2003 Ford Focus DTC 2274

Don't replace any O2 sensors. They're just reporting the condition. That means they're working. The Engine Computer constantly adjusts the fuel / air mixture from too rich to too lean, with the average being perfect. Something has caused the exhaust gas to be constantly too lean in spite of the computer's attempts to adjust it back to rich a couple of times per second. It sets that code when it has lost control and can't add enough fuel.

The most common cause of a lean code is a vacuum leak. One way to find that since you have a scanner is to watch the O2 sensors switch from rich to lean or watch the voltages under "live data" while the engine is running. Pinch off various vacuum hoses to see if that makes a sensor start switching properly. You can also introduce extra fuel with a propane torch or a can of carburetor cleaner. Spray that around intake gaskets and injector o-rings.

Low fuel pressure will also cause a lean condition. The Engine Computer can compensate a little by increasing the injector on-time, but there is a limit to how much fuel it can add. You could have varnish buildup in an injector that is limiting fuel flow. That can cause a misfire too. You can find that by switching two injectors like you did with the ignition coils when you're still getting the misfire codes.

A spark problem will often set a lean code too due to the unburned oxygen going into the exhaust. No matter how much fuel the computer adds to all the cylinders there will still be that unburned oxygen, (and unburned fuel), from the misfiring cylinder. You will typically smell the piles of extra fuel at the tail pipe but you'll be getting codes for running lean. Oxygen sensors only measure oxygen, not fuel.

A less common but very elusive cause of lean codes is an exhaust leak ahead of the front oxygen sensor. Between the pulses of exhaust gas flow, the momentum creates little puffs of vacuum that can draw in outside air. The oxygen in that air gets detected as a lean condition. No matter how much fuel the computer adds, it won't change how much outside air continues to sneak in.

All cars other than Chryslers need a mass air flow sensor to monitor incoming air. Check for a loose hose between the sensor and throttle body. Just as with a vacuum leak, if any air sneaks in that doesn't go through the sensor, the computer won't know about it and won't command the correct amount of fuel to go with it.

The mass air flow sensor would not be my first suspect because that will cause a change in the amount of fuel to all cylinders equally. You'd have low power but not a misfire code to only one cylinder. You might get a "random cylinder misfire" code.

At the mileage you listed I would start with the inexpensive basics meaning spark plugs in this case, then move on to diagnose it further if necessary. There is no common failures of ignition coils on most Ford engines so if further diagnosis becomes necessary I'd look at the injectors and fuel supply system next.

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.

When you pulled spark plug wires you watched how much the engine speed dropped for each cylinder. You're doing basically the same thing by unplugging one injector at a time except now you have a computer that is going to adjust idle speed back to specs. That means you can't use the rpm drop as a comparison but you can use the amount of correction the computer inserted as a comparison. That's where the scanner comes in.

What you can try is disconnecting the idle air valve so the computer can't adjust engine speed. You will likely need to prop the throttle blade open a little, but then you can unplug the injectors and watch idle speed changes. GM and Chrysler use a "stepper" motor to set the position of the idle air valve, and if you unplug it, it won't move. The idle speed will stay right where it is. Fords, as usual, do things differently. They have a spring-loaded valve and they vary the voltage to it to adjust it. When you unplug it the spring will pull it to a real low idle speed. That's why you'll most likely have to prop the throttle open a little.

Disconnecting the idle air valve will set a diagnostic fault code and may turn on the Check Engine light. That code should self-erase later or if you disconnect the battery for a minute. I'm always leery of telling people to disconnect the battery on newer cars because there are so many tricks being designed in by the manufacturers. I don't think you'll have a problem on your car, but regardless, only disconnect the battery if the Check Engine light will not go off by itself after you reconnect the idle valve and restart the engine.

The valve is controlled by voltage in this case but I never experimented with one to see how they vary that voltage.

The Chrysler / GM system uses a stepper motor which is not a motor with brushes like we normally think of one that spins. The coils of wire are pulsed with varying voltages and polarities to bump the armature to various positions. There are no voltage pulses when the computer wants the motor to stay right where it is. That's why it will stay where it is when you unplug it.

On the Ford system it is a solenoid that has a spring-loaded plunger. Higher voltage makes a stronger magnetic field that pulls the plunger more to expose more of the air passage around the throttle blade. There's two ways to vary that voltage, and that's what I never looked at. The easiest way to understand is simply applying more and more voltage to open the valve more and more. That will work fine but if you understand electrical theory, that means that at any point other than fully-open and fully-closed, there is going to be some voltage across and some current flow through the controlling transistor. Voltage times current equals power, (watts), and power results in heat. (We have to do it that way in home audio amplifiers to prevent distortion, and that's why they have the huge finned aluminum heat sinks on the back). Heat is the deadly enemy of transistors. If we switched everything on cars that way, every computer would have piles and piles of heat sinks on them and each one would be many times larger than they are now.

Instead, most switching circuits on cars use "pulse-width modulation". An injector and an ignition coil are perfect examples of "PWM". The circuit is switch full-on for a period of time and full-off the rest of the time. When the circuit is switched full-on, all the system voltage is placed across the device leaving 0 volts across the switching transistor. 0 volts times any current equals 0 watts and no heat buildup. When the device is switched off there's 0 current flow. 0 amps times any voltage equals 0 watts, so again, no power.

I suspect your idle air valve uses PWM to control its position. The ratio of on-time to off-time is varied to adjust the average amount of voltage thereby adjusting its position. Either way, you need that varying voltage to hold the valve in a specific position. When you unplug it, the return spring is going to retract it to a setting much lower than idle speed. At that point you would need to prop the throttle blade open a little if you want to do the injector test.

As an example, prop the throttle open and lets say you find the engine running at 1200 rpm. Unplug one injector and the engine speeds drops to 1000 rpm. Same with the next three injectors when they're unplugged one at a time, but it doesn't drop at all when you unplug the fifth one. That's the one that isn't contributing any power. At that point you still don't know if you have an injector problem or a spark problem. All you know is that cylinder isn't producing any power. The easiest thing to do is switch the spark plug, and the coil when there's individual coils, with those from a different cylinder, then do the power test again. If you switch the coils and spark plugs from cylinder two, lets say, with cylinder four, and now cylinder four is dead when it was cylinder two before, you know it's the spark plug or coil. If cylinder two is still dead, suspect the injector. You can swap two injectors the same way but that takes a little longer to do.

There is no practical way for you to test an injector. You have no way of learning any more about it than it dribbles a little fluid. You also need to be concerned with volume at a given pressure, and spray pattern. Volume is critical for equal fuel to every cylinder to prevent one from running too rich or too lean, (a huge problem that GM hasn't learned yet). Spray pattern is critical for proper atomization of the fuel. Liquid fuel doesn't burn or contribute to power generation. It causes higher emissions and lower fuel mileage. We used to have intake manifold runners over a foot long that gave the fuel more time to vaporize. Now they get an inch or two from the injector tip to the intake valve to do that. The higher pressures help but if the spray pattern causes the fuel to hit the intake valve, when it is cold yet it can cause "puddling" where the fuel condenses again back to a liquid. That results in cold stumbling and hesitation problems. If you want to learn more about injector testing and the common problems with them, do a Google search for Jim Linder. He has a shop in Indianapolis that specializes in rebuilding injectors, and flow-matching them to solve a lot of GM's problems.