You could unplug and swap injector leads between 3 and 4 to see if it is weak injector current due to @Dave WM's post #11.

They should just reach.

 

Edited June 12, 20196 yr by 240260280

In your initial post, you write that you checked all 6 dropping resistors. The car only has two dropping resistors- one for injectors 1, 2, 3....and one for injectors 4, 5, 6.  I have a 75 280 but have never seen dropping resistor problems and therefore I don't really know whether they either work 100% or fail 100% ...or whether they reduce voltage to the injectors and lower output to cylinders. EF-23 in the 75 FSM covers the testing. The dropping resistor though is the only thing I know of that isolates cylinders 1,2,3 from 4,5,6....unless there is something in the ecu (mentioned above).

There are 6 resistors split 4/2 in heat sinks.

 

@240260280...clearly you are correct. I thought I had read 3/3 somewhere but clearly it is 4/2. Probably just an assumption I made with two casings / harnesses. Apologies for the incorrect info.

 

2 hours ago, jonathanrussell said:

@240260280...clearly you are correct. I thought I had read 3/3 somewhere but clearly it is 4/2. Probably just an assumption I made with two casings / harnesses. Apologies for the incorrect info.

 

It could be 3/3 for the power transistors in the ECU though. I am trying to find a schematic.

btw I will guess the 4/2 came about  like how they added 2 cylinders to an existing 4 to make the L24.

Dave you are correct there are 2 transistors in the control unit that are associated with the pulse to the injectors (FSM EF-51).One transistor is associated to injectors 1-2-3 and the other with injectors 4-5-6. I did this check with the 3 volt lamp as indicated in the manual and both transistors check out. This isn't an exact performance evaluation , but rather an overall output check.

Funny I remember many years ago I had a lamp that was provided with a FI  meter I purchased and no explanation of what it was for.  I don't know what happened to it, but that was the lamp as mentioned in the manual.

I also pulled the dropping resistors and cleaned everything including the connector contacts on both sides.  All the resistors measured 6.5 ohms. After I reinstalled I checked that the power relay was working and there was battery voltage at the control unit pins for each injector. 

The car is running MUCH better and no backfire thru AFM. The drop outs are farther apart and very small compared to previously. I went back and retested with my Colortune unit and the visual blue flame is much better on cylinders 1,2,3. There are still some drop out or miss fires, but not as many. The intensity of the Colortune flame is much brighter. Cylinders 4,5,6 are unchanged, bright in color intensity and no drop outs. 

So, I think part of the problem may have been with the connectors on the dropping resistors or the control unit connector. What I am going to do is remove and clean all connections on the distributor, transistor ignition unit, and  injectors. All my injector connectors were replaced about a year ago and I still have to replace the AFM connector. It looks good but the boot has deteriorated.

I would like to dig into the transistor ignition unit and see what's going on there. I need to hunt down an OEM unit for testing. Anyone have a spare they would like to part with?

I still believe there is a slight problem, but I think I'm on the right path. 

Thanks for all the inputs it's really helped.

Ron

 

Edited June 13, 20196 yr by zclocks
additional info

Great news Ron! Yes all of us familiar with Bosch EFI systems have had to deal with electrical contact corrosion. 

Inspecting, cleaning, and dielectric-greasing every connector you can see on a 280z is actually a wonderful grenade approach to trouble shooting! It seems to do wonders every time!

I pulled one apart a long time ago, IIRC it was all mostly hi reliability type parts (metal film resistors, film caps, maybe some IC's but I bet they were not custom, prob things like 555 or 556 etc...

I do not recall seeing any caps electrolytic types (they tend to dry out and open over time). So if anything I would guess maybe a physical problem, pin connections broken trace (if abused) or electrical damage from reverse polarity or some kind of short of the pins.

I can see the power transistors leaking or shorting since they are under the greatest current draw. But I think its a long shot.

Yes,  I should know better. I've seen so many corroded connectors on clocks that I just replace every wire /connector. The thing that fooled me is that the connectors didn't look bad, but corrosion is resistance. 

Thanks again for the input. 

Ron

Dave, 

Yes, caps have a limited life and so do power transistors. No telling what material they used. If I can get a transistor unit I'll see about replacing the transistors and caps. 

Ron

Hey Dave, 

I was going thru a few boxs of Z parts and found a 75 transistor Ignition. It's completely made by Hitachi . They were one of the big part swingers of the day. I'm impressed with  the design and construction. Looks like the main power transistors were also from Hitachi. 

As soon as I can i'm going to install it and see if it makes any difference in my car. If not then I could have a spare or something else to repair.

Edited June 15, 20196 yr by zclocks