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Pelican Technical Article:

Fuel Pump Testing

Nick Czerula

Time:

3 hours3 hrs

Tab:

$0

Talent:

*****

Tools:

Phillips & flathead screwdriver, socket set

Applicable Models:

BMW Z3 Coupe/Conv (1996-02)

Parts Required:

Fuel pump, fuel pump sealing O-ring

Hot Tip:

Check fuse before digging too deep

Performance Gain:

Properly functioning fuel pump

Complementary Modification:

Replace fuel filter

The fuel supply system in a BMW Z3 consists of: 

  • Fuel tank
  • Fuel filler with associated breathing and venting pipes
  • Evaporative emissions control valve and pressure pump
  • High pressure fuel pump assembly with fuel filter and fuel level senders
  • Fuel pump relay
  • Fuel pressure regulator
  • Fuel rail and electric fuel injectors

A faulty fuel pump can cause poor engine performance and hard starting problems. When it fails completely, you may be left with an engine that doesn't start. If you suspect your fuel pump has failed, you can confirm the fuel pump has received the correct voltage when commanded ON by the fuel pump relay using a digital volt ohm-meter (DVOM). If the correct voltage is present, but the pump does not activate, it is likely faulty. Use the DVOM in conjunction with a wiring diagram to determine the correct terminals for testing.

Accessing the fuel pump terminals can be done without removing the fuel tank. There are access panels underneath the rear seat cushion that allow servicing. The fuel tank is two-lobed. There are two fuel level senders, one in each lobe on the left and right sides underneath the rear seat cushion. The left fuel tank lobe holds the fuel level sender and fuel filter, while the right tank lobe holds the fuel pump.

To avoid marring the paint and trim, work with a plastic prying tool or wrap a screwdriver tip with masking tape before prying out body or interior items.

Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches. 

When performing electrical tests, be sure to use a digital instrument such as a DVOM. Using an analog device may lead to damage to sensitive electronic components.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability. 

Vehicle models change and evolve, as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Pressure testing:

Remove engine cover over fuel injectors. See our tech article on engine covers, removing.

Models with 4-cylinder engine - Remove fuel test port cap (green arrow).
Figure 1

Models with 4-cylinder engine - Remove fuel test port cap (green arrow).

Models with 4-cylinder engine - Install fuel pressure gauge.
Figure 2

Models with 4-cylinder engine - Install fuel pressure gauge. Be sure all test hoses are installed and routed properly.

Models with 4-cylinder engine - Next, start or attempt to start engine.
Figure 3

Models with 4-cylinder engine - Next, start or attempt to start engine. Fuel pressure should be 3.0 bar +/- 0.2 (44 psi +/- 3). Once you test pressure. Recharge fuel system by cycling key or running fuel pump, note fuel pressure. Then allow fuel system to sit under pressure for 15 minutes. Pressure should drop no more than 0.5 bar in that time period. If it drops you could have a faulty fuel pump check valve or a leak in the fuel system.

Models with 6-cylinder engine - Remove fuel test port cap.
Figure 4

Models with 6-cylinder engine - Remove fuel test port cap. (green arrow)

Models with 6-cylinder engine - Install fuel pressure gauge.
Figure 5

Models with 6-cylinder engine - Install fuel pressure gauge. Be sure all test hoses are installed and routed properly.

Models with 6-cylinder engine - Next, start or attempt to start engine.
Figure 6

Models with 6-cylinder engine - Next, start or attempt to start engine. Fuel pressure should be: All 6-cylinders except S54: 3.5 bar +/- 0.2 (50 psi +/- 3): S54 6-cylinder: 5.0 bar +/- 0.2 (72.5 psi +/- 3)

Electrical testing:
Electrical testing - Working behind right side seat peel carpet away from body.
Figure 7

Electrical testing - Working behind right side seat peel carpet away from body.

Electrical testing -Remove four Phillips head screws from the access panel (green arrows).
Figure 8

Electrical testing -Remove four Phillips head screws from the access panel (green arrows).

Electrical testing - Remove access panel lid from body by lifting up.
Figure 9

Electrical testing - Remove access panel lid from body by lifting up. Move away enough to access electrical connector on fuel pump module.

Electrical testing -Disconnect fuel pump electrical connector (green arrow) by sliding locking tab using a flathead screwdriver, then pulling connector off fuel pump module.
Figure 10

Electrical testing -Disconnect fuel pump electrical connector (green arrow) by sliding locking tab using a flathead screwdriver, then pulling connector off fuel pump module.

Connect DVOM across fuel pump terminals (green arrows).
Figure 11

Connect DVOM across fuel pump terminals (green arrows). These are the two terminals on my subject vehicle. You will be testing voltage across the large wires at the connector.

Turn key ON, DVOM should read battery volts (around 12 volts).
Figure 12

Turn key ON, DVOM should read battery volts (around 12 volts). If no voltage is found, check relay and fuel pump fuse. With key ON, fuel pump will receive voltage for a 3-5 seconds to prime system, (if engine doesnÂÂ't start).

