weak spark some backfire

[zclocks]

 

Ok, I am officially stumped on this one and need some help.

Car

75 Calif 280z with only 80 miles on a rebuild 14 yeas ago. I know it's along story .

Problem:

1-I have a small back fire thru the AFM only some times when I crack the throttle.

2- cylinders 1,2,3 have a small drop out(loss of spark). 4,5,6 look very good using Colortune.

What was checked:

1.       Originally the car would not start. Fuel pump was bad and replaced.

Idle pressure 32psi and pump head pressure is 40.

2.       Checked pressure regulator and not leaking at vacuum port.

3.       Plugs replaced  as they looked very dark.

4.       Vacuum is 17mmhg and steady.

5.       AFM boot cracked and was replaced.

6.       Adjusted valves (cold and hot).

7.       Checked timing , 10 deg btdc

8.       Air/fuel meter indicates I am now running slightly lean (at operating temp) and rich at start up.

9.       Checked timing chain mark on cam shaft  sprocket . Is just to left on mark #2

10.   Cap and rotor inspected and no cracks or discoloration.

11.   checked resistance of plug wires and all within 500ohms.

12.   checked all 6 dropping resistors. All measured 6.5 ohms

 

Observations:

1.       I have a shut off valve on the fuel line and can isolate pump pressure from fuel rail. The gauge is after  the shut off so I can check pressure between the gauge and injectors.  I might have one or several injectors leaking as the pressure drops off in about 30 min.

2.       Using Color tune plugs 1,2,3 have a weak spark , some misses in spark and are slightly blue(good color)

3.       Plugs 4,5,6 have a bright spark, consistent firing and no misses.

So what it looks like is that plugs 1,2,3 have a weak spark which could be causing my backfire. The only common point  I see between the plugs would be the dropping  resistors, which all measure the same 6.5 ohms, spark plug wires, coil, and distributor.  

At this point what am I missing? Any help would be appreciated.

Ron

Edited June 11, 2019 by zclocks
forgot car info

[Zed Head]

4 hours ago, zclocks said:

Car

75 Calif 280z with only 80 miles on a rebuild 14 yeas ago. I know it's along story .

Problem:

1-I have a small back fire thru the AFM only some times when I crack the throttle.

 

Does the engine stumble or lag when it backfires?  Some people call it "frontfire".  Sounds a bit like the "lean" problem that many of us have fixed by adding a potentiometer in to the coolant temperature sensor circuit.

http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

 

[zclocks]

I have not driven the car . The problem is the miss at idle and I know it will be a problem when I do get it on the road. It should have a smooth idle. Problem is the miss in 3 cylinders.

 

[Mark Maras]

 A miss in three cylinders has to be in the cap, rotor, plug wires or plugs. It would seem that you've eliminated wires and plugs  (NGK BP6ES?) from the list.  Don't know what brand the cap and rotor are nor how old they are but I'd replace them with a quality brand. Nissan and Bosch are the two I'd investigate. Jalex in Panama cleared up a similar problem with Bosch parts.

[zclocks]

Thanks Mark. 

I'll take a look at all 3 again. Yes, the plugs are BP6ES, NGK plug wires and Nissan cap / rotor.

 

[Zed Head]

I think that the EFI system engines always sound like they have a miss at idle.  Because all 6 injectors open at the same time and each cylinder is at a different point in it's 4 cycles.  They don't all get the same type of fuel-air charge.  I think that it leads to incomplete combustion in some cylinders and that's why the manufacturers developed sequential injection for idle and low RPM.

You'll know more when the engine is under load.  Good luck.

[Dave WM]

did you try swapping the injector plugs 123 with 456 to see if the lean run follows the injector plugs.

Also did you could try swapping the injectors themselves if no change with the plug swap.

the richness of the AFM will have an effect on both idle and vacuum. a rich setting will increase both. I recently tested this and found that adjusting the AFM bypass screw could be used to optimize idle quality and vacuum, prob at the expense of emissions.

[zclocks]

I haven't swapped injectors, not my favorite thing , but that's just about all that's left. My car use to idle very smooth and had quick throttle response.

I know there's a definite miss in 1-2-3. Could be the injectors not working correctly although they sound good . What are the chances that just 1-2-3 injectors are bad? 

Ok, thanks for the input and I will proceed with my quest.

Ron

[Dave WM]

I would think its more likely a ECU thing than and injector thing, since its a 123 vs 456, hence the easy thing to do is swapping the injector harness plugs around.

[240260280]

Odd for first 3 out of 6 independent items.... not at all like carbs. Could be problem with fuel rail as it is only item splitting front and back 3.

[Dave WM]

for some reason I am thinking the ECU has two power transistors for grounding the injectors, 1 for 123 and 1 for 456. I could be all wrong about that, but if not, then its a valid test of the output from the ECU.

[Dave WM]

I wonder if the show the lean/rich condition? Unless its just an idle thing. Did the color tune look the same above idle on all plugs?

Edited June 12, 2019 by Dave WM