FAQ
TL;DR: 3 common leak points were flagged (filter, check valve, lines). “Air in the pipes indicates a leaking fuel system.” Fix leaks or restrictions, and check the tank strainer and heater before chasing the pump. [Elektroda, daro31ie, post #16845263]
Why it matters: This FAQ helps Audi A3 8L 1.9 TDI owners fix air bubbles in fuel lines after filter/valve work, preventing rough idle and stalling.
Quick Facts
- Reported symptoms: rough running, irregular idle, stalling, and visible bubbles from filter to injection pump. [Elektroda, gredekk, post #16844890]
- Suspected causes: leaking filter/valve/lines and restrictions at the tank strainer or heater. [Elektroda, daro31ie, post #16845263]
- Access tip: the fuel tank strainer is under the trunk lid (tank access cover). [Elektroda, andrzej20001, post #16847018]
- Restriction risk: blockage can cause “cavitation,” i.e., gas bubbles precipitating in diesel. [Elektroda, zyga c, post #16846494]
- Edge case: even after cleaning the sieve and replacing parts, bubbles can persist—recheck for leaks and restrictions. [Elektroda, gredekk, post #16868420]
Why do I see air bubbles in the clear fuel line after a new filter and check valve?
Contributors point to leaks at the filter, check valve, or lines. A restriction upstream can also draw in air. As one expert put it, “Air in the pipes indicates a leaking fuel system.” Inspect clamps, seals, and the filter drain or heater seals. [Elektroda, daro31ie, post #16845263]
What symptoms confirm the issue on an Audi A3 8L 1.9 TDI?
Owners report rough running, erratic idle, engine cutting out, and visible bubbles from the filter to the injection pump. If bubbles increase before a stall, suspect leaks or fuel pickup restriction. Document changes after part swaps to isolate causes. [Elektroda, gredekk, post #16844890]
How do I access and check the tank strainer on the A3 8L?
Open the trunk and remove the tank access cover (lid). The strainer sits at the in-tank pickup. Look for sludge or debris and clean it, then reassemble with good seals. This is the first place to check for restrictions. [Elektroda, andrzej20001, post #16847018]
What is cavitation in a diesel fuel system?
Cavitation is the formation of gas bubbles in fuel when a restriction causes low pressure. One poster explains it as “precipitation of gas bubbles in the fuel.” Remove restrictions at the pickup, strainer, or heater to prevent it. [Elektroda, zyga c, post #16846494]
Could bacteria in diesel cause air bubbles?
Yes. A contributor notes bacteria can grow in diesel (ON) and create sludge that clogs the strainer. That blockage leads to suction-side issues and bubble formation. Clean the strainer and consider draining contaminated fuel. [Elektroda, andrzej20001, post #16846660]
I already replaced the filter, check valve, and clamps. What next?
Verify the tank strainer and fuel heater are clean and sealing. The OP did this and still had bubbles, indicating a remaining leak or restriction. Reinspect each joint and the filter drain/thermostatic valve for hairline leaks. [Elektroda, gredekk, post #16868420]
Can I eliminate all visible bubbles, or is a small amount normal?
The thread consensus is clear: air means a leak or restriction. Eliminate those and bubbles should stop. Persistent bubbles point to unresolved sealing issues or a partially clogged pickup/strainer. “Air in the pipes indicates a leaking fuel system.” [Elektroda, daro31ie, post #16845263]
Where should I look first if bubbles increased after the filter change?
Recheck the new filter’s seals, the drain/thermostatic valve, and every clamp you touched. Then inspect the tank strainer via the trunk lid access. These were the first failure points highlighted by contributors. [Elektroda, daro31ie, post #16845263]
What is the tank strainer (sieve) and why does it clog?
It’s a mesh pickup filter inside the tank. Debris and bacterial sludge can accumulate on it, restricting flow. That restriction causes suction-side pressure drop and visible bubbles. Cleaning the strainer often restores normal flow. [Elektroda, andrzej20001, post #16846660]
How do I clean the tank strainer on the A3 8L? (3 steps)
- Open trunk and remove the tank access lid.
- Lift the pickup unit and expose the strainer.
- Clean off sludge/debris, refit with sound seals and clamps.
[Elektroda, andrzej20001, post #16847018]
What if bubbles persist after cleaning the sieve and replacing parts?
Document what changed, then pressure-test or visually inspect each joint for micro-leaks. Revisit the heater and filter drain seals. The OP still had bubbles after cleaning, which signals an unresolved leak or restriction. [Elektroda, gredekk, post #16868420]
Is the check valve (Bosch/Boshe) a known culprit?
Yes. A contributor replaced a new filter again within a week due to similar issues. Faulty or poorly sealing filter valves can admit air. Swap in a known-good unit and verify seals and clamps seat correctly. [Elektroda, daro31ie, post #16846378]
How many suspect points did this thread surface?
Five: filter assembly, check/thermostatic drain valve, fuel lines/clamps, tank strainer, and fuel heater. Addressing all five restored many systems in similar cases, according to contributors. Track changes after each fix to isolate the cause. [Elektroda, zyga c, post #16846494]
Could a clogged pickup cause the engine to die at idle?
Yes. The OP’s engine ran irregularly and would cut out as bubbles increased. That behavior aligns with suction-side restriction or leaks letting air enter. Resolve those to stabilize idle and prevent stalling. [Elektroda, gredekk, post #16844890]
What quick checks should I do before blaming the injection pump?
Confirm tight clamps, intact lines, and correct filter valve orientation. Inspect and clean the tank strainer and heater seals via trunk access. Only after these checks should you suspect pump issues. [Elektroda, daro31ie, post #16845263]
