280Z AFM troubleshooting / Running rich.

[Sigurd]

Hello all,

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.

 

 

[siteunseen]

If it looks like it's been opened I'd pop the black cover off and make sure that weighted sweeper vane is freed up and moving nice and smoothe. Maybe clean the contact pad too.

Check the tps also. It could be stuck on the wide open side.

The multi meter guys will be all over this, you've actually done something.

 

[Zed Head]

2 hours ago, Sigurd said:

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.

Check your fuel pressure regulator (FPR).  They can blow a hole in the diaphragm and dump a lot of extra fuel in to the system.  Pull the vacuum hose off and see if it has fuel in it.  This problem won't show up through the normal tests.

[Sigurd]

Thanks for the tips. I already checked the TPS and it was OK I think. Not sure exactly when the full throttle switch is supposed to activate, I thought mine was a bit early at around 50% pedal travel. Idle switch deactivated at 5-10% travel, so was fine. Just went down and pulled the FPR vacuum line, it was dry and tight.

So I went ahead and opened up my AFM, found it had been tampered with. The locking nut glue blob was gone, so I'm going to try and calibrate it according to http://www.atlanticz.ca/zclub/techtips/afm/calibration/index.html

Tried hanging a 160 gram weight on the flap, which should pull a stock calibrated AFM to 50% open position. Mine was pulled to 85% open. If this calibration method is indeed correct, this would explain a lot...