I suggest load testing using a test light and a DVOM.
Figure 13

I suggest load testing using a test light and a DVOM. Connect DVOM across fuel pump electrical connector terminals and take a reading, it should read battery volts when key is turned ON. Then connect an incandescent bulb style test light to battery ground and touch the test light probe tip the positive wire you are back probing with DVOM. Your reading should hold steady, a maximum drop in voltage of 0.5 volts is OK. Anything more is a problem. In this photo, voltage head steady. You can also jump out your fuel pump relay (to bypass it) for testing. The vehicle used in this tech article was a 1998 Z3 with a 4-cylinder engine. The fuel pump relay position varies depending on year. See list below for relay locations. Double check your fuel pup relay location with the latest BWM repair information. Next to DME main relay in E-box mounted near DME. Under left side of dashboard on relay panel (see photo below). On a late M Roadster, the fuel pump control module behind parking brake lever. Remove the lower dashboard trim.

The fuel pump relay is green and located at the end of the under dash relay panel (green arrow).
Figure 14

The fuel pump relay is green and located at the end of the under dash relay panel (green arrow).

Remove fuel pump replay.
Figure 15

Remove fuel pump replay. Connect a fused jumper wire between terminals indicated by arrows. Green arrows point to terminal 30 (battery positive). Yellow arrows point to terminal 87 (fuel pump feed). This will activate your fuel pump. Check for voltage at fuel pump as you did earlier. Battery volts should be present. Terminals may vary, double check your model against a wiring diagram. This photo shows the bottom of the relay, it has the terminals noted, it will help you identify the correct terminal.

Alternately, you could connect a fused jumper wire between terminals indicated by arrows.
Figure 16

Alternately, you could connect a fused jumper wire between terminals indicated by arrows. Green arrows point to terminal 30 (battery positive). Yellow arrows point to terminal 87 (fuel pump feed). Keep in mind, it is the too larger wires you are jumping. This will also activate your fuel pump. Check for voltage at fuel pump as you did earlier. Battery volts should be present. Terminals may vary, double check your model against a wiring diagram. This photo shows the bottom of the relay, it has the terminals noted, it will help you identify the correct terminal.

You can also check the relay circuit using a digital multi-meter (DVOM).
Figure 17

You can also check the relay circuit using a digital multi-meter (DVOM). Connect the black lead of your DVOM to battery negative, and use the other to back probe the terminals. Use the following list to cross reference your voltage readings. Keep in mind wire colors and positions may vary depending on year and engine. What you see below if from my 1998 Z3 4-cylinder, other readings should be similar. Green arrow: Terminal 30 - constant battery positive. Red arrow: Terminal 86: switched battery positive, present when key is in ON position. Blue arrow: Terminal 85: DME relay control, close to zero volts when DME turns relay on. Yellow arrow: Terminal 87 - Feed to fuel pump, battery positive when relay is activated.

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Comments and Suggestions:
Ray Comments: Gas gauge was working then it failed in front of my eyes while I was filling up. I fill it up and overflowed on floor., first time that ever happen. It was not full. I had fill it very slowly and can only get about 4 to 5 gallons.I want to replace my f pump and sending unit together???
What are my options, 2000 z3 roadsters. 100150 mile. I don't trust fule pump. I need to move seats too stuck from water.
October 7, 2016
  Followup from the Pelican Staff: See this page:

http://www.pelicanparts.com/cgi-bin/ksearch/pel_search_2014.cgi ?SUPERCAT_FLAG=Y&command=DWsearch&description=fuel%20pump%20 - Nick at Pelican Parts
 
Gene Comments: I figured it out, removed the drive cables from the motor to the rails. Turned them with a drill motor opposite directions from each other, the seat slowly moved up the rails. Continued switching from one side to the other to keep seat even and now can access the fuel pump cover, figure I'll tap the cover a few times to try and jar the fuel sender which I believe is stuck. Tank reads full when ignition is on. Otherwise will wait till I use some gas since the tank is full right now, just filled it.
July 22, 2016
  Followup from the Pelican Staff: Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
 
Gene Comments: I have 2000 z3 roadster 2.8, my passenger seat motor is deadboth of them, how can I move my seat forward to access the fuel pump, short of a cutting torch. My fuel gauge reads full all the time.
July 21, 2016
  Followup from the Pelican Staff: You will have to remove the motors or drive cables and manually activate them. You can use a hand tool that will turn the drive. - Nick at Pelican Parts  
skip Comments: What are the other two relays used for?
June 10, 2016
  Followup from the Pelican Staff: Depends on the model. I would get a wiring diagram for your vehicle and check the application data. - Nick at Pelican Parts  
BluesFan44 Comments: Hi - I'm having issues with my fuel gauge always reading empty. Is there a way that I can test the resistance that the sender unit is sending to the gauge without completely removing the pump/sender assembly?
June 22, 2015
  Followup from the Pelican Staff: Not really, because you have to move the float up and down to measure the full range. - Nick at Pelican Parts  

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Page last updated: Mon 12/5/2016 02:07:12 AM