Wont start. Fuel pump/relay

The glow plugs are not operative in current temperatures so bit of a red herring there. The engine needs diesel injected correctly, air and compression.

The fuel will never be completely bubble free as it froths as it travels from the tank but not a great deal. What is paramount is EVERY pipe and connection is completely air tight, the filter housing and drain is air tight and all the correct clamps are used. Pressurising the tank will achieve nothing unless a large split is present. The fuel line runs at negative pressure and that is what lets the smaller air particles into the fuel but does not show as a fuel leak.

The cam shaft rotates the tandem and also depresses each injector in turn, so a cam belt failure would give no injectors or tandem pump.

The injector loom could be the failure as the engine is not trying to start. You can check the resistance value of each injector on the rear of the knurled loom to head connector. It is controlled by the ECU.

Diesels run full bore air so if there is a restriction such as a rag in a pipe or the ASV in the incorrect position this would prevent running.

This brings me to compression again cam shaft or valves or piston rings although this is unlikely. Loose or missing glow plugs will also have an effect on compression.

The tandem appears to be functioning, we just do not know if fuel is in the manifold to the injectors. Very difficult to prove. If there was a restriction between the tandem and manifold this could perhaps cause the fuel to bypass and go straight to the return. This is where a fuel manifold pressure sensor would have been useful.
 
Thanks @depronman

When I tested my tdi using the clear pipe before the tandem pump there were zero bubbles, you cannot tell the fuel is moving at all but it must be because the engine was running.

Screenshot_20240318_101254_DuckDuckGo.jpg


If you break the fuel supply into 3 clear pipe test sections which are

  • Pre tandem pump
  • After tandem pump
  • Return at the tank
Its important to know as your cars are not starting the answer to this question.

Once you have manually pulled fuel through the return line using the vacuum pump and refitted the hoses.

When you crank the engine is fuel moving past the first clear section of pipes you fitted?

You'll be seeing the movement as the bubbles stream by. If there were no bubbles you wouldn't know the fuel was moving as I said above.

Fit one bit of clear plastic pipe at a time or fit all three at once it's up to you what you choose.

Start at the first clear section and if it has bubbles in it.....

Don't bother moving onto the second section of pipe until you have fixed the leak that exists in the line before the first clear pipe.
Once there are no bubbles in that first bit of pipe move on to the next to check that your tandem pump isn't introducing air and so on.

Good luck


At this point in time I guess your both at a low ebb but hopefully the tide is turning to a swell of diesel.
 
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johnyfartbox said:
I have given them a spray with penetrating oil,
Click to expand...

Just to reiterate this is a high risk strategy that will prove nothing
The chances are those glow plugs have been in for 20plus years and removing them on a cold engine is highly likely to relit in a broken off plug with no means of removing it from the cylinder head - and to gain what knowledge?
If diesel is present at the injectors and the loom is providing power to the injectors then there is fuel being injected into the cylinders and the engine will run

My money is on no fuel at the injectors due to it air locking from the tandem pump to the injectors. I know of no pay to prime the injectors other than cranking as the tandem pump will be providing diesel into the cavity which feeds the injectors with diesel but there will be a pocket of air between the injectors and the diesel coming from the tandem pump so this air must be pushed through the injectors before diesel is present at the injectors. At this point it will splutter one cylinder at a time into life. Once the first cylinder (likely to be no3 because it is closest to the tandem pump) gets diesel it will start to partially run on that cylinder which means the starter motor as less to do and the engine speed will increase which will in turn speed fuel priming of the other two injectors

If the fuel system is air tight then no air is present and therefore fuel is always available at the injectors ready to inject and the engine starts on the first crank

However is air in entering and fuel is being allowed to drain from the injection cavity in the cylinder head the air bleeding process described above needs to be repeated to get fuel to the injectors

Sound tandem pump related to me
 
There's no injector fuel bleed screw therefore the injectors can not be air locked.
They can however not get enough air free diesel.

