Starting Issues (injectors?)

[Blinkerfluid22]

Well after rebuilding the head after dropping a valve the car was up an running again. One night a few weeks ago I had let the car warm up and was backing out of my garage when it just stalled out and i haven't been able to get it to restart. When I try to start it the starter just cranks and once in a while there is a loud bang from the tail pipe. I have confirmed spark to all plugs, fuel pressure is around 40psi while cranking, cylinders have compression (thank God) and the battery is fully charged. One odd thing I noticed was that the plugs were dry after trying to start the engine. Shouldn't they be covered in fuel if the injectors are firing? What should I focus my attention on next? 

 

The car is a 76 280z, with 63k miles and only about 60 miles since the rebuild. I also replaced the AFM with a remanned version at the time of the head rebuild.

Edited August 10, 2015 by Blinkerfluid22

[siteunseen]

Is the afm plugged in good and the ground wire tight on the bottom mount?

[Blinkerfluid22]

Is the afm plugged in good and the ground wire tight on the bottom mount?

 

the AFM plug is plugged in but the connector clip is broken, so I doubt it has the best connection. The ground for the AFM is this black wire shown, right? Its supossed to be attached to the  AFM under that mounting bracket and is grounded to the bracket correct? I did drive around 2-3 times without it grounded, could that have fried the AFM?

Edited August 11, 2015 by Blinkerfluid22

[siteunseen]

I'm not too sure. I'm hoping the ground would've found another path. There's 3 bolts holding It to the plate.

Maybe some more knowledgeable folks will jump in.

I would think It would smell funny if it's burnt.

[Zed Head]

 

the AFM plug is plugged in but the connector clip is broken, so I doubt it has the best connection. The ground for the AFM is this black wire shown, right?

 

I think that ground wire is an extra ground path.  The mounting screws should allow the same path.

 

The plug hangs below the AFM.  Gravity might have pulled it loose.  First place to check.

[siteunseen]

Here's where the AFM ground goes, thanks atlanticz for the picture.

 

[Blinkerfluid22]

After some more diagnosis i found constant a 12v to the injectors even on the ground side. At some point the previous owner wired in new injector connectors, could it be the wiring has just gone bad? Could it be the ECU?

[siteunseen]

I put new connectors on my '77 a few years ago.  If my memory serves me right, when I wired them in the instructions said that the wires could be swapped around without damage, they were both 12 volts.  The ECU sends a ground pulse? that tells them to fire.  Not 100%, but pretty sure.  Maybe one of our electrical gurus can say for sure.

[Driver_X]

Injector connectors are not polarity specific.

[Zed Head]

After some more diagnosis i found constant a 12v to the injectors even on the ground side. At some point the previous owner wired in new injector connectors, could it be the wiring has just gone bad? Could it be the ECU?

CO explained this way back in someone else's thread.  The injector circuits are wired in parallel and have a common connection for power, and at the ECU transistors, so there's potential to ground on both sides of a plug if you measure one at a time.  I think that if you unplugged all of them (or some smaller sub-set) and measured you'd only see power on one side.

 

Anyway, it's normal.

[SteveJ]

When measuring the voltage on the ground side of the injectors, you will see 12 VDC with the key in the START or ON position until the ECU gives the command to fire. At that point, the ECU is grounded, and the circuit goes from the injector, through the dropping resistor and on to ground. When I measured voltage on a 280ZX one time, the voltage would drop to about 10VDC when the injectors were firing.

 

If you want to "test fire" your injectors, I would ground the circuit downstream of the dropping resistors.