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Fuel pressure regulator test E28

Symptoms of a bad fuel pressure regulator

  • Engine misfire
  • Black smoke from the exhaust
  • Excessive fuel smell in the engine bay
  • Poor engine performance

Learning context

The fuel pressure regulator controls the pressure of the fuel system while the vehicle is running. The fuel pressure regulator is located near the front of the engine and attached to the fuel rail.

Why am I testing it?

I am testing the fuel pressure regulator to rule it out as the cause of excessive fuel consumption in my E28. As well as an excessive fuel smell coming from the engine bay while the engine is running.

Summary

  1. Relief fuel pressure
  2. Check the fuel pressure regulator
  3. Fuel pressure regulator reading at idle (with the vacuum hose attached)
    • 325, 325e, 325es(motronic)= 36-37 PSI
    • 325i, 325is, 325es(motronic 1.1)= 43-45 PSI
    • 318i(1984 and 1985 L-Jetronic)= 43-45 PSI
    • 528e. 533i (up to 1987 motronic)= 33-34 PSI
    • 535i, 535is(motronic 1.1)= 40-41 PSI
    • 525i(motronic 1.1)= 35-37 PSI
  4. Fuel pressure regulator reading at idle ( with the vacuum hose detached)
    • 325, 325e, 325es(motronic)= 39- 40 PSI
    • 325i, 325is, 325es(motronic 1.1)= 46-47 PSI
    • 318i(1984 and 1985 L-Jetronic)= 46-47 PSI
    • 528e(up to 1987 motronic)= 39-40 PSI
    • 535i, 535is(motronic 1.1)= 46-47 PSI
    • 525i(motronic 1.1)= 42- 43 PSI
  5. Tools required

Warning

Gasoline is extremely flammable, take proper precautions when working on any part of the fuel system. Don’t smoke, allow open flame, or use bare light bulbs near the area exposed to fuel. If you spill fuel on your skin, rinse immediately with soap and water, wear safety glasses, and have a class B fire extinguisher on hand.

fuel pressure relief procedure

Before you begin testing any part of the pressurized fuel system, you need to relieve any pressure that remains in the system.

  1. Locate and remove the fuel pump fuse from the Main fuse panel. If you are not sure where your fuel pump fuse is located consult your Owners manual, or take a look at the blog post with a fuse box diagram
Fuel pump fuse E28 fuse box
E28 fuel fuse location

2. Start the engine and wait for the engine to stall, then turn the vehicle off.

3. Remove the fuel filler cap to relieve the fuel tank pressure.

Remove fuel cap to relieve fuel pressure

4. The fuel system is now depressurized

Procedure

  1. Disconnect the negative cable on the battery

2. Disconnect the fuel line and attach the fuel pressure regulator in line with the return fuel line.

3. Turn the ignition on pressurizing the system. Check for leaks on or around the area before starting the vehicle

4. With the vehicle at idle note the pressure reading and compare it with the vacuum line attached list above.

5. Detach the vacuum line that connects to the pressure regulator, and note the pressure reading. Compare it to the vacuum line detached list above.

6. If the fuel pressure is low, pinch the return line shut and watch the gauge:

  • If pressure does not rise, the fuel pump is defective or there is a restriction in the fuel feed line
  • If the pressure rises sharply replace the pressure regulator.

7. If the indicated pressure is too high disconnect the fuel return line and blow through it to check for blockage. If no blockage is present replace the pressure regulator.

Conclusion

I was not able to perform the test becuase the pressure gauge is faulty and was not reading any pressure at all. I will be getting a replacement and perfomring the test again.

work in progress will update.

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