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E28 Fuel system pressure check

Symptoms of bad fuel pressure

  • Engine running rough
  • Poor fuel economy
  • Black smoke from the exhaust
  • Lack of horsepower (if fuel pressure is too low)
  • The engine having trouble starting
  • Engine stalling after starting

Learning context

An electric fuel pump supplies fuel under pressure to the fuel rail, which distributes fuel to the injectors. When the pressure is low or high it can cause the vehicle to run poorly, and cause some of the issues listed above.

Why am I testing this?

As I have mentioned in a few previous posts, I am diagnosing an excessive fuel consumption issue with my E28. I am testing the fuel system pressure to rule it out as the cause for the excessive fuel consumption in my car.

summary

  1. Relieve fuel pressure
  2. Check for proper fuel pressure (engine off, fuel system energized)
  3. Fuel system pressure readings
    • 325, 325e, 325es(motronic)= 36-37 PSI
    • 325i, 325is, 325ic(motronic 1.1)= 43-45 PSI
    • 318i(1984 and 1985 L-Jetronic)= 43-45 PSI
    • 528e, 533i(up to 3/1987 motronic)= 36-37 PSI
    • 535i, 535is(motronic 1.1)= 43-44 PSI
    • 525(motronic 1.1)= 39-40 PSI
  4. Hold pressure reading- 30 PSi
  5. Tools required for the job

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 this blog post about Fuse location

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.

1987 535I fuel cap removal to remove fuel pressure

4. The fuel system is now depressurized

procedure

  1. Disconnect the negative cable from the battery
Negative battery terminal
e28 12v battery

2. Detach the fuel feed line between the fuel filter and the main fuel line for Motronic 1.1 system. If your vehicle is equipped with the L-Jetronic or motronic detach the feed line from the fuel rail.

Filter Fuel feed line

3. Using a tee (three-way) fitting, a short piece of high-pressure fuel-rated hose and clamps to attach a pressure regulator in line with the fuel system. Warning: Do not use a plastic tee fitting for this test. It won’t be able to withstand the fuel system pressure.

Fuel pressure kit.

4. Attach the negative cable to the battery

5. Bride the terminal of the fuel pump relay using a jumper wire. (Locate the fuel pump relay and bridge pin #30 and #87 together.)

E28 fuse relay pin #30 and #87 jumped to power the fuel system

6. Turn the ignition on

7. Note the fuel pressure and compare it to the list above. After turning the ignition off wait five minutes keeping an eye on the hold pressure reading.

8. If the system fuel pressure is less than specified:

  • Inspect the fuel system for leaks
  • If no leaks are found install a new fuel filter and perform the test again
  • If the pressure is still low, check the fuel pump pressure and the fuel pressure regulator.

9. If the fuel pressure is too high, check the fuel return line for any obstructions. If the fuel return line is not obstructed replace the fuel pressure regulator.

Fuel return line crushed.

Summary

The pressure gauge malfunction and I coult not finish the test, I will get a replacement and get the pressur ereading. While I was checking the rest of the fuel system I did notice that the fuel return line was crushed. I think the jack slipped whil ejacking up the car and it crushed it, so i cut and replaced it with fuel ratted hose and double clamps on either side.

If your vehicle passed the fuel system pressure test, continue with the fuel pump pressure test, and the fuel pressure regulator test.

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