Damage to spark plug wire

[joelittel]

I’ve been trying to track down the cause of an odd stumble my car developed about a year ago.

When pulling away from a stop it would stumble, lose power, nearly die and occasionally buck a little bit.

Last weekend I discovered a damaged area on the insulation of one of my spark plug wires. Swapped it for a spare and continued with my investigation.

To my surprise the symptoms went away immediately. Does this make sense to you guys or was this more likely a pleasant coincidence? I drove the car quite a bit and have not experienced the symptoms since.

Obviously the wire needed to be replaced but do you guys think it could have been the cause of my car’s odd behavior?

[SteinOnkel]

Absolutely. If your car goes from 4 cylinders to 3 and back multiple times in the span of a few seconds, you'll notice.

[18Fiatsandcounting]

Steiny is absolutely correct, yes, this likely was the cause of your issue. Damaged spark plug wires can do two things, both with the same end result: 1) if the inner conductor is broken, your "spark" can't jump both this gap and the gap in the spark plug, or 2) if the "spark" leaks out through a damaged outer insulator and shorts to ground, that's where the spark will occur, rather than inside the cylinder as it should.

Electricity always takes the easiest path to ground, and if the gaps are too large, well, the energy is returned back to your ignition coil.

-Bryan

[joelittel]

Thanks guys.

I had been scratching my head for way too long, and looking everywhere else for the cause of my issues. I’m really hoping to have fixed my problem, but won’t know for sure until the weekend.

In the end it was something someone said on this forum years ago that lead me to the damaged wire. Someone had given the advice to go back to the last thing you had tinkered with before the trouble started. In my case the last thing I tinkered with was painting my plenum. I didn’t see how my stumbling could be related to painting but I decided to check it anyway. Checked the grounds, checked the seals, checked the sensors and their connections.... and there it was. A small warn spot on the wire insulation that had probably been rubbed raw by my cold air intake tube not being completely inserted onto the plenum flange when I put it all back together. I’m only guessing but I’d bet the air intake tube made contact with the wire as I was pulling away from a stop because the engine was shifting a bit as the rpms increased. Our engines move more than you’d guess when you pull away from a stop.

That’s the current theory anyway.

I went ahead and purchased some spiral core and very well insulated wires to replace all four of my current ones. Spiral core are recommended for use with fuel injection and higher compression engines requiring a bigger spark. Less resistance and more insulation to protect against any EMI with my sensors.

Fingers crossed. I need my Fiat fix.... badly.

[18Fiatsandcounting]

I'm glad it appears you've figured it out. When I run across a Fiat that "stumbles", one of the first things I do is put a timing line on the wire from the coil to the distributor (or the equivalent). Not on the plug wires. The timing light will flash 4 times as fast since you're triggering it each time one of the 4 plugs fires, so it's easier to see a miss in the almost steady stream of light. If I see a miss, I then go to each individual spark plug wire to find which one(s) is missing. Several times it HAS been a bad plug wire. One of the easiest fixes to make!

-Bryan

[joelittel]

To be honest plug wires were never even on my radar. At a glance they look brand new so checking them never occurred to me.

Knowing a little more about them now has opened my eyes to the extreme conditions they operate under. Seems obvious now but....

In hindsight I cannot believe I didn’t have a similar problem sooner, I neglected my plug wires for years!

But, it only appears to be fixed. I won’t feel comfortable with the car until it has been put through it’s regularly scheduled weekend duties. Ie a trip here, a quick stop there, a long drive to a restaurant two towns over, grocery getting etc etc.

[DieselSpider]

New looking can be misleading since the center conductor can still have failed or the insulation become porous and grounding out any where they might make contact.

Stumble on take off requires the center wire going and primary coil trigger wire to the dist to be viewed as a prime suspects. That's one reason so many manufacturers have gone to Coil On Plug designs eliminating the common ignition coil and all the high tension spark plug wires. Pricey on my V10 with 10 individual COP units that might need to be replaced over time.

With my daily drivers being an Electric Car and the Diesel Spider I am getting spoiled though by no longer having any old fashioned spark plug wires to deal with.

[joelittel]

Coil on plug sounds like an interesting upgrade. Whenever a part has failed I’ve done what I can to replace it with something better. Thanks for the information, I’m going to enjoy reading about this.

[RRoller123]

Really interesting discussion, good stuff. Mine has a subtle miss at most speeds, but not when full on the throttle. Plenty of power, pulls tremendously. Haven't found the source yet. Computronics ign, with 2 coil packs. Any ideas?

[SteinOnkel]

Could be the wires. Apart from a visual inspection, you can also measure the internal resistance with a voltimeter.

On my hot rod VW I decided to go for Wasted Spark (that's what the engine came with stock). Could've gone COP, but then you're stuck trying to find the right coil packs.

Not sure if this approach is even viable on a Fiat Twincam, as they need to sit super snugly, have the right length etc.

[18Fiatsandcounting]

Really interesting discussion, good stuff. Mine has a subtle miss at most speeds, but not when full on the throttle. Plenty of power, pulls tremendously. Haven't found the source yet. Computronics ign, with 2 coil packs. Any ideas?

Someone please correct me if I have this wrong, but I vaguely recall that it's harder to ignite an air/fuel mixture when the engine is under full throttle and at higher rpms. So, for your subtle miss at most speeds but not full throttle, it could be a spark issue but I'm leaning towards an air/fuel issue. Running too lean? Minor intake air leak? Emission control device coming on or turning off when it shouldn't (like EGR)?

Does the engine miss at idle?

-Bryan

[RRoller123]

Hmmm..... Could be lean, I get no moisture out the exhaust, even when cold. I think the O2 sensor may be suspect, and I will put a meter on it. A digital meter is tough to read, as it just shows a number that jumps around, but an analog meter will probably stabilize the arm at an average, so I will try that. I believe the reading should be somewhere around 0.5VDC.... ?

Almost positive there are no air leaks, have really inspected and secured for that. Might try blocking off the AAV after warm up and seeing how it runs, to be sure there is zero air passing through. See what that shows. Can probably just put a clamp on the hose.

It does run rough at idle, but I have a 274FI cam in it, large valve head and very light flywheel. Not sure if it is a miss or not. I think swapping out a spare old set of spark wires one at a time may be a good way to help isolate the problem. Will try that too.

[SteinOnkel]

FYI, your car most likely has a narrowband oxygen sensor. These are notoriously unreliable outside of ~14.0-15:0 AFR. So I would not read too much into what your voltmeter (analog or otherwise) is saying. You can't tune an engine with them and you can't really diagnose them this way either.

In German we call them "Sprungsonde" Sprung means to jump and Sonde is probe or sensor. The reason they are called that is because the voltage is supposed to jump or oscillate in a set manner when they are functioning properly. In order to diagnose them correctly, you will need an oscilloscope (a cheapo amazon usb one will do just fine).

Or, just swap it out. It's not like they're pricey.

Cheers
Steiny

[RRoller123]

That makes sense, I will prob just replace it. I have had the car since 2011, hasn't been changed by me in. I did notice exactly what you are talking about with the digital VM, it had readings jumping around between about .4 and .6 or 7.