Bleeding the fuel System

[saltysplash]

I need to replace my fuel filters and while doing that aint a problem, bleeding the system afterwards is my concern.

 

 

On the old tub, I used to turn off the stopcock from the tank. undo the banjo nuts and replace the filter, then, turn on the stopcock and the ole Gardner would fire up as if nothing had changed.

 

Reading the manual for my Perkins 3HD46 it seems i would need to bleed the low pressure system and high pressure system when i change the filters

 

Q1. why is this, if i shut off the fuel tap why would i get an air trap on this engine and not my old Gardner?

Q2. If i have to bleed the high pressure system i assume this is all after the fuel lift pump, and would have to turn the engine over to do it, so, do i undo every nut to each injector and have plenty of rags placed around the lunp to catch the excess and is it best to do it by starter or by hand?

[RobinJ]

I need to replace my fuel filters and while doing that aint a problem, bleeding the system afterwards is my concern.

On the old tub, I used to turn off the stopcock from the tank. undo the banjo nuts and replace the filter, then, turn on the stopcock and the ole Gardner would fire up as if nothing had changed.

 

Reading the manual for my Perkins 3HD46 it seems i would need to bleed the low pressure system and high pressure system when i change the filters

 

Q1. why is this, if i shut off the fuel tap why would i get an air trap on this engine and not my old Gardner?

Q2. If i have to bleed the high pressure system i assume this is all after the fuel lift pump, and would have to turn the engine over to do it, so, do i undo every nut to each injector and have plenty of rags placed around the lunp to catch the excess and is it best to do it by starter or by hand?

It depends on the setup. Engines are often fitted with a filter which is quite low in relation to the engine, in marine installations these are often mounted higher up on the top of the engine or adjacent bulkhead.

Provided the fuel cannot run out when the filter is removed, no problem, but if the fuel pipes drain off, then you will have to bleed the low pressure side.

Unless your unlucky, and don't get all the air out, I cannot see why the high pressure side would need bleeding, although you may have to use the inlet bleed on the pump!

Mine is mounted at the back of the block and I find I can bleed from the top of the filter, using the lift pump, and everything works fine.

[Tony Brooks]

I need to replace my fuel filters and while doing that aint a problem, bleeding the system afterwards is my concern.

On the old tub, I used to turn off the stopcock from the tank. undo the banjo nuts and replace the filter, then, turn on the stopcock and the ole Gardner would fire up as if nothing had changed.

 

Reading the manual for my Perkins 3HD46 it seems i would need to bleed the low pressure system and high pressure system when i change the filters

 

Q1. why is this, if i shut off the fuel tap why would i get an air trap on this engine and not my old Gardner?

Q2. If i have to bleed the high pressure system i assume this is all after the fuel lift pump, and would have to turn the engine over to do it, so, do i undo every nut to each injector and have plenty of rags placed around the lunp to catch the excess and is it best to do it by starter or by hand?

 

 

There is a very good chance that you will not have to bleed the high pressure side but if you have to you do loosen the injector unions and I would spin the engine on the starter until and tighten each one as they drip. You can also wait until all drip and the tighten them. If you need loads of rags there is a problem. At full power most diesels inject less than a pinhead of fuel each time, so you only get a few drips. A bit of tissue should Eb fine. Also only slacken the unions about half a turn - 2000 to 2500 psi will easily find its way out!

 

I do not see how anyone can chnage a filter without trapping at least a bubble of air in the filter heda - and the there is teh question of a possible strainer in the lift pump (not familiar with that engine). Then the issue is how can the bubble of air escape without going through the system. Modern engines tend to bleed ("leak") a significant amount of fuel back to the tank from the inlet side of the injector pump, so any air is sent back to the tank before it can enter the injector pump. Older ones like the BMCs only had a 0.5mm hole in the top of the filter and no bleed back from the pump inlet so air trapped in the pipe between filter and pump can not escape.

 

So it all boils down to engine design. I afraid comparing a Gardner to a Perkins is a bit like comparing a BMW to a Ford. Gardners would have probably felt it worth while to spend a little money making their engines easier to maintain. Perkins were/are more concerned by cost.

 

Even on modern "self bleeding" engines I think it is well worth while to learn how to bleed them manually because it helps with faultfinding and of course the manufacturers fail to add "as long as you have well charged batteries" to the statement about being self bleeding.

[Malarky]

The main problem is an air gap caused by the new filter containing air,or pipes to it as mentioned. The filter system can be rigged to self bleed and fill via the return pipe, so I expect the Gardner fired up on the diesel in the pump, then filled the filter for you.

Possibly best to bleed diesel into the filter, then run the engine off what is in the pump, to draw any residual air up to the injectors before putting a spanner to them, otherwise you may have to pump the initial diesel in the injector pump out before hitting the air. Alternatively just bleed the injector pump next.

[Orca]

I've got a 3HD46 and was planning on changing my fuel filter over the next fortnight or so - never done it before though, but have had some experience on 4 cylinder Perkins lumps. These normally fire straight away after filter changes, usually just by filling the filter with diesel prior to installation. If not, then I have just cracked off the banjo nuts on the main fuel pump and bled via the lift pump. This has usually worked and I have never as I recall had to crack off individual injectors. The 3HD46 will no doubt prove to be a different beast entirely though... Would like to know how you get on sir!!

 

ex-Copernicus

[saltysplash]

Thanks for the replies folks, Im always a bit scared about touching something that is already working coz i have previous for sheering bolts and making a simple job into a major job. last time i bled an engine was an old 1940 4 pot lister on a dutch barge and was surprised how easy it turned out to be, but, that was then, and it was somebody elses boat.

 

My Gardner was a 1945 4L3 and no matter what i did to it it just kept running.

 

I love the Perkins 3HD46 as its so simple but know full well, if i touch it, it'll never start again.

 

Owners manual is quite clear so maybe i should just doooooooo it!....be brave grasshopper

 

 

Orca, will let you know the result when i pluck up courage

[Orca]

Righty ho - best of luck!!

[saltysplash]

I've got a 3HD46 and was planning on changing my fuel filter over the next fortnight or so - never done it before though, but have had some experience on 4 cylinder Perkins lumps. These normally fire straight away after filter changes, usually just by filling the filter with diesel prior to installation. If not, then I have just cracked off the banjo nuts on the main fuel pump and bled via the lift pump. This has usually worked and I have never as I recall had to crack off individual injectors. The 3HD46 will no doubt prove to be a different beast entirely though... Would like to know how you get on sir!!

 

ex-Copernicus

Finally got round to changing the filter yesterday.

 

All went fine till it came to bleeding the system.

 

Followed the manual and loosened the banjo nut then the two bleed screws on the injection unit and began pumping away with the fuel lift pump. Nothing.

 

Then spent the next 2 hours pumping and faffing and removing the fuelout line from the pump to see if the pump was actually working and no it wasnt......it was then that i took another glance at the Manual and found that the lift pump wont work if the cam is in its top position.

 

I turned the engine over a couple of time and went back to the lift pump which now had 'real' pressure on it when pushed down. within seconds fuel bled from the loosend nuts and i tightend em up in the required order. 2 more minutes to then bleed the injectors and the engine fired up a treat.

 

So the job is actually very simple providing you dont waste 2 hours pumping a lever that doesnt do anything :/