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zclocks

weak spark some backfire

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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 by zclocks
forgot car info

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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

 

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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.

 

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 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.

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Thanks Mark. 

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

 

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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.

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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.

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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

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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.

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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.

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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 by Dave WM

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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).

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@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.

 

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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.

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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 by zclocks
additional info
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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!

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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.

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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

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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

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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.

ti-1.jpg

ti-2.jpg

Edited by zclocks

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yep, that's the same as mine. I got and aftermarket unit of ebay for 50$ IIRC, pulled my OE off to test it, left the aftermarket unit in place so I carry the hitachi as a spare. The aftermarket unit seemed to have fewer parts IIRC. I can see several electros in there that prob should be checked at least of not just shot gun replaced. I can't believe the engineers ever expected this kind of long term life.

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