Simple question.... bubbles - montag
This is about my VW (again).

Should you see air bubbles in the fuel return pipe at idle? I've got lumpy idle with lots of bubbles!!!!
Simple question.... bubbles - Peter D
What model but I think the answer is no anyway. Where is your fuel pump, if it is engine mounted i.e. up from then you are definately pulling air into the system but how did you view the air bubbles or are we talking diesel here. Peter
Simple question.... bubbles - DL
If it's a diesel then air bubbles in the return line are normal - in my experience!
Simple question.... bubbles - montag
Its a 1800 VW Scirocco, petrol. The return line is see through plastic so it is easy to see. Only really happens on idle.
Simple question.... bubbles - DL
Nothing to worry about IMO aslong as the flexible fuel pipes are in good condition.

The fabric-braided hoses on these cars did have a habit of going soft, so check them carefully for any tell-tale signs of fuel seepage. Smell is probably the giveaway.
Simple question.... bubbles - Peter D
You can only get air in the system if you are sucking it in prior to the pump. If this model is injected then you are not going to gey low speed evenness. Regards Peter
Simple question.... bubbles - pastyman
DL

If your seeing bubbles in a diesel system, then something is wrong, a diesel engine is a closed system, i.e. no air, or it won't work, thats the idea of bleed screws and hand primer pumps.

Pastyman..
Simple question.... bubbles - DL
Well I say that as I have observed small air bubbles in the clear fuel lines of many VAG/GM Diesels over the years - Yes, I'd admit any air to be bad news but these engines were (and still) run without incident.
Simple question.... bubbles - David Davies
Some bubbles or 'aeration' in the return line of the Sirocco fuel system are fairly normal.This is because the returned fuel discharges from a jet approx 0.5mm diameter into a return pipe of approx 3mm.diameter.The pipe is open at the tank end so it will never completely fill with fuel.Aeration in the inlet line can indicate a porous fuel line between tank and pump.A good test is to install a transparent fuel filter in the line before the pump(there may already be one) and then with the engine running, hold the filter with the inlet at the bottom.If you now see a constant stream of air bubbles you can be sure you have a faulty line betwwen filter and tank.Quite often the pipe fails at the connection to the tank pick up.
David Davies (Tune-Up Raglan)
 

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