I'm struggling with a recently acquired, stock 78 280Z running very rich. As in 13-15 mpg combined, and all plugs are getting carbon fouled to the point one or more start misfiring completely. I changed the previous owners BPR6ES with fresh BPR5ES plugs, but after around 1000 miles I could again hear the 5 cylinder "brrt-brrt-brrt" and soon after the 4 cylinder "cho-cho-cho" ?
I've been troubleshooting using my multimeter and the 1980 EFI bible, it's all well within specs except for the test 1- (3b) - Air flow meter resistance #2 (ECU pins #7 and #8). It gave a reading of 360 ohms, well above the 150 suggested by the bible. I pulled the AFM to test it more thoroughly according to the FSM, which gives the following instructions:
4.: 12v DC to terminal #6 and #9
5. Test voltage between terminals #7 and #8
6. Gradually open the flap and if there is a proportional decrease in voltage, the potentiometer checks out.
And it did check out, the transition was really smooth. But I have no idea if the start and final voltages were within spec, as they arent mentioned in any of the manuals. With closed flap the differential was 8 volts. +8 at #7 and 0 at #8. At full open it was +8 and +8 for a 0 volt difference. Still not convinced everything was as it should I tested the resistance again. And this time it was 180 ohm, almost within spec. Weird. So I gradually opened the flap fully, and the resistance jumped erratically back and forth between 200-500 ohms. Then I closed the flap and made a new measurement: 260 ohm. Repeat: 230 ohm. Also confident the fault is not in my multimeter, it's brand new and doesn't hesitate measuring the correct resistance of anything else.
So naturally I'm wondering - is my AFM actually working as intended? Or is it broken, but perhaps fixable? And if broken, is this the likely source of running rich?
I haven't yet opened the AFM to fiddle about inside, but it certainly looks as if someone else has in the past.