Got a few minutes? Part 2.

• OK. So you want to know about the laws in your state (country), well then either go to law school and study until the end of time, or consult a lawyer and hope that he/she is truly up on the subject at hand. Now if you want to know about auto repair and how cars really work I guess you should go to tech school and study until the end of time or else consult a lawyer!
  
  OK, maybe that's not fair but then again that's the point, being fair. In the linked podcast Mr. Lehto describes buying a used car and even with a blank check the shop only replaced the spark plugs. It's notable right here that we don't know anything about this car, make, model, year, mileage, previous repair history etc.
  
  Over the last few years if you have paid attention to the information that has been shared the first thing that you should realize is that you can't take any given symptom on any given car and automatically know what is wrong, and especially what part to replace. People often guess with respect to a given symptom what part(s) may be causing an issue and like any guess they just might be right. However when it comes to really being proficient one has to test, not guess because when it comes to service and repair the guesses will quickly lead to one of Mr. Lehto's top five rip-offs where part after part get's replaced. Experience has taught top techs that nobody can just tell you what is wrong based on a symptom, but we can tell you how to test and prove what is wrong.
  
  In some of the threads here in the forums we have discussed the issues about only replacing the spark plugs on today's cars and this actually dates back into the mid 80's once computer controls hit the scene and the engines were designed to run a leaner air fuel ratio. (I promise that I'll try and do this without writing an entire book on engine performance so there may be some portions of this that deserve to be explained in greater detail). In the podcast Mr. Lehto praises the shop for only replacing the plugs and not the plug wires. That is actually a mistake on the shop's part and anyone who listens to the podcast who doesn't know better could listen to Mr. Lehto and then stop trusting a shop or the technician who correctly advises to replace both as a set.
  
  There are several things that happen when someone tries to do only the plugs, or only the wires. As the boots deteriorate from the engine heat and simple aging that not only weakens their insulating capability but it also causes them to harden. In that condition when they are removed and re-installed on the plugs they cause microscopic scratches on the insulating porcelain of spark plug. Any damage to that surface can allow spark to start to jump outside of the cylinder instead of across the gap of the plug. When that happens it results in carbon tracking of the plug and the plug-boot. We learned this the hard way back in the 80's and 90's when we used to still pull plugs to inspect them. Back then it was quite common to pull and inspect the plugs and quite often we found nothing wrong with them, so we would re-gap them and put them back in. Then, and it could be a few days to a few months later the car would come back and now it had a misfire that it didn't have only a few days before and we would then find an external carbon track on the plugs. The real fun then was that if you didn't replace both the plug and the wire at that point was you ended up re-transferring that carbon track from which ever component you didn't replace to the one that you did and the car would keep developing repeated misfire conditions until someone finally came along and replaced all of the plugs and wires at the same time.
  
  This didn't happen in the 70's and earlier cars but it happened frequently when leaner engine designs became the norm. Today when you are driving your car down the highway it takes anywhere from around 9000v to some 20,000v to fire the spark plugs depending on the operating conditions.(avg. 12000v-15000v) The richer air fuel ratio's of the older cars only required 5000v to 12,000v so even though the same scratches could occur to the older model plugs the spark demand inside the cylinder simply wasn't usually high enough to force the spark to get to jump outside of the spark plug like newer cars can.
  
  The things that contribute to the spark demand voltage that is required to ionize the gap of a plug are primarily the air fuel ratio in the cylinder, the compression of the cylinder, the plug gap, and the timing of when the plug is fired. It gets pretty complicated but essentially the leaner the air/fuel ratio the higher the voltage demand will be. The same goes for compression, the higher the compression the greater the demand voltage as well as the closer the ignition timing is to TDC and the wider the plug gap. As a technician road testing a car many misfires can be proven to be ignition spark leakage related by simply understanding this and operating the vehicle during a road test in ways that cause the spark demand voltage to be raised and lowered to see if the tendency for the cylinder to misfire changes. When you take all of this into perspective and especially since the number one reason to try and prevent misfires is to protect the catalytic convertor one should avoid inspecting and re-installing spark plugs or replace only the plugs or the wires (plug boots coil on plug) individually.