Heretical thoughts, Perkins 4108

[huldah]

After having fixed major problems, I was surprised to find more air getting into the fuel system. The only place left was the copper run off pipe joint to the engine. This has been prone to problems, because the pipe was fixed at a right angle to the engine, and was stressed every time the engine rocked. I am rerouting a new pipe to overcome this. I did not expect a leak in the fuel run off pipe to cause problems.

However, while driving, I suddenly realised that as the fuel pump only sent fuel on the upstroke, the engine would be starved on the downstroke. (Should sound like a doodlebug)! So how is this problem overcome in a sealed fuel system? The only possibilities that I can think of is whether there may be a bellows device on the injector pump, or the fuel in the fuel run off pipe is utilised. If the latter, then a leak here would allow air in.

I feel a bit like the guy who said the world was round. I will be interested to know what others think!

Philip

 

[supermalc]

The fuel pump has an unlimited supply from the fuel tank, and is designed to keep constant pressure (even on the down stroke, as the up stroke pumps more than the engine can use) to feed the fuel injection pump. This should purge any air in the system.

I read somewhere fitting a temporary clear flexible fuel line will show up any air bubbles entering the system....a visual way of determining where the air is entering the system before the injection pump.

 

[pappaecho]

Purging the lift pump system on a 4108 is not easy. I had a similar problem with a copper washer - reused - which was leaking air ( fit new copper washers on banjo unions). Using the hand cranking lever on the lift pump, you should crack the union on the inlet side of the injector pump and pump the lift pump until no air is coming out of the
joint - only diesel.
Then you should tighten this union and then crack at least two of the incoming injector unions and crank the engine - again until there is no air coming out. Tighten the loose unions on the injectors, preheat as normal and the engine should start first go. Run it for good few minutes.

Bear in mind that the lift pump delivers more fuel than is needed, the excess is returned by the injector pump to the fuel tank. Presumably there is a filter/water separator on a bulkhead somewhere. Ideally you should have a wire reinforced diesel proof flexible pipe from it to the inlet of the lift pump. No solid pipes because they are bound to fracture sooner or later due to flexing and vibration - the ideal time for this is in a Force 7 on a lee shore

 

[rjweatherhead]

Two years ago we had a very slight leak at the water separator - not enough to release a noticeable amount of fuel but enough to allow air in when our 4108 was running. This resulting in the engine speeding up for a few seconds - if we had had a clutch I would have said it was slipping! This first happened on a night crossing from Mallorca to Barcelona - and caused my heart to race a bit too!
I note that we have a solid copper pipe from the separator to the pump on the engine itself - I will look carefully at this after pappaecho's comment.
I agree it is very difficult to bleed the 4108. After half an hour and air bubbles still appearing (with me on one side of the engine and my wife working the pump on the other!), I said sod it (so did my wife!) and we tried starting the engine. It worked fine. Could this damage the engine - how purist must one be?
Rick

 

[huldah]

Thanks for the answers.

I noticed a slight dampness on a injector run off banjo, so will do this next.

On the weekend, I took the water separator off to try and get a new rubber seal from a tractor supplier. (No luck yet, it is probably very old). I then noticed that the glass bowl was a size too small. This meant that it went on in a different position each time. I also was surprised to see how much the metal bent if over tightened, i.e. over tightening increased the risk of a leak. I roughed up the glass bowl edge with a diamond sharpener and built it up in three places with resin, so that it always fits centrally. I also rubbed the bowl on a larger diamond sharpener, to make sure it was completely flat. So far no more leaks. The engine is already running better. I do have a flexible pipe to the fuel pump. This was my first leak, the joint fell apart as I lifted it! Unfortunately, the new fuel filter has decided to leak instead. Oh well, I will win in the end!

I have spent my life sorting complex mechanical problems, so I tend to notice things that any normal person would take for granted. That was the reason for my second paragraph. On a petrol engine the pulsed pumping of fuel is compensated by the float chamber which acts as a reservoir. What occurred to me as I was driving, was that without a float chamber, the diesel engine must have another means of keeping a constant flow inside the injector pump. I wonder if any one knows how this is achieved? Sorry, it's just the way I was built!

Philip

 

[TheBoatman]

One method I have used!

Over the years I have bled many diesels and I must admit that one method that seems to work MOST of the time and stood me in good stead is
1. Batteries must be in good condition (Fully charged).
2. Loosen all bleed screws.
3. Open throttle 50-75%.
4. Turn engine over on key.
5. Go to first bleed screw in the line and when diesel comes out cleanly re-tighten.
6. Work your way through bleed screws in order until you re-tighten injector pump screw.
7. Engine normally fires up.
8. Important point,, at no time must you stop turning the engine until you have re-tightened the injector pump screw.
9. Warning, who ever is "on the key" must keep the engine turning until it fires up and revs away.
10. It helps if you use the lift pump to basically prime the system first.

Try it it works for me 99% of the time. /forums/images/graemlins/smile.gif

 

[Aja]

Re: One method I have used!

Using this method you must be careful not to llet the starter motor overheat and turn the water inlet off or you could flood the engine with sea water.

Donald

 

[pappaecho]

With regards to float chambers in petrol based carburettors, these have been largely replaced by injector systems which are similar for both diesel and petrol. As i stated above, the system works on an excess of fuel, which is sampled by the injector pump, the rest being returned to the fuel tank. That is why moren cars have a flow and return pipe, and why older cars simply had a flow