ) then I'll order one. In the meanwhile I'll use the motor with caution keeping a close watch on the level. if its not the diaphragm then I understand next suspect is the HP pump or piston rings. lets hope its neither of these its a pity becuase she starts and runs beautifully.

I fitted an electric lift pump to my 4108. ASAP sell them for about £35. Its fitted below the level of the fuel - just after the primary filter. I left the engine lift pump plumbed in as well. It makes bleeding the air out a lot easier.

 

[CalicoJack]

Thanks for all the thoughts. Every time I have tried to change the filters and only bleed to the fuel return the engine runs for a few seconds and then stops. I'm sorry that I'm not alone in this, perhaps the way forward is the electric lift pump?

Nigel