You can feel a misfire because the crankshaft's rotational speed slows down slightly when that misfire occurs. That's also how the Engine Computer detects it and that's how it can determine which cylinder has the misfire. There's two things that can prevent the code from coming back. One is if there aren't enough misfires in a row. When they occur randomly the computer will count the number that occur within a specific amount of time. When the number reaches the predetermined threshold, a code will be set.
The second thing that prevents any code from setting is that a long list of conditions must be met for that to happen. One of those conditions always is that certain other codes can't already be set. For example, the computer knows that after the engine has been off for more than six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature when you start the engine. No two sensors are ever exactly the same, and that is one strategy the computer uses to learn the characteristics of a new one. If something were to happen to the coolant temperature sensor, ... Typically someone unplugged it while the ignition switch was on, ... And it set a code, the computer would know it can't rely on it to compare to the intake air temperature sensor. Now if you change the intake air temperature sensor and the new one provides slightly different voltage signals, the computer won't know it. It will think the values it's reading are from the old one. The only way a code for the intake air temperature sensor would be set is if it was unplugged.
The same is true with misfire codes. Those normally set when there are no other problems detected. In this case when you have multiple ignition coils, if you unplug one the computer will see that and set a code related to a break in the electrical circuit. On some cars a misfire code will not be set because the computer already has a code in memory pointing to the problem. You would expect there to be a misfire when there's a break in that circuit, so the computer doesn't have to tell you twice that cylinder has a problem.
One misconception that a lot of people have is there is only a code stored in the computer when the Check Engine light is on. In fact, there are well over 1000 possible fault codes and only those that could adversely affect emissions must turn the light on. About half of the codes will not turn the light on. That's why we have to check for codes as the first step when diagnosing a running problem.
The most common way to test injectors involves disabling one cylinder at a time and watching the drop in engine speed on a tachometer. That worked fine on engines with carburetors but with fuel injection the computer will compensate for that rpm drop by adjusting the idle speed motor on GMs and Chryslers, or the idle air valve on Fords. You would use your scanner to view live data and watch what the computer is doing to control idle speed. This is real easy on Chryslers. The computer can set the idle speed motor to any of 256 settings called "steps". Step 32 is about average for a properly running engine. When you disable one injector the engine will slow down and the computer will react by adjusting the idle speed motor to lets say step 50. That lets more air in through the passage around the throttle blade. At the same time it increases the amount of time each injector is held open.
In this case you would expect to see step 50 every time you unplug one injector. If an injector is not spraying enough fuel, (or there's any other cause for a misfire), there won't be that expected rpm drop when you unplug the injector, and there won't be any increase in the idle steps. Instead of steps, a lot of manufactures list their idle speed in percent. That valve is open a certain percent to achieve the desired idle speed. You would expect to see it increase to a higher percent when a cylinder is disabled, and it should be nearly the same amount of increase for every cylinder.
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Wednesday, August 21st, 2013 AT 1:00 AM