Pulling spark plug wire while engine running = bad?

I remember reading a thread here some time ago where one of our resident auto mechanics claimed pulling a spark plug wire with the engine running could cause damage to one or more of the electronic ignition components of modern day vehicles. I can’t find this thread now and I’m not finding anything definitive via Google searches. Anyone know the straight dope?

If it is bad, doesn’t that mean a fouled plug that fails to fire could cause the same damage?

I don’t know about damaging the electronics (in theory, an unloaded secondary could change the impedance of the primary in a way the coil driver electronics doesn’t like) but I’ve not personally encountered a problem. Not that I do it much.

the thing I would worry about is the excess fuel overheating the catalyst. plus you’ll probably throw a misfire DTC as well.

Fouled plugs and plug wires that short out require less energy for the spark to find ground, which means less stress on electronic ignition components.

A badly worn plug or a disconnected plug wire presents a large gap for the spark to jump and requires more than the normal energy to find ground, which means more stress on electronic ignition components. I wouldn’t phrase this as “cause damage to,” which suggests a fairly short-term cause and effect, but rather as “shorten the life of.”

With a spark plug in place and grounded to the engine block, the secondary coil will only produce so much voltage before a spark jumps the plug’s spark gap. As you’ve noted, that peak voltage is even less if the plug is fouled, since the fouling material provides a lower-impedance path to ground.

If the secondary coil is connected to a very, very large air gap - example, if you disconnect the plug wire, or connect it to a spark plug and deliberately don’t ground the threaded part of the spark plug to the engine block - then the secondary coil can build up very large voltages. Problems arise when the voltage in the secondary is so great that it arcs through the potting material around the coil, creating a new low-impedance path to ground. Now your ignition coil may not even be able to produce enough voltage to fire the spark plug. In cases of modest damage, the plug will only refuse to spark at high engine loads (when dense mixture in the combustion chamber results in high spark gap impedance); in case of the worst damage, the plug won’t even fire when the engine is idling.

This assumes a case where the ignition coil is separate from the spark plug, and the wire being disconnected is the high-voltage one between the coil and the plug. Some engines are coil-on-plug: there is no high-voltage line, and the wire you disconnect is the low-voltage connection between the coil and the ECU. In that case, damage to the electronics is unlikely.

As jz78817 notes, passing unburned mixture into the exhaust system is likely to warm up the catalytic converter quite a bit. If you do this at elevated engine load or for significant lengths of time, you may overheat and damage the cat.

Plug wires go bad and become open. I never heard of that damaging anything but performance and millage.

I can’t imagine limited pulling the plug wire to identify a bad cylinder hurting anything.

I’d worry more about getting the piss shocked out of you when you pull that wire off the plug.
I speak from painful experience.

I’ve Never done it with a car made before 1990. Seems unlikely to damage the car. Mechanics have been doing it since the Model T’s.

Well, I did find this article (see the last paragraph of the first column on page 106) from 1979 before starting this thread, so I thought damage may be more prevalent in more modern day vehicles.

Thanks for the replies everyone.

back then electronic ignition was still in its relative infancy, and it was hard enough getting the ignition modules to live under normal use.

Why would you want to pull the wire from a running engine? I must have forgotten this diagnostic technique.

The primary reason you would pull a plug wire from a running engine would be to see if that cylinder was in fact firing. Cat converters, ECMs and coil packs aside, if you have a multi-cylinder engine (think 8-12 cylinders) and something’s off, quick and dirty diagnosis (think in the pits) would involve pulling a plug wire. It’s too loud to hear anything, so you have to watch the tach. If lifting a wire produces no drop in idle speed, it means you found a dead cylinder. A more elegant, new-tech way of checking this is to use an IR thermometer on each header. The cold one isn’t firing. Old school was to put a wet finger on each and feel the sizzle or lack of same.

Assuming you found a dead cylinder, the first thing a race mechanic would do is replace the plug lead “almost” on the plug, leaving an air gap of 1/8" or so. As previously mentioned, the increased voltage demand - prior to damaging the coil - would give extra spark to the dead plug and - if it was simply wet with fuel or the plug was too cold and there was no mechanical issue - it would light up. Or you would learn that there was no spark at the lead and go in that direction.

A good mechanic can pull the leads on a V8 at idle and tell you (on a single carb) if you have an ignition or compression problem, and on a multi-carb engine whether you have jetting issues in less time than it took to type this. A very valuable tool.

I thought that would be the answer. In most cases (outside of a pitstop in a race), there are other means to diagnose, or the engine doesn’t have to be running when the wire is pulled. But if I were a mechanic, I’d probably do it anyway just to save time.

With all those electronic diagnostic software built into modern engines, and the codes they display to indicate problems, wouldn’t they identify which cylinder is not working?

Seems like this ‘technique’ is something done by a cowboy mechanic, who is not using the proper repair tools & manuals.

You suspect one of your cylinders is missing. (not physically, just not firing)

You pull and reseat all the wires one at a time. You get a worse idle on every wire pulled, except for one of them. That is the cylinder that is giving you issues.

Because, on the other plug wires you pull, your engine runs more roughly, or just dies. Obviously, that cylinder was pulling it’s load, and when you pull the wire, it is no longer working. But on one of the wires you pull, there is no discernable difference. That is your problem child.

Could be anything from a broken cam lobe not allowing gas-air intake, to an actual issue with the wire or the plug, or any other number of things. But, at least you have it narrowed it down to one cylinder.

Of course, back when I worked on my own car, you were expected to change the plugs every year. Seriously, when is the last time you changed your cars plugs? I don’t even think that todays engines use their spark plugs. I think they are magical!

And if you are driving anything that is 40+ years old (like my wife), this is a perfectly good troubleshooting method…

In a nutshell, this is position is why the question was asked in the first place. This is the equivalent of not knowing which end of the horse the feed goes into 100 years ago.

Yup, I pulled one from a crashed ultralite with a running engine back in the late 1970s, and yessirree ZINGGGGGGGG! :smack:

Does hurting anything include you?
There are some ignition systems out there that are capable of producing lethal voltages.
GM DIS can produce up 90,000 Volts. It is my understanding that there had been fatalities from working on these. GM issued very specific instructions on finding bad plug wires or plugs and I can assure those instructions did not include pulling a wire off a running engine.

There’s nothing like getting shocked by a magneto. Makes your while arm tingle if the mag is in good shape!

As others said, pulling off a plug wire while running is a good and quick way to diagnose a “dead” cylinder. You can tell if the plug is slightly fouled if it begins to fire when you have the wire partially off (increasing the air gap as others said)

The funny part is the two cylinder engines I play with…they actually idle more evenly when you pull one wire. They quit if you pull two though :wink: