Spark Issue. Can Someone Help Me Diagnose

[blu260z]

Hi guys.

 

Any help would be appreciated. Recently took the car for a drive and notice it ran rough abouve 3500rpm and when returned home it was hard to start and ran rough even at idle, and sometimes didnt even start. Thought it was time a did a bit of a tune up so got some new leads, plugs, rotor cap and button re-assembled and its still the same.

So I started more checks, checked fuel, timing, compression and spark thats when I found something strange!! I pulled a plug and ground it on the valve cover and got no spark however when I lifted it off a few millimeters I got a great spark. Leading me to think when the plug is screwed in it wont be getting a spark.

 

What could cause this ? The car has the 280zx distributor upgrade could it be the module?

I have also checked the engine and body grounds and all have checked out with less than 0.5 ohms resistance so I dont think it is a grounding issue.

 

Thanks Chris

[GregTas]

Running rough, not misfiring? Could be lack of fuel, not ignition. Fuel filter or pump? Have you checked the flow? You said 'checked fuel' so perhaps you have.

 

You've replaced most of the ignition system. So if electrical it could be a coil.

 

Checking sparks like you have that is an easy check, but inside and engine under pressure it take a lot more energy to ionise the air across the plug gap. An ignition coil can appear to be ok but under load can not have the grunt need to do the job due to an internal fault.

 

When I used to test coils we had a device that allowed you to open the gap up between a set of contacts. If the coil  had the power to run an arc across 12 mm or so it was doing well. Some of the high energy coils cold to 15mm or so.

 

Igniter modules usually  either work or they don't, but they can play up and fail if they get too hot which is why they need the heatsink of the distributor body.

 

Perhaps try another coil if you have one handy.

Edited September 28, 2017 by GregTas

[blu260z]

Thanks Greg. I have pulled the fuel lines and I definitely have fue. The coil is aprox 1 yr old Pertronix 3 ohm coil so it shouldnt be gone but that will be something I will keep im mind. It still doesnt answer why I get no spark when I ground the plug and when I remove it from the ground it will spark?

[HKSZ]

use a timing light on all six leads to see if you get a consistant spark

[GregTas]

Thanks Greg. I have pulled the fuel lines and I definitely have fue. The coil is aprox 1 yr old Pertronix 3 ohm coil so it shouldnt be gone but that will be something I will keep im mind. It still doesnt answer why I get no spark when I ground the plug and when I remove it from the ground it will spark?

 

Is it possible that the spark was still there but in the bright light just not seen?

 

For the spark to jump across to the rocker cover then it has either  gone from the metal plug lead end down the centre electrode and jumped across the plug gap then across to ground, or it has tracked down through the porcelain to the plug end before jumping across.

 

If it jumps a big gap to the rocker cover then coil probably is ok. To check the plug you can measure the resistance from the plug lead end to tip which should be 0 ohms. Then plug lead end to thread which should be O/C.

 

If in doubt change the plug and see how that goes.

[blu260z]

Hey guys had a quick look today and found no spark with the timing light on the leads. The lead from the coil however is producing spark. So I have spark upto to distributor but no further. This is further making me think it is the 280zx module. Can anyone else confirm this?

[GregTas]

If coil lead is pulsing then module is working. The igniter module just makes the spark triggered by the wheel in the distributor. The distributor cap and rotor directs it to where it has to go. Suggests the fault is in the cap or rotor button.

 

Is the little carbon tip in the middle of the distributor cap where it should be?

[blu260z]

Thanks Greg, ok then I will check out the cap. The cap and rotor are brand new so should be ok. I will double check it though.

[Petertil]

I had a similar problem on my 260Z and found that there was leakage of the spark voltage from the plug centre electrode across to the screw thread outer case so that even though the timing light showed that there was a spark voltage going through the lead it wasn't getting to the spark plug gap. It was getting shunted to earth before it got to the gap.

 

I tested with a multimeter and found a resistance across the plug when it should have been open circuit. A new set of plugs fixed the problem although I'm sure the old ones can be cleaned and made serviceable again. 

 

I think maybe it was caused due to a build up of carbon inside the plug due to it running rich when I was having trouble with the carbys.

[blu260z]

Turns out it was a faulty brand new distributor cap. So that fixed my starting issue but not my running issue... small steps at a time

Edited October 7, 2017 by blu260z

[GregTas]

Are the dashpots in the carbies moving up and down ok?