Do plug wires get "better" when warmed up?
 

The short story: Is there any evidence that a "bad" spark plug wire that causes missing can come around and work just fine when the engine warms up?

I have wondered about this over the course of several cars I have owned and I am beginning to believe this is true...please set me straight on this.

Many thanks,
John

PS: I should add that the wires are not wet at all -- the cars were alway indoors at 65degrees plus. I am wondering if the wires simply become more conductive due to heat.

In a round about way yes.

I knew my wires needed doing, car would run differently depending on the weather, humid, raining, etc...

Have a brand new set of wires on now. If the car sits over the weekend and gets rained on it generally runs like crap until it warms up/dries out.

If you are having problems just replace them as soon as possible so you don't get stuck away from home in really bad weather.

In my experience wires don't get progressively worse, they just up and stop working one day.

Scott

Re: Do plug wires get "better" when warmed up?
 

sometimes just the opposite happens with the Beru connector

it's possible that higher underhood temperatures could drive out some moisture in the ignitiion. I can't imagine that the resistance of the wire itself would change sig'ly based on the temp. changes involved.

Or maybe it just takes that long for the drugs to kick in....

Yeah, I think drugs are the answer.

Heat generally does electrical circuits no favors. Heat does however make engines much happier. I once said that the hotter an engine is the better it runs, and with just a very few exceptions, I am sticking to that assertion. That's why we no longer have air cooled engines in todays' cars. Todays' cars have HOT combustion chambers. My car runs better when warmed up.

Along the lines of Superman, I think your plugs are clearing up as the engine warms up. I've noticed that in my car - it gets a bit crisper. But, then my engine has some 100K and runs (MFI) rich...

I had a Subaru with silicone leads on it and when it was cold it would run fine. Once the engine bay temp was up to operating levels however, the car would start to miss and chug. This was all down to the conductive core of the leads breaking down. Just old age. This fault was diagnosed for me by a friends brother (Wrench) AFTER I had replaced the fuel pump arrrrgh! Rule 1: When you don't know the answer, ask the Pro.

Ya know -- come to think or it -- if it is running better hot c'uz of getting water out of the wires somewhere, that almost surely means you are headed for trouble. I might replace evey ign. wire, cap & cover in the whole engine just to be safe.

Since you are in the PNW, moisture is very likely -- but hten so are drugs.

You guys are killing me. Okay, I must confess, this post is a stalking horse. The story is a bit long and is OT, but it is a German car...

I was given a very straight and clean 1990 VW Golf GL. All the usual suspects in this car that had sat in a parking lot for 8 months. Old gas, plugs, rotor, wires, and cap. Air, gas, and oil filter as well.

Took the car, replaced rotor, cap, plugs, air, gas, oil filter and oil. Put in 20oz of Techron and filled with Premium. Replaced all small vaccum hoses as well and checked all connections from oxy sensor, coolant temp sensor, fuel rack, hall sender, etc, etc. to the ECU. The system is Digiphant II, VW's inhouse solution to K/L Jetronic. I am now driving the heck out of it to "wake it up" and it has come a long way.

However, when I first took this in, you would start the car and it would misfire at idle *as if* one plug wire were bad. Hook my timing light to wires and sure enough it appears #1 is misfiring. Swapped plugs with #2 but still #1. Checked resistance of plug wires against known and #1 is out of range, rest are OK. So you'd think it was the plug wires, right?

But, you know what? Get the car warmed up and it runs like a champ. The vicious misfire goes away. I mean it purrs like a kitten, perfect! Don't touch anything!

So is it the plug wire, a bad head (there is no oil in the antifreeze) that is heat sealing,a warped manifold that is heat sealing (the 1.8 is famous for this), or what? I no longer believe replacing the wires will make any difference.

So that's the story. Since I MUST keep costs down and new wires are $65.00 I am reluctant to spend the money (and the wife agrees, if you know what I mean and I think that you do).

Many thanks for your replies,
John

You don't say how many miles are on the car and if the wires are original or not.

As a rule I treat wires as a maintenace item at every 50k.

I suggest you go buy a set of wires, put on clean gloves and replace the questionable wire only.

If the problem goes away then change the rest and eat the $65.

If not put the new wire back in the box and return them.

My local places will take returns on wires as long as they have not been used.

Just make sure the packaging is intact and the "test" wire is nice and clean.

If it were the head sometimes you can hear air escaping, should sound like a soft exhaust leak, try putting soapy water around the head at #1 where it meets the block, see if you get any bubbles.


Scott

John,

In general, the answer is NOPE!

However, in the electronic ignition arena ... it may have an 'intermittent' connection such as a solder joint that is heat sensitive! Happens every once in a while ... thermal expansion during the underhood warmup process allows the wayward component to make a positiveconnection again, and the ignition starts working properly again. I don't think spark plug wires normally fit into this scenario, but since some have a 'lump' resistance with connections, it <b><i>COULD HAPPEN!</b></i> Bosch has made spark plug coonectors with resistors for VW for several decades, so it could be one of the connectors, but my money is on the 'electronics!'

