Moss Miata

Tips for Diagnosing Fuel Delivery Problems

by Tommy Samuels


You may have a problem similar to what I went through with my '99. Here are some tips that can help you along the way.

Tools Needed:

A multimeter -- you'll need this to be able to measure resistance and DC voltage.


Check the fuses with the meter. You want continuity.

Check (listen) for the main relay clicking in when you turn on the switch. It's on the passenger side fenderwell and it's pretty loud. you should hear it in the cockpit of the car. Then, move on to:

1. Pull the access panel in the package tray so you can get to the fuel pump assembly.

2. Disconnect the electrial plug to the fuel pump and gas gage sensor.

3. The plug will have 4 wires. You're interested in a red wire with a blue stripe and a black wire.

4. You want to measure voltage across these wires. You should have 12VDC when you crank the engine over. It will stay at 12 VDC for 3-5 seconds after you stop cranking the engine IF you leave the ignition switch in the ON or RUN position


5. Install a jumper between the Fuel pump and Ground pins on the Diagnostic connector. This will pull in the fuel pump relay and you SHOULD get 12VDC at the fuel pump connector with the ingnition switch ON. If it passes this test, the fuel pump relay CONTACTS AND COIL are working. If it fails this test , you probably have a BAD Fuel Pump RELAY. Remove the jumper from the FP and Gnd terminals of the diagnostic connector.

6. Disconnect the plug from the fuel pump relay. There are 4 wires. Red with blue stripe goes to the fuel pump. Light green goes to the ECM.

6A. The other 2 should measure 12vdc with the switch ON.

6B. The light green wire should have continuity to ground when trying to start the engine. This will stay on for 3 to 5 seconds after trying to start the engine if you keep the switch ON (AS IN TEST 4). If you get an ERROR on your meter doing this test, try test 6C

6C. Change the meter to measure VOLTAGE. measure voltage between the light green and the nearest ground while trying to start the engine. IF YOU GET 5VDC, YOU HAVE A PROBLEM WITH THE ECM.

Fuel Pump Troubleshooting :

Remove the fuel pump assembly from the tank

REMEMBER: gas is highly flammable. Remove the pump, drain ALL the gas and let it sit out in the open for 20 or 30 minutes before doing the tests.

If you have access to a 12 volt power supply you can test the fuel pump by touching the 2 terminals on the left side of the 4 pin connector with 12 volts. It SHOULD come on . If not don't give up yet!

Remove the short jumper wire from the 4 pin connector and the fuel pump. Measure continuity from end to end on BOTH the Black and Blue wires. Wiggle it around to be certain there aren't any breaks in the wire or loose connector pins.

Test the pump by applying 12 vdc directly to the 2 pins on the fuel pump. If it runs, the pump is good. If it doesn't the pump is bad.

Note from Daniel Stewart:

I just wanted to let you know that I recently replaced a faulty fuel pump in my '99 miata that was running, but not putting out enough pressure for the car to start. I thought it might be helpful to let the readers know that if the pump runs they should still try a fuel pressure tester to make sure the pump is doing what it's supposed to.

Back to the Garage

28 May, 2009

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