Air in fuel

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qwerty
Posts: 24
Joined: 03 Nov 2002, 01:10

Air in fuel

Post by qwerty » 02 Mar 2003, 04:51

Can anyone tell me if it is possible for diesel injection pump to let air into system via faulty seal behind drive gear as I have tried just about every other diagognsis to get rid of air from fuel system.
Vehicle is Peugeot 405 diesel

Dave Burns
Posts: 1916
Joined: 14 May 2001, 05:30
x 2

Air in fuel

Post by Dave Burns » 02 Mar 2003, 16:05

Which pump is it, and what are the symptoms.
Dave

qwerty
Posts: 24
Joined: 03 Nov 2002, 01:10

Air in fuel

Post by qwerty » 02 Mar 2003, 17:07

Hi Dave
Sorry it is a bosch injection pump.Symtoms bad starting and bad idling .Ok on road. I fitted clear plastic piping between fuel filter and injection pump and also on return to tank pipe.Found no air on feed pipe but there was air bubbles in return pipe.I also bypassed injector spill pipes.

Dave Burns
Posts: 1916
Joined: 14 May 2001, 05:30
x 2

Air in fuel

Post by Dave Burns » 02 Mar 2003, 18:12

Well the incoming fuel is used to lubricate the shaft before it reaches the lift pump, so there is suction being applied at that point. I don't however think your problem stems from this area for several reasons.
If air could get in then its reasonable ot expect that fuel can get out, is there any evidence of this.
The injection pump can deal with small amounts of air getting in while the engine is running, the air naturally finds its way to the top of the pump where its expelled through the return restrictor.
If it were possible for air to get in via the shaft seal in the volume necessary to upset the idle, then the drive shaft bearings would have to be worn to a point where the pump wouldn't work in my opinion.
Air getting in at this point over night or any point in the delivery tract high on the engine or in the engine compartment such as priming bulb, filter and so on wont prevent the engine starting with this pump, but can produce eratic idle untill proper fuel delivery is re-established.
The reason being the injection pump remains full of fuel, even if air is being drawn in to it by the weight of fuel in the return line running back to the tank, as air enters the pump it goes out again at the top, it cant take fuel with it, fuel can only leave via this route if fuel is also entering at the inlet.
If there is a complete refusal to start because of air in the pump after being left overnight, then IMHO this has to come from somewhere after the lift pump, only then can the fuel be syphoned back to the tank from inside the injection pump.
If the fuel level drops below the inlet to the high pressure cylinder, then the engine can take a good few turns before the lift pump raises the fuel level enough for it to start.
The centrifugal lift pump is not very efficient at starter motor cranking speeds which is why starting becomes so difficult.
If the fuel level has dropped until the lift pump is empty then starting on the key without hand priming is virtualy impossible.
Anywhere on the injection pump body after the lift pump, where air can get in, is also at pressure when the engine is running, meaning fuel will deffinately get out.
If you are sure the bad starting comes from lack of fuel, then crack open the injector unions, no fuel here while cranking will confirm your diagnosis or point you in another direction.
Dave

qwerty
Posts: 24
Joined: 03 Nov 2002, 01:10

Air in fuel

Post by qwerty » 02 Mar 2003, 23:06

Thanks Dave for info.
I will remove timingbelt covers and check for fuel leak at pump drive shaft seal.