Some wires spark faster and skip/misfire

[leoJr]

The '99 5.0 auto started a low rpm stumble.

Check Engine light eventually came on. Friendly local car parts store said the code is Cyl 3 misfire. Suggested I swap wires with another cylinder and come back and for a rescan.

I put a timing light on various wires. Some spark slow without a skip. Others spark much faster but also skip occasionally.

I suspect whatever is causing the multiple sparks is also causing the skip?

Ideas? I did some searching and didn't this problem.

Thanks!

 

[leoJr]

This occurs at idle, seems to get word as it warms to operating temperate and is notable at small throttle openings. Push the peddle hard and it seems to go away. Not noticeable at steady speeds, just small speed increases.

 

[Gadget X]

How long has it been since you replaced the plugs and wires? Could also be a symptom of a "weak" coil pack. You can check the primary and secondary resistance on the pack to see if it's bad.

 

[leoJr]

Plugs and wires are long overdue for a change.

Thanks, I'll find the resistance spec and check that.

Local parts place said each wire was $80…I need to double check that too.

 

[leoJr]

I think watching the timing light triggered by the wires may be a distraction. I may revisit it later.

I didn't check the coil voltage as suggested earlier yet. The wires that flash fast and the ones that flash normal change… Thought I had a pattern, one side of the shared coil was fast and the other slow, until the last pair checked the opposite. I scratched my head and checked them again and fast wires were now slow, etc.

I ordered a new set of wires and plugs, they should be here Friday.

I put my AutoEnginuity on it last night and the B1S2 O2 sensor voltage stayed steady high. As this is after the catalytic converter it doesn't makes sense that it would read rich while the S1 sensor moved normally. So I'll order an O2 sensor too.

The AE scanner confirmed the #3 cylinder misfiring.

Here's the scan of the for sensors…

 

[2000StreetRod]

which plugs?

You have a waste spark ignition system. The cylinders are paired. That is why there are only 4 coils and 4 trigger wires from the PCM to the coils. When one cylinder is firing on its compression stroke its pair is firing on the exhaust stroke. Current flows from center to outer plug electrode for one cylinder and from outer to center plug electrode for the paired cylinder. Your plugs should be dual platinum or dual iridium otherwise one electrode will wear much faster than the other in one bank of cylinders. Many forum members have reported early failures with Bosch plugs in the 5.0L V8.

 

[leoJr]

Thanks 2000StreetRod for the insight on the plugs!

I am familiar with the shared coil firing two cylinders. But both spark plugs wires should be firing the same rate. One wire will fire normal, and the other will fire much faster, and also have a noticeable skip to it. The waste spark system design or failing coils explain this faster rate. I think my timing light pick up is getting triggered by neighboring wires. Which means they could also be leaking into adjacent wires causing mis-fires. New wires may solve this.

Anyone have insight on failed O2 sensors? The one seems to have failed at full voltage output…seems counterintuitive?

 

[2000StreetRod]

timing light trigger reference

Below is a schematic for my V6 ignition wiring. The V8 is similar.

A timing light has a power positive and negative connection and a spark connection. I think the spark connection triggers the light on a positive voltage value relative to ground. All of the plugs on one bank of cylinders fire on a positive voltage relative to ground. The plugs on the other bank fire on a negative voltage relative to ground. The voltage required to bridge the spark plug gap is significantly less for the cylinder on the exhaust stroke than the cylinder on the compression stroke due to low pressure in the combustion chamber. Your timing light may be triggering on the exhaust and compression strokes for the bank with positive voltage and only on the compression stroke for the bank with negative voltage.

I would think if your post cat O2 sensor was stuck high the PCM would set an associated DTC.