924S - Fuel pump howling - but fuel pressure regulator OK ?

  • Hello everyone,

    I'm currently putting a 924S in its original condition (deregistered since 1995) back on the road as a first-time project. With the help of all the information in this forum, I'm already pretty far along. :)

    So far I have only "driven" the car in the driveway and have the problem that after 8-12 minutes of engine running the fuel pump becomes very loud and constantly "howls". However, the engine runs well, revs up cleanly, etc. The noise does not change with the engine speed.

    Here are the key data relating to the issue:

    • Tank emptied, strainer cleaned, new fuel in it, flushed forward several times, as well as back into the tank
    • Fuel hoses (rear axle area), filter and fuel pump (initially cheap model) replaced
    • To replace the oil and oil filter, the car was initially left running for a total of over 1 hour when stationary, and the pump was not yet noticeable.

    After researching here in the forum, I proceeded as follows:

    • Fuel pressure measured at the rail:
      • Cold engine, DME bridged: Fuel pump stopped!!!
        • 2.8 bar (should be 3.0 bar according to the forum)
        • When the return line is disconnected, the pressure rises above 4 bar
      • Idle: 2.2 bar (should be 2 +/- 0.2 bar according to the forum)
      • 3000 rpm: 2.0 bar (should be 2.7 +/- 0.2 bar according to the forum)
        • when revving up, the pressure drops from 2.2 to 2.0 bar and therefore does not increase (3500 rpm also 2.0 bar)
      • after switching off, the system maintains the pressure very well, after 30 min. still over 1 bar

    • Vacuum hoses
      • traced in the engine compartment and seat checked:
      • all hoses apparently OK and securely attached
      • pulled off the fuel pressure regulator and damper:
        • dry, no gasoline visible and no smell of it either
        • if you disconnect the hoses while the engine is running, the fuel pressure does not change

    • Fuel filler cap
      • when you open the fuel filler cap, regardless of whether the car is switched off or running, there is a hissing sound with air coming out, i.e. excess pressure in the tank (should be OK)
      • Pump noise or fuel pressure unchanged

    The pump noise, if it occurs after 8-10 minutes with the engine running, is always present regardless of the test above.

    As I understand it, the fuel pressure regulator is not necessarily the cause of the noisy pump, as the pressure drops and does not rise when revving up and the vacuum connections are dry?

    I am unsure about the pump, as it sometimes makes "choking" noises when the DME is bridged, but still reaches over 4 bar when the return line is depressurized.

    Can you give me any hints as to the possible cause or further tests?

    Many thanks in advance for your help!

    Best regards

    Ignition wool

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  • This whining probably comes from a damaged bearing in the fuel pump. The noise only occurs at operating temperature (temperature, expansion, etc.).

    According to your statement, the fuel pressure at 3000 rpm is not ok. Check this again with the information in the workshop manual. The fuel pressure must be correct. But one thing at a time After the pump, the regulator is suspect. Even if this is tight, this is no proof of function - this is only the fuel pressure.

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  • Thank you for your advice!

    I looked again in the English WHB for the 924S/944.

    I can only find the following information here:

    - Pump bridged: 2.3 - 2.7 bar (in my case 2.8 bar, possibly still OK)

    - Idle: approx. 2.0 bar (in my case 2.2 bar, OK)

    I haven't actually found any information on the 3000 rpm speed or other operating states.

    Does anyone know this from any source or is it rather an observed value?

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  • Hello everyone,

    I still haven't found anything about fuel pressure at 3000 rpm or similar.

    I then installed an original Bosch pump.

    The pressure was unchanged as in the above post.

    The other general conditions also remained the same.

    The pump was then very quiet in some tests (only when stationary).

    I thought the problem was solved and went to the hole in the battery box.

    Then I drove about 30 km to the TÜV and back. (Passed :))

    The pump became louder again on the journey.

    After another 30 km, the pump is still loud. A fairly constant noise that is clearly audible and annoying when driving.

    I think it is unlikely that the new pump is also defective.

    It is more noticeable that both have increased their volume immensely after a certain time.

    Is there anyone here who can give me a tip as to a possible cause?

    Many thanks in advance!

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