slightly rough idle - odd spark pattern

[JHZR2]

Hi,

My 98 S-10 ZR2 has 56k miles. It has been great, really reliable, and very useful, while being a perfect off roading vehicle, and a lot of fun.

Since about 2002, Ive not used it regularly. It has been for reasons ranging from fuel prices to needing to drive a LOT. Anyway, since about then, Ive noticed that the idle, while not rough, is a bit "shaky", i.e. I can feel the engine shake at idle.

Ive tuned the truck up in ~2003, it has MSD wires, the correct spark plugs, I even swapped another OE coil, it gets good mileage, etc. But yet at idle the engine shakes a bit more than I remember.

I got a blue point spark tester - the gadget that you put onto a spark plug wire and it flashes a lightbulb when the wire has current running through it. I noted that some of the wires had a much higher frequency of flashes, and one of the wires had a noted slowdown of flashing that was well correlated to when I feel the engine shake.

The question is, are these spark testing tools reliable? Is a variation in the reading from cylinder to cylinder normal? I have no CELs, never have. I pass emissions testing with flying colors, I dont think it is a big issue, but then again it was alarming how well the change/pause in flashing of the tester correlated to the engine shake.

So, question #2, what could it be? As I mentioned, I have MSD wires, they are in great shape. Delco platinum OE plugs, correct gap, etc. Could it be the distributor? Some of the positions are dirtier/eroded more than others? The plug wires are all either parallel or at 90 degree angles. They are all on really tight. Could one connection being marginally "looser" than the rest cause such behavior without a misfire or notable issue?

Any thoughts? any tests?

Thanks in advance,

JMH

 

[chrome]

I just wonder what you are seeing is "cross firing" with the ignition system? Sometimes, parallel and crossed cables can cause what you are seeing. Try rerouting the ignition cables and see if you notice an improvement?

 

[Eddie]

You may have "induction coupled" several ignition wires. You will not see arcing when this happens. As crome suggested, separate or re-rout the ignition wires.

 

[Tosh]

Originally Posted By: JHZR2
I got a blue point spark tester - the gadget that you put onto a spark plug wire and it flashes a lightbulb when the wire has current running through it.

How is this different from a 'timing light'?

 

[mechtech2]

If the wires are causing misfires, you will see differences in the spark plugs color. Magnecores are good, but may have been yanked or something like that, and are damaged.
Check that first.
An injector may not be flowing properly, also, so run a bottle of good cleaner through a tank. The common good ones are Chevron Techron valve and injector cleaner, Redline SI -1, and Gumout Regane.

 

[JHZR2]

Ive run cleaner and use additive regularly. How would you tell if they are damaged? If a test with the multimeter reads good, is there anything else that I can do?

Thanks,

JMH

 

[Quest]

meter testing does not warrant the wires condition for we are dealing with high voltage here and under such situation, anything could happen (do you know how challenging it is to deal with multi-kV electrical stuff?)

under high voltage (proper term is"high tension"), it can break through certain insulating materials: e.g. aged silicone rubber sheath, rubber of insufficient thickness, etc. Also: many spark plug wires cannot take repeated heat soak over the years w/o experiencing some insulation breakdown (causing spark leaks).

If you suspect that your spark plug wires are at-fault, try a cheep new set of wires and see if it cures your problem. Also: check to see if your spark plugs developed spark leaks due to carbon tracking on the electrodes, the outer insulator body, etc.

also check your distributor cap and rotor for carbon-tracking and when in doubt, start with a fresh d-cap and rotor and go from there.

(*had some issues with OE delco caps in the past RE: phenolic cap body developed micro-cracks and moisture/carbon tracking caused spark leaks.On the surface: the symptom is exactly the same like yours...*)


Quest-TD