Diesel Filter Problems: Air Bubbles & Performance Loss

Hi,
'I've been experiencing a loss of power in my '99 Golf IV TDI with 90 hp for quite some time – it's been tuned to 115 hp. The Volkswagen workshops aren't putting in any effort and are just incompetent.'

I've noticed that there are always air bubbles in the hose leading from the diesel filter. This doesn't change when I accelerate; the bubbles just move faster and there are more of them. Is this normal, or could something be wrong?

The diesel filter itself was recently replaced. However, the loss of power had already occurred before and has not changed since.

Thank you in advance.

in the line that comes out of the diesel filter.


Which way should the diesel filter be oriented? The fuel line that carries fuel from the tank towards the engine (the supply line) must not contain any air bubbles. Air bubbles in the return line, on the other hand, are not a problem.

If you have air bubbles in the fuel line, then your fuel line is (somewhere) leaking slightly. This would cause the engine to inject air instead of diesel... Assuming that air has a lower calorific value than diesel, this could be the reason for your loss of power. }

Regards,
Private.

Hello,

Your performance loss is likely directly related to the bubbles/foam.
Hopefully, the O-rings of the return valve were also replaced when the diesel filter was changed. Otherwise, only the fuel lines need to be checked for leaks. Don't forget the tank ventilation; if it's blocked, excessive vacuum will build up and air will usually be drawn in somewhere.

Best regards, Rainer.

'...thank you for your answers. So, it's the transparent hose that goes directly into the engine... there are bubbles inside (otherwise I wouldn't see anything). Is this where the fuel supply for the injection system is connected? - If so, then there's probably a problem right there...'

tk3003 wrote:

...is this where the fuel is supplied for injection? - if so, then there's probably a problem there...

Hello, yes. Before you start researching other causes of the problem, it's important to make sure there are no more air bubbles present. Depending on the number of air bubbles, your car might feel like it's being driven with brakes applied .
A small bubble (approximately 5mm in size) that forms after a long period of standing and with the engine off is actually normal; it's probably never completely airtight.

Best regards, Rainer.

I had the exact same problem.
'When I was parked on the incline at my driveway and had very little diesel in the tank (almost empty), I had even more air than the 5mm, and my TDI started very poorly.'

After draining (?) the filter, it was better, but I only experienced a complete improvement after replacing the filter and raising its position slightly (I loosened the screw on the clamp and pulled the filter out a few centimeters, then re-tightened the screw). I'm not sure if that was the right solution, but I haven't had any problems since.