Mr Bean Posted February 4, 2008 Share Posted February 4, 2008 As i am new to the world of heavey oil fuel can some-one give me some advice? Why do you need to bleed the air out of the fuel system if it runs out? What happens if you don't do this, will it break anything? Thanks in advance. Trev Quote Link to comment Share on other sites More sharing options...
disco_al Posted February 4, 2008 Share Posted February 4, 2008 i'll have a go......... diesels work on compression ignition, as opposed to a petrol that use a spark. if there is air in the fuel lines, then not enough fuel will be atomised by the injectors to allow the engine to fire, hence the need to bleed the air out of the system after working on it (ie filter change) to ensure that enough fuel goes in. most non starting problems with diesels are usually fuel related, as there is not much else to go wrong (on old diesels anyway, new ones have too many electronics) someone will correct me if i am wrong Quote Link to comment Share on other sites More sharing options...
Gromit Posted February 4, 2008 Share Posted February 4, 2008 After all the theory, I've changed fuel filters, fuel tanks and run out of fuel - a few times. I've never had to bleed it. I just cranked it for a bit and it fired right up, spluttered a few times and then was grand. This is a 200tdi, not sure on an NAD. Quote Link to comment Share on other sites More sharing options...
Mr Bean Posted February 4, 2008 Author Share Posted February 4, 2008 So would damage occur if the bleeding wasn't carried out? Quote Link to comment Share on other sites More sharing options...
Pete Attryde Posted February 4, 2008 Share Posted February 4, 2008 So would damage occur if the bleeding wasn't carried out? I am prepared to be wrong but the worst I can see happening is the battery will go flat as the engine will just crank over and not start. Why do you ask? Pete. Quote Link to comment Share on other sites More sharing options...
Mr Bean Posted February 4, 2008 Author Share Posted February 4, 2008 I was told by someone that if it's not bled it damages the engine. I couldn't see how damage could occur so i thought it better to ask some experts than go around spouting false information. Thanks for the replys so far everyone. Quote Link to comment Share on other sites More sharing options...
billypikey Posted February 4, 2008 Share Posted February 4, 2008 battery will go flat as the engine will just keep cranking over and over Quote Link to comment Share on other sites More sharing options...
rtbarton Posted February 4, 2008 Share Posted February 4, 2008 If there are any air bubbles in the fuel they will compress so pressure will not build up enough to overcome the injector springs so no fuel will get into the cyclinders. The fuel system is lubricated by the diesel so it is not wise to turn the engine over too much with no fuel in the system, although you do have to a bit to bleed the high pressure pipes. Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted February 4, 2008 Share Posted February 4, 2008 Only in the case of Lucas pumps do you have any risk of damaging the engine...and this risk is infinitesimally small. Diesels need to be bled for a number of reasons... Firstly, there is a continuous flow of fuel from tank, through pump and back to tank. This ensures a constant supply of fuel to the injection pump. In the case of a petrol, the lift pump is able to push fuel through the fuel system without the engine running (even in the case of petrol injection models where there is also a return to the tank) this allows air to be bled out of the system. This is not so in a diesel - the engine needs to be turning over to allow flow through the injection pump. So, if you were to run out of fuel in a diesel, you would have to crank the engine over long enough to draw fuel out of the tank, and into the injection pump. Secondly, in normal operation the injection pump squirts a small amount for fuel down the high pressure lines and causes a wave of pressure to propagate along the pipe to the injector, which then opens, squirting fuel into the engine. If air is in this system, in much the same way as air in a brake line, the fluid in the pipe is made compressible. This absorbs the high pressure wave and the injector fails to fire. This is why you should slacken the unions at the injectors to bleed the system - in practice, not necessary with a beefy enough battery. Lucas injection pumps differ from most other kinds in that they use the fuel as hydraulic fluid to transmit forces within the pump (this is also the reason why they don't like cold vegetable oil). There is the possibility that air may enter the speed regulator allowing the engine to overspeed and self destruct. However, if air gets into the regulator, chances are that it is already in the high pressure lines meaning that the engine won't run anyway! This is the reason for the different fuel systems on Lucas and Bosch pumps. Lucas pumps have a more robust fuel supply, using the fuel filter as a swirl pot (This can be seen on any LR diesel pre TDi). Bosch systems don't bother with this as the injection pumps don't suffer from the same problems. The correct way to bleed a diesel fuel system would be to: Crack off the bleed screw on the fuel filter and pump the hand primer on the lift pump until fuel comes out the bleed screw. Tighten bleed screw. Crack off the injector unions and crank the engine with the throttle wide open until fuel appears at all of them. Tighten unions. Crank engine with throttle wide open until it fires. In practice...you can get away with just cranking the engine with throttle wide open till it fires. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted February 4, 2008 Share Posted February 4, 2008 .......Crack off the bleed screw on the fuel filter and pump the hand primer on the lift pump until fuel comes out the bleed screw. Tighten bleed screw. Crack off the injector unions and crank the engine with the throttle wide open until fuel appears at all of them. Tighten unions. Crank engine with throttle wide open until it fires. Sometimes there are bleed screws on the body of the injector pump, air should be bled from these between bleeding the filter and high pressure pipes. Quote Link to comment Share on other sites More sharing options...
David Sparkes Posted February 4, 2008 Share Posted February 4, 2008 I can see the content of this thread being slightly confusing to other newcomers to the world of diesel, so I'll stick my neck out slightly, in an attempt to clarify. Some people will call this pedantry. Which answer fits largely depends on the engine you are concerned with, without that information, all the different answers are correct. With respect to Land Rovers, the 'TDi' engines, 200 or 300, can probably be cranked to start, with the only risk of damage being to the battery or starter motor. Bleeding these, by opening intermediate joints, will reduce the workload on those two items, but bleeding is messy and smelly, so turn the key is the normal order of the day. The pre-TDi engines, (2.25, 2.5 NA, 2.5 Turbo) using Lucas or CAV pumps, are much more likely to need manual bleeding, initially with the manual lever incorporated in the pump on the engine block. The manual bleeding may well include opening the system at several places, such as the feed into the injector pump, bleed screws on the injector pump, and the delivery pipes at the injectors. Even after all that, some electric bleeding, via the starter motor, may still be called for. Note that manual operation of the engine mounted lift pump can be ineffective if the cam stops in the 'wrong' place, thus holding the pump mechanism in the fully operated position. In this circumstance the lever always moves easily, because it is not moving the pump itself. I'm not venturing to comment on the TD5 and more modern engines, although I'd expect them to be self-bleeding. Cheers :-) Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted February 4, 2008 Share Posted February 4, 2008 Well said David. A little more clarity than my short essay Quote Link to comment Share on other sites More sharing options...
Mr Bean Posted February 4, 2008 Author Share Posted February 4, 2008 Thank you all for those who posted and it makes sense now. I can go forth and pass on the knowledge! Quote Link to comment Share on other sites More sharing options...
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