Ummmm, here's a suggestion. Warm the car up before you drive it.

Sounds like you're doing important research into the area of pennies-per-mile efficiency using an ordinary passenger automobile. Only by following the strategy I outlined above can you achieve maximum results. Oh, and used tires can be procured at salvage yards. On wheels, with air.

Well gentlemen, God hates a coward (and a cheapskate). So I guess I'd better prepare for the fiery pits (though Dante tells me to bring a parka?).

But enuf pinheading, I will take your advice and run with it, looking closer at those dist. and trying the plug wire "lend."

Guys, give me a break here... a 1K sale for this car after putting $120.00 into it means fresh SSIs for my little Pcar. All hail the Porsche, for which this man will eat mac and cheese for years and wear underwear until you can shake it to dust (thank you Dennis Miller...).

Again, many thanks to all. This 'board is a true oasis of knowledge.

John

John,

To narrow down the wires ... start the engine cold on a dark night or in a dark garage ... and pop the hood and look for a 'lightning show' ... sparks and corona effect will be present if the wires are in need of replacement.

Warren,

That's a good test but not conclusive.

Once saw a wire arcing on my car at night.

Knew right then that they needed replacing.

Car was still running fine so I procrastinated.

Drove another 2 months and I forgot all about it.

Did some work on the car that involved removing the wires from the plugs.

Next day after a bit o rain the car drove like garbage into the office. Sat under my building and dried out.

Drove great all the way home.

Next day, after more wet weather the car ran so poorly I could'nt drive it to the end of my street.

Oddly no light show.

Replaced wires and it was fine.

Scott

Scott, I've learned to believe the night lights inspection. But you may not see the light(s) if the center tap lead (coil HT to distributor) is grounding via a nearby path (say, the OEM type braid sheath). BTDT. Suggest you make sure you have enough gap around wires to do the night inspection for arcing.

I know this may be slightly off topic, but I saw a movie as part of a High Rise Fire Safety Director class that demonstrated how gasoline burns. They had a puddle of gas on a small plate and put a match to it and instead of exploding, as I thought it would, it just burned the vapor rising off the liquid portion. But as the liquid heated and more vapors formed, the flame increased in size. The point that they were making was that gasoline in liquid form does not burn, only the vapor.

In addition to the vapor burning, they did an experimint to show that the vapor will only combust at a very narrow window of concentration and that it will not burn if concentrations were too weak or too lean.

They had a spark plug mounted in a glass chamber, adding one drop of gasoline at a time, shaking the cantainer, then igniting the spark...nothing until several more drops were introduced and finally a flame. Next they added more drops beyond what caused the flame and that mixture would not burn no matter how many times they fired off the spark, demonstrating how difficult or impossible to ignite an over rich mixtrue.

This might explain why a cold engine with cold wet and poorly vaporized fuel will tend to sputter and generally run rough when first started but smooth out later as the fuel in the heated combustion chamber vaporizes making it easier and requiring less ignition energy to ignite.

This might also explain why ignition cables capable of delivering the full secondary voltage to your spark plugs from your coil are so important, especially during cold starts where conditions for combustion are not favorable.

Joe

Joe nailed it on the head.

You aren't experiencing improving plug wires when they warm up. You are experiencing a mixture that requires less spark energy to ignite in a warm engine.

Fuel more readily turns to vapor with heat. Since it takes a given air/fuel ratio to be combustible, a cold engine must dump in extra fuel to get the same amount of vaporized fuel as a warm engine has.

Think of the gap of the spark plug as a canyon. The microscopic fuel particles are stepping stones across that canyon. If it's cold, the droplets are larger and further apart. When it's warm they are smaller and closer together. It takes less energy to jump the gap with a well atomized fuel mixture. Just like if you had thousands of small stepping stones to cross the canyon, versus 2 or 3 large ones that were far apart.

The more atomized fuel there is between the spark plug gap, the less voltage it takes to jump the gap. If you have too much fuel, then the voltage doesn't even heed to make a spark to pass from one electrode to another, and you get a misfire and a fouled plug. A wider gap increases the necessary voltage, as does higher compression ratios and forced induction.

So basically, a warm engine is just making life easier on the bad plug wires. Plug wires actually tend to get worse as they get hotter, but if their work is made easy enough, then they won't pass their spark to ground trying to find an easier path.

Great analogy!

A very worthwhile study -- thanks you guys!

I have to agree that the additional fuel is washing out my spark. The Digiphant II system also like to retard the ignition at cold start to "richen" things up a bit.

I am also just a bit suspicious of the Champion plugs I put in. I am going to go with the Bosch OEM and a new set of plug wires this weekend -- I'll report back on Monday on the car's condition.

Thanks much to all for bearing with me on this situation,
\John