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Audi A3 8L 1.9 TDI: Air Bubbles in Fuel Lines After New Filter & Boshe Valve Installation

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  • #1 16844890
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    Hello, I have a problem, I recently installed a new fuel filter with a Boshe check valve. Because the engine started to go crazy I didn't walk regularly and then went out. Probably from the fact that there was a large amount of air in the ducts. so I changed the filter and the valve like a car goes well no more going crazy but I noticed that in the pipe going from the filter to the injection pump there is a small amount of air bubbles. My question how to remove them? and Can you get rid of them completely? Returning to replace the filter and valve I did because because the engine went crazy and then the number of these bubbles in this pipe was greater than now.
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  • #2 16845263
    daro31ie
    Automation specialist  
    Posts: 2425
    Help: 366
    Rate: 645
    Air in the pipes indicates a leaking fuel system.
    Most often the problem is with the fuel filter, valve or only the lines.
  • #3 16846287
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    The filter, the new valve, the straps too ... the wires look ok so where does this air come from?
  • #4 16846378
    daro31ie
    Automation specialist  
    Posts: 2425
    Help: 366
    Rate: 645
    buddy, what does the filter drain valve look like?
    Once after a week after changing the filter for a new one I changed to another.
    Just for the reason you describe.

    Pozdr. daro
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  • #5 16846391
    andrzej20001
    Level 43  
    Posts: 17698
    Help: 1568
    Rate: 6611
    A sieve in the tank may be banded or a cord there.
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  • #6 16846414
    daro31ie
    Automation specialist  
    Posts: 2425
    Help: 366
    Rate: 645
    andrzej20001 wrote:
    A sieve in the tank may be banded or a cord there.


    If it was similar before the exchange, then certainly yes
  • #7 16846427
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    The filter I gave today a new band and valve also and still the same.
  • #8 16846494
    zyga c
    Level 17  
    Posts: 364
    Help: 10
    Rate: 74
    If the fuel system is tight. and a stuck dragon in the tank - this is the phenomenon of "cavitation" or the precipitation of gas bubbles in the fuel.
  • #9 16846660
    andrzej20001
    Level 43  
    Posts: 17698
    Help: 1568
    Rate: 6611
    Bacteria grow in ON and they clog the strainer. Such sludge.
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  • #10 16846960
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    how to unseat this sieve?
  • #11 16846988
    andrzej20001
    Level 43  
    Posts: 17698
    Help: 1568
    Rate: 6611
    Blow your mouth. Joke. No clear
  • #12 16846997
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    but some locations of where it is and how to disassemble?
  • Helpful post
    #13 16847018
    andrzej20001
    Level 43  
    Posts: 17698
    Help: 1568
    Rate: 6611
    The lid in the tank in the trunk
  • #14 16847021
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    Sory that I am so green but you can specify exactly how to do it, where to look and by?
  • #15 16868420
    gredekk
    Level 7  
    Posts: 7
    Rate: 4
    Hello, I cleaned the sieve, the wires, I looked at the new fuel filter and the heater .. bubbles are still what can I cling to now? Thanks in advance for your help .
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Topic summary

✨ The discussion revolves around an issue with air bubbles in the fuel lines of an Audi A3 8L 1.9 TDI after the installation of a new fuel filter and a Bosch check valve. The user initially experienced erratic engine behavior, which improved after replacing the filter and valve, but air bubbles persisted in the line from the filter to the injection pump. Responses suggest that air in the fuel system may indicate leaks, possibly from the filter, valve, or fuel lines. Other potential causes include a clogged strainer in the fuel tank or cavitation due to gas bubble precipitation. Users discuss methods to address the issue, including cleaning the strainer and checking for leaks, but the problem remains unresolved for the original poster.
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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

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)

  1. Open trunk and remove the tank access lid.
  2. Lift the pickup unit and expose the strainer.
  3. 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]
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