1990 Mazda B2600 Engine missing

Hi 90mazda,

Thank you for the donation.

If the igniter is bad, you would not be able to start the engine so that should not be the cause.

The more likely cause would be a bad injector that is stuck open. Try to determine which cylinder is not working correctly by pulling out the spark plug wires and noting which cylinder causes the least change in idling speed.

Get a compression test done to verify if they are within specs.

If compression is within specs, swap the injectors between the 2 good ones and retest if it is still # 2 and 4 that is misfiring.

All four injectors are powered from the same source, via the FI main relay. Cylinder 1 and 4 works in tandem with a common ground at ECU. Cylinders 2 and 3 is another pair. That eliminates the possibility of power supply failure and you shoulc check the voltage at cylinder 2 and 4 to determine if it is a broken circuit.

Recheck the ground circuits between the engine and body, it could be insufficient and adding another might help.

If it is the computer, then either you replace it or you try using a piggy back. Connect the 1 and 4 cylinder injector ground circuit to the computer. Likewise with 2 and 3.

For a cheaper alternative, you can try getting a reman.

Remanufactured computers are cheaper than new unit and locally you should be able to source them. Check with your local parts store for that.

After rechecking through the wiring diagram, I realised I made a mistake previously. For earlier models, the injectors would work in tandem, injector # 1 and # 4 are linked together whereas Injector # 2 and # 3 are a pair. This would rule out the computer as the culprit. If it is, you would not be able to start.

At the injectors, unplug all the connectors and test the Black/Yellow wire for battery voltage with ignition switch on.

If voltage is not available at #2 and #4, check for broken wires between the wires.

If battery voltage is available, test for continuity between the Light Green/Black wire of injector # 1 and # 4. If continuity is not found, you have a break in the wire and reconnecting them would work.

Likewise, perform same task with Light Green/Red wire between injector #2 and #3.

By the way have you tested and confirmed that the injectors are good?

For continuity test, you need an ohmmeter. If you do not have the ohmmeter, disconnect the ECU harness.

Attach the test lamp to Lt Green/Black wire of injector # 1 and battery voltage. Use another wire to ground the Lt Green/Black wire of # 4. If test lamp lights up, the wire is good.

Yes, that is correct.

Which wire did you test? The one with battery voltage when igniton is on would also light up if battery voltage is applied with a test lamp connected, unless the ignition switch is turned on.

Did you unplug all the injectors?

Remove from 1st and try with 4th injectr. What is the result?