Spark Plug Lead Breakdown - What Happens?
 

I am currently faced with replacing my HT lead set due to a misfire on the lower plugs. Took the exhaust off and tested for a spark on each plug, I expected one or two would not be firing. However, all 6 lower plugs sparked OK, so I am left thinking there must be an occasional short on the leads. I found this a bit confusing. I did initially find the leads a bit crossed over each other underneath as a couple of clips were missing, which are now on order, but there are no obvious signs of deterioration in the insulation.

I have read what there is to read about HT lead problems but I do not fully understand. There are no visible faults with the current HT lead set or connectors which are probably originals, so 21 years old.

What actually causes them to fail and breakdown? I wondered if they tend to breakdown internally where they are the most bent - around the dizzy caps.
Can anyone shed any light? Cheers.

The stock wires are copper wires . If you have an old wire it can leak the spark out the insulation , the spark will go to ground before it gets to the plug gap .
Have you checked the inside and outside of the cap the tracks ?

mist the leads with water from a spray bottle and watch the light show they emit ...as the insulation gets old and hard it cracks and high voltage leaks out..

As Indycam and Silvertarga stated. If you want a great light show start the car at night in your garage open the deck lid and mist the air a bit, turn the lights off so you are in total darkness. It will seem like the aurora borealis just moved into the engine compartment. It also works great on a humid/rainy night.

There are two failure points on the plug wire harnesses:

1) Insulation breakdown of the jackets on the wires causing crossfires to another wire and/or ground, and misfires.

2) Failure of the internal RFI suppression resistors inside the plug connectors. This causes misfires, as well.

Well thanks for your explanations. I didn't know the plug caps contained "RFI supression resistors".

I don't know whether to buy a set of DIY wires from Pelican Parts and use the old caps, or whether to buy a whole new HT lead set. The full set seems rediculously expensive.

However, it'd be a big job to change all the leads whilst retaining the old caps to find I still have a misfire from a cap, once I have put all the exhaust back on etc. Its a PITA.

Is there any way of checking the cap resistors?

Cheers.

Lead insulation just goes over time. Heat, elements, high voltage etc etc. They'll miss under load, hard to see any visual degradation unless it's real bad, the lead will burn where the spark is jumping. Given the length of the leads it will take you for ages and may even be impossible to find visually.

You can check the resistance with an ohm meter to isolate the issues. Most auto or electronics shops will have a digital volt-ohm meter and they're super handy.

I'm new to Porsche so it may be different but the lead or cap point that's giving you issues (causing the miss) will have a much higher resistance than the other leads. Bit like a compression or leak down test.

This will also help you isolate the 1 or two leads that are causing the miss and maybe you can buy just those to keep your costs down.

Just need to reiterate I'm new to Porsche but this is how I've done it on my other cars so it should work, and the $20 you spend on a Volt-Ohm meter won't go to waste on future projects.

Usually as mentioned the first failure is the resistor in the spark plug end connector which you can replace seperately

and a Multi meter is an indespensable tool.

http://www.pelicanparts.com/cgi-bin/...0Each%20%20%20

If you need or want to replace the wires just get the Beru set.

The DIY wires are a PITA and need a special crimping tool to build and they come with no connectors (which could be your problem) and no holders and there is room for error.

You will spend a long time build them and putting them in the looms before ever even start to get them in the car.

You're all giving some good views on this. This is what I'll do:

1 Check the caps for evidence of tracking, although I saw nothing obvious before.
2 Check the resistance of all the lower leads.

I guess I'll have to remove all the caps for the dizzy cap to then pull the leads through from below. Is this the best way? In any case my multi-meter test leads aren't long enough to go from underneath and into the engine bay I don't think.

Regarding Pelican Parts DIY leads, they are already prepared to Beru spec I understood, and have the brass ends crimped on, are cut to length and are numbered. So maybe not quite as bad as you reckon Makmov. Still a general PITA though. Just need to get the old caps off and screw them into the new leads, providing the caps are OK.

maybe so, I just remember make older 911 wire sets up, which were steel braded shells and about a $1000 for six wires and it wasn't much fun. Got to becarefull with the end so you don't brake them and check resistance in the wire than in the end and then as a completed wire.

Well I've removed the lower leads and checked the resistances. The multi-meter was turned to ohms and "200", and this is what I got:

1 3.3
2 3.4
3 3.4
4 3
5 3
6 3.3

So I'm guessing this means the plug cap resistors are ok, so I just need to change the HT cables.
Please would anyone have any comments on the resistance readings from experience?
Cheers.

I believe 3 is normal.

Or 4 across the entire wire assembly 1 in the wire and 3 in the connector for a total of 4k

What happened in the end ??