Slight diesel knock

Wasn't sure whether to put this in the 300tdi bit, but as its a more general defender question now, I put it in here.

300tdi, with an intermittent diesel or engine knock under load. This has been going on for a year now, I have fitted a new Delphi lift pump, new injectors, checked pump timing, changed fuel filter again which was spotless.

As the fuel cost has come down so much lately, I nearly filled it up today. And the result of the high fuel level, was that the motor was running sweet as a nut, no knocking, more power.

I did a 30mile round trip, and on the way home the fuel level was dropping a bit and the knock seemed to be coming back.
This is making me think it could be fuel supply related. Perhaps an air leak in the supply pipe or a pickup filter issue in the tank. Perhaps causing the FIP to knock.

How difficult is it to remove and clean the pickup pipe? Is there any way of checking for air in the fuel system.

Cheers

Fuel pick up pipe is easy if its a 90. Driver seat base and panel out, its the round fitting with about 6 bolts (from memory) and one pipe. I don't think there is sufficient free play in the pipe to let you get the whole thing out of the tank without disconnecting the fuel union on top of it. Might be worth getting hold of a new gasket first as I think they were cork so may fall to bits on removal.

Kwakerman said:

Fuel pick up pipe is easy if its a 90. Driver seat base and panel out, its the round fitting with about 6 bolts (from memory) and one pipe. I don't think there is sufficient free play in the pipe to let you get the whole thing out of the tank without disconnecting the fuel union on top of it. Might be worth getting hold of a new gasket first as I think they were cork so may fall to bits on removal.

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Cheers, I'll get it in the garage and give it the once over. Glad you mentioned the gasket.

Interesting post. I'll be monitoring this to see the results.

My fuel tank breather hose is actually broken off completely close to the tank and I am going to replace it to see if this makes a difference in performance. I can also hear a slight difference in the noise of the engine when the tank is full or close to full as well.
I understand that the breather hose affects performance of the fuel system as it relates to pressure so I believe there is some relationship there. Even though I cannot confirm that the "noise" I'm hearing is diesel knock, I believe it might be related to the fuel system. Lift pump confirmed working and fuel filter changed recently.

LeftFoot said:

Interesting post. I'll be monitoring this to see the results.

My fuel tank breather hose is actually broken off completely close to the tank and I am going to replace it to see if this makes a difference in performance. I can also hear a slight difference in the noise of the engine when the tank is full or close to full as well.
I understand that the breather hose affects performance of the fuel system as it relates to pressure so I believe there is some relationship there. Even though I cannot confirm that the "noise" I'm hearing is diesel knock, I believe it might be related to the fuel system. Lift pump confirmed working and fuel filter changed recently.

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Not sure where you get that idea from? a breather is just that it allows whatever its plumbed into to breath to atmosphere.
Plus have to add if its broken off at the tank end which if I remember correctly is at the same height as the main tank filler pipe as you say it must be leaking fuel onto the road?

Well honestly as I said i'm not sure. In my mind the broken breather hose means that a part of the tank is "open to atmosphere" as it were. So technically when the pump pulls fuel the tank is no longer pressurized the same way it would be if the tank system was sealed. Kinda like a beverage package with a straw in it. If there is a small hole in the package when you draw the liquid through the straw by suction the hole affects the pressure in the package and hence hinders the liquid coming out. That's my interpretation, I might have it all wrong. And no, my fuel is not spilling out, perhaps if the tank is totally full I might lose some.

To keep an eye that there are no bubbles in fuel line I have a clear pipe by the lift pump.

LeftFoot said:

Well honestly as I said i'm not sure. In my mind the broken breather hose means that a part of the tank is "open to atmosphere" as it were. So technically when the pump pulls fuel the tank is no longer pressurized the same way it would be if the tank system was sealed. Kinda like a beverage package with a straw in it. If there is a small hole in the package when you draw the liquid through the straw by suction the hole affects the pressure in the package and hence hinders the liquid coming out. That's my interpretation, I might have it all wrong. And no, my fuel is not spilling out, perhaps if the tank is totally full I might lose some.

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Fuel tank shouldn't be pressurised as the fuel pump sucks more fuel out the tank than is returned. As the fuel level drops in the tank it has to be replaced with air otherwise you would end up with a vacuum lock. Its the breather/vent system that lets this air in. That said if the tank gets hot in the sun and the fuel expands it will also let excess pressure vent to atmosphere.

No progress on this yet, been out all day trying to re-map a rally car for my dad and then need to get my EVO 6 in the garage to replace some seals, hoping to get my 90 in the garage for the weekend and check out the fuel system, and replace the rear a frame bushes.

Will report back.