The passage into the engine has a plastic tube inside that helps the flow in and out of the passageway so also doesn't suffer from being air locked.

pictone.png
picttwo.png


self study programme 223
The 1.2l and 1.4l TDI engines
With Pump Injection System
Design and Function
 
Last edited by a moderator:
depronman said:
Just to reiterate this is a high risk strategy that will prove nothing
The chances are those glow plugs have been in for 20plus years and removing them on a cold engine is highly likely to relit in a broken off plug with no means of removing it from the cylinder head - and to gain what knowledge?
If diesel is present at the injectors and the loom is providing power to the injectors then there is fuel being injected into the cylinders and the engine will run

My money is on no fuel at the injectors due to it air locking from the tandem pump to the injectors. I know of no pay to prime the injectors other than cranking as the tandem pump will be providing diesel into the cavity which feeds the injectors with diesel but there will be a pocket of air between the injectors and the diesel coming from the tandem pump so this air must be pushed through the injectors before diesel is present at the injectors. At this point it will splutter one cylinder at a time into life. Once the first cylinder (likely to be no3 because it is closest to the tandem pump) gets diesel it will start to partially run on that cylinder which means the starter motor as less to do and the engine speed will increase which will in turn speed fuel priming of the other two injectors

If the fuel system is air tight then no air is present and therefore fuel is always available at the injectors ready to inject and the engine starts on the first crank

However is air in entering and fuel is being allowed to drain from the injection cavity in the cylinder head the air bleeding process described above needs to be repeated to get fuel to the injectors

Sound tandem pump related to me
Click to expand...
Im going to retry the vacuum pump a while longer , and hope it is an air lock issue i havent cleared ... I have nothing to lose whilst I wait to move forward with the filter. im reluctant to disconnect or disturb any other connections until I have confirmed filter is ok. have just ordered a @depronman drain screw.
 
The clear pipe on the return showing air and the lack of success with the suction priming prove beyond doubt that the reason for the non starting is lots of air in the fuel feed. When this is fixed I'm confident that suction bleeding will start it. Fuel filter is prime candidate so I would deal with that and then retry starting with suction.
 
tnpali01 said:
Im going to retry the vacuum pump a while longer , and hope it is an air lock issue i havent cleared ... I have nothing to lose whilst I wait to move forward with the filter. im reluctant to disconnect or disturb any other connections until I have confirmed filter is ok. have just ordered a @depronman drain screw.
Click to expand...

Your drain screw will be in the post tomorrow
75dd8323788a18b63bea1ada26d75e26.jpg



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I've been having similar problems, and when using a vacuum pump on the return line I couldn't get rid of the air. I tried cranking it anyway, and after a very painful 20 seconds it does eventually fire up. With a fully charged battery, and my booster connected, it will always eventually start.

I think mine is also sucking air in via the fuel filter bleed plug, but hopefully I will have a new depronman one soon too!
 
DaveySprocket said:
I've been having similar problems, and when using a vacuum pump on the return line I couldn't get rid of the air. I tried cranking it anyway, and after a very painful 20 seconds it does eventually fire up. With a fully charged battery, and my booster connected, it will always eventually start.

I think mine is also sucking air in via the fuel filter bleed plug, but hopefully I will have a new depronman one soon too!
Click to expand...

Yours will be being posted tomorrow. The undertray mounts where the holdup but printed now

Paul


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johnyfartbox said:
O ring, sorry.
Click to expand...

Yes ideally fit the new O rings that come with the filter (Mann filters come with o rings, lesser filters may not)
Or swap the old ones rings from the plastic drain screw to the metal one
I did look at supply the screw with the o rings but the min order qty was 1000 of each size so I gave up on that idea

Paul


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I also meant to say that my vacuum pump works for vacuum and as a pump pump.. I think they all do this so it could be used to pressurise the system to check for leaks?
 
Do not pressurise the system you will gain nothing. The fuel line works at negative pressure i.e. the tandem sucks the fuel through it. As air particles are smaller than diesel they can enter the line through bad pipes, joints or drain screws.
 
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