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Basic Rough Idle Diagnostic

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One of the most common issues in a more modern car is noticeably rough idle or lack of power as the car ages. Two of the most common maintenances which get neglected are the fuel cleaning service and a common tune up. A simple reason is two fold. First and foremost most cars on the road are designed with 60k to 100k which means people are less likely to be concerned with performing this service. The other service, a fuel service, is also one that is supposed to be performed every 15k per most manufacturers. It usually consists of either 3 or 4 stages. One stage is an additive that removes deposits from the crank cakes (where your oil goes), a second stage into the gas where it cleans the injectors, gas tank and other fuel delivery systems. The third stage is delivered into the intake through a vacuum line/system to clean deposits that accumulate on the valves, pistons and other components. That being said, we can now get into the nitty-gritty.

One thing most people assume with a spark system on a modern car is that when the plugs wear out the car will run rough, a code will pop and they can take it to the service station and voilà it will be fixed. What actually is occurring and why it takes so long past the MSI (maintenance service interval) for the car to act up is simple. The ECU is designed to read in real time a myriad of systems from fuel, air flow and even air temperature to ensure all the fuel is burned. This means the system can increase the voltage output of the Coil Pack, which we will discuss later, until it runs as ideal as possible. This becomes a problem because no system is truly designed to operate for prolonged time periods at or near 100% duty cycle. Over time the components, the coil pack, will begin to or completely burn out resulting in a more expensive fix later on down the road.

Step 1Edit

Tools needed:

  • Spark plug wire pullers with rubber handles and tips (The rubber parts are EXTREMLEY important for this test)
  • Rubber gloves and Rubber soled shoes
  • Spark plug socket and wrench

If your car is running rough and you either have no idea if a tune has been performed (I.E. you purchased a used car with no repair history) or you have neglected the tune up past the MSI you need to examine the spark delivery system in a little better detail. The first step and is/can be the most dangerous is checking the spark output. There are special tools you can purchase in order determine the exact output of the coil pack. However, most people don't have access to such expensive equipment so there is a way to figure out the output. You will need a few basic tools available at any auto parts stores which are listed just under step 1.

Now that you have the appropriate attire and tools on, a rubber mat under your feet if available will help as well, you will need to have the car running and clear access to the coil pack and its wires. First take a look at the wires and using your spark plug wire pullers see if you can remove the wire without popping the others off. If you can't carefully twist them so you can remove then reinstall each wire one a time. Now that you have clear access and can remove the wires start the car and one at a time remove the wire carefully without holding the wire itself more than about a half an inch away if possible. You are trying to get a good look at the spark coming from the pole down in the hole of the coil pack. A good coil pack will arc outward and either reach bare metal under/near the coil pack or the wire along with an audible clack or popping noise. Observe each of the wires to see if any of them are different from one another. Some systems may not produce a spark that long, but if any of the poles are not producing a spark similar to the rest, I.E. it doesn't reach the wire or is visible at all, you have a bad coil pack. The other thing to watch is to see if the engine runs rougher than normal when the wire is pulled off the pole. If you pull the wire and no change is noticeable you can bet that that is your problem.

Step 2Edit

Tools Needed:

  • Multi-meter or Ohm meter
  • Tape Measure
  • Calculator
  • something to write or take down some numbers with
<p>You will need to now inspect each of the wires. See what kind of texture the rubber has. Anyone who has seen or left something plastic or rubber in the sun can tell how dark black or grey rubber will start to turn white and become brittle. It won't have as much bounce or be easily removed without damaging the wire. I always recommend replacing the wires with the rest of the service regardless of weather they have been replaced or not recently. After completing a visual remove the wires one at a time. This is important because on V6 model engines or even some 4 cylinder or straight 6 motors the wires need to go back in the exact order or you will spend all day trying to plug and unplug until you put them back in the right order.

Now you will need your tools. There is resistance built into wires to help build the voltage to make a large spark. First measure the wire's distance in feet. Now use your multi-meter or Ohm meter to measure the resistance. Now use the calculator and divide the resistance by the length in feet. Do this for all of the wires. This will give you the resistance per foot or Ohm/Ft using the length only multiply that by the Ohm/FT you got from each of the wires. This will result in the Ohms you should have for that wire. If any of these are not lining up closely to each other then you have a bad wire.

Step 3Edit

Tools Needed:

  • 3/8 Wrench and maybe some extensions and a wobble
  • Spark Plug Socket

Now that you have verified the coil pack and the wires you'll need to inspect the plugs themselves. The wire will only need to be removed from the plug itself then remove the plug using your tools and inspect it. There are 2

--GCarlis289 (talk) 06:30, October 1, 2014 (UTC)

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