Hey all,

Ive been lurking for a while and thought I would pop in and say hi. Hello

Its an embarrassing problem as I am a mechanic but now stumped on my own car lol

The problem Im having is that after leaving the car over night in the cold

Its starts fine but after a few seconds it stops and throws frothy fuel back from the pump into the clear pipe. (air and fuel mixed).

It will only then start if I crack off the injector tops and purge air and fuel from the system.

It would seem that air is getting in the pump? over night..

This is at the pump end of the fuel system I'm sure as I have so far:

Replaced in tank lift/sender unit

Replaced fuel filter with cav296 system (no t piece to worry about).

Fitted a non return valve in line on the inlet side

Replaced injector leak off pipes.

Its strange as it starts so not the pressure side of the pump?

Ive worked on cars for many years and this is a new one on me.

Can anyone help?

You sure it's not the check valve in the sender?

It's been quite a well known fault...

Try drilling the check valve out, re-attach the lines and see if the bubbles vanish.

The fuel pipe runs bubble free when the engine is running.

Its getting the air from the pump side.

Seals in pump? allowing air to return through the pump from the return line?

Just to help anyone else who has this problem...

When the skoda has done a fair few miles then the injection pump splines wear.

When its very very cold the diesel will thicken making it harder to draw through the intake lines. Add this to the fact that metal also shrinks and you can see where the problem lies.

The pump rotor with the splines shrinks very slightly and coupled with the wear and the very thick oil it them creates a large vacuum inside the pump when starting and draws air in through the pump internals from the outlet pipes and past the splines.

Hence a build up of air.

Easier for the pump to do this than it is to draw the thicker freezing cold fuel once warn.

Fit a decent non return valve on the outlet side as the in-tank non returns barely work much below freezing.

This was fitted with a drop loop of pipe to make sure its always full of fuel and hey presto. It starts!

Edited 3 December, 201213 yr by illmanors

Thanks for the info, ive never heard of this, and mines on well over 200k miles.

Are you sure that the pipes to the injectors were properly fitted? Sounded like they werent torqued correctly, no matter how much vacuum if the system is air tight it wont let in air.

Oh and i also live in scotland so very very cold is familiar too.

Glad you got it sorted though

Thanks for the info, ive never heard of this, and mines on well over 200k miles.

Are you sure that the pipes to the injectors were properly fitted? Sounded like they werent torqued correctly, no matter how much vacuum if the system is air tight it wont let in air.

Oh and i also live in scotland so very very cold is familiar too.

Glad you got it sorted though

Mines on 400k now so its pretty worn lol

The return path/lines often have air in them just by design. Ive never seen an inlet fully air free and that air gets thrown into the outlet where it moves much slower due to the fuel that has been taken and used by the engine. The return path often has 4-5x more air than the inlet by flow and then couple that to the design and you find the issues.

Hence why people get problems on hills with the air/fuel returning through a worn pump.

People change the T piece and this will help stop the flow back but its not the root cause.

Well in engines with my type of miles anyway

The injector lines are joined to the return path via a union on the end on the pump. I changed the leak-off lines just incase and also checked the tops etc.

I've had people complaining of air in their lines due to the poor design of the in-tank sender...

...Quite a few people have just drilled their check valves out of the sender resulting in no air bubbles - which is why I suggested it as a quick simple thing to do/check.

I've had people complaining of air in their lines due to the poor design of the in-tank sender...

...Quite a few people have just drilled their check valves out of the sender resulting in no air bubbles - which is why I suggested it as a quick simple thing to do/check.

Thanks,

I will do this to aid with the cold starting due to thick fuel as the valve looks restricting.

Will update when this is done.

Mines on 400k now so its pretty worn lol

The return path/lines often have air in them just by design. Ive never seen an inlet fully air free and that air gets thrown into the outlet where it moves much slower due to the fuel that has been taken and used by the engine. The return path often has 4-5x more air than the inlet by flow and then couple that to the design and you find the issues.

Hence why people get problems on hills with the air/fuel returning through a worn pump.

People change the T piece and this will help stop the flow back but its not the root cause.

Well in engines with my type of miles anyway

The injector lines are joined to the return path via a union on the end on the pump. I changed the leak-off lines just incase and also checked the tops etc.

Makes sense.

P.s i too will be drilling out the valve

I did it about a month ago now.

Much nicer - and no bubbles... And a bit more power at the top end