I am not exactly the best driveability expert, although I'd never admit it, but I can offer some guidance that might affect your train of thought. First of all, I am not clear on how "running too rich" fault codes are set, because that is a result of the front oxygen sensors' readings, and they measure oxygen, not fuel. My understanding is they measure lack of oxygen, but I don't know all the conditions necessary to set those codes. Under normal conditions the exhaust gas mixture switches between too rich and too lean, by design, about two times per second. My guess is the computer has to fail to see the too-lean condition for a specific period of time.
My reason for bringing this up is when you have a spark-related misfire, you get excessive unburned fuel and air in the exhaust. It's the oxygen that gets detected as a lean condition, but it's the gas you smell as a rich condition.
I have read a lot of posts where the ignition coils were the cause of the problem. That seems to affect Ford trucks more than any other vehicle. What I would do when there are individual coils, (or with injectors), is to switch the suspect coil with one from a different cylinder. Erase the fault codes, then drive it until the problem occurs again, then see if the fault code set for the cylinder you moved the suspect part to. Unfortunately you have a single coil pack that makes swapping coils impossible. Given the history of the coils, I wouldn't mind buying a new coil pack to try, and if it doesn't solve the problem, hold onto it for the future.
Be aware that each ignition coil fires a pair of spark plugs at the same time. One fires on the end of the compression stroke and the other gets the "waste spark" at the end of its exhaust stroke. If you short one spark plug wire to ground, that spark plug will be bypassed but its mate will fire very nicely. If you open the circuit anywhere, as in pulling a spark plug wire away from its spark plug, both spark plugs will not fire. If one coil arcs near one spark plug wire's tower, it will bypass just one spark plug. If the coil's wire is breaking apart inside the assembly, it will kill the spark to both spark plugs. For that reason, you can't make a diagnosis based solely on whether just one or both spark plugs stop firing. The arcing at a coil tower always involves carbon-tracking, just like we used to see inside distributor caps that got moisture inside. Once that starts, carbon is a conductor of electrical current, and the carbon tracking will get progressively worse very quickly. It will eventually lead to the tower burning away, if you can stand to drive the truck that way. Ignition coil failures are rarely intermittent for very long. They usually fail completely rather quickly.
Since you brought up the mass air flow sensor, its job is to measure the weight of the incoming air. That is the primary factor in fuel metering calculations on all brands of vehicles except for Chrysler products. No air must be allowed to sneak in that doesn't go through the mass air flow sensor. That includes loose hose clamps or cracks in fresh air tubes between the sensor and the throttle body, as well as other vacuum leaks. When air does sneak in the computer doesn't know about, it won't command the correct amount of fuel to go with it. Leaks will cause running lean fault codes, but they have to be pretty bad before it results in a misfire.
There is always a long list of conditions that must be met for a fault code to set, and one of those conditions is that certain other codes can't already be set. For example, with your original problem, if a code is set for an electrical problem related to the Engine Computer's coolant temperature sensor, the computer will never set the code for "running cold too long". In this case it's because the sensor circuit isn't working, but in general terms it is because the computer knows it can't rely on that sensor's readings, so it aborts any self-tests that use those readings. Knowing how those fault codes relate to each other in this way can provide clues when you have two and you aren't sure if they're caused by the same thing. Let me clarify that before my mind gets more 'wrapped around the axle". You have a code for running too rich on bank 2, that's the passenger side. You also have a code for a specific cylinder misfire. Each of those is not prevented from setting when the other is there. To a trained specialist, that is an important observation. Next, the side with the rich fault code is found by which O2 sensor is taking the incorrect reading, but the misfire is determined quite differently. When a misfire occurs, you can feel it, and you can feel it because there is a momentary lapse between cylinder power pulses. That lapse causes the rotational speed of the crankshaft to slow very slightly. That slowing is what is detected by the Engine Computer. Since it knows which cylinder should have fired during that lapse, it knows which one is misfiring.
Now that you know that wondrous piece of trivia, that can be related back to when you have two misfiring cylinders, and both are mates to the same ignition coil. If both misfire, you should see excessive unburned oxygen coming from each cylinder, and here's the secret. Each cylinder is on a different side of the engine. You should get lean codes for each bank. If you get a lean code on just one side, expect to find just one cylinder misfiring. That is more often caused by a spark plug or wire when you have a coil pack. It can also be caused by the ignition coil when you have six or eight individual coils.
Spark-related misfires cause the cylinder to totally miss a power pulse, and we can easily feel that, especially on six-cylinder engines, and even more so on four-cylinder engines. Fuel-related misfires often cause only reduced power. Spark initiates the burn or it doesn't. Fuel has a whole range of incorrect amounts it could have. The neat thing is a reduced-power pulse is still detected by the computer because it still sees reduced crankshaft rotational speed, but we often don't feel those misfires. This is when we are surprised to see those fault codes when we thought the engine was running fine. Low fuel volume to a cylinder can be caused by low fuel pressure or a partially-clogged injector. These are bigger problems with GM engines. GM is one of the few manufacturers that just grabs a handful of injectors from a big bin, and stuffs them into the engine as it comes down the assembly line. Most other manufacturers "flow-match" their injectors so they install a matched set into each engine. With age and mileage, fuel-related misfires become real common with GM engines. Fuel pressure that is just five pounds below specs can start to reduce flow through an injector enough to set lean codes or cause fuel-related misfires.
We can also get a misfire from an air problem, more specifically, lack of sufficient oxygen. We think of obstructed air flow more so with carburetors because plugged air filters were a common cause, but when we have a mass air flow sensor, we will still get the right amount of fuel to go with whatever air makes it into the engine. We might have reduced power, but usually not a misfire problem. The lack of oxygen comes from a malfunctioning EGR system that lets in too much inert air, (exhaust gas) that displaces fresh air. This has been a big problem for Ford, but to my knowledge, it only affects V-8 engines, not the 4.0L. To identify this, there are eight tubes for the EGR system, one running to each cylinder, and those tubes plug up with carbon. Normally a plugged tube would block EGR flow to that one cylinder, and therefore reduce the total flow by one eighth. With the Ford system, total flow is the only thing that is measured and controlled, and that is expected to divide up evenly among all eight cylinders. When a tube is blocked, that cylinder gets only fresh air, . . . and it runs just fine. It's the other seven cylinders that get too much EGR flow. Over time, more tubes become plugged with carbon, but the total EGR flow stays the same. The typical complaint shows up when seven tubes are plugged. The clinker is the only clear tube is feeding the one cylinder that is misfiring. That is the cylinder that is getting eight times the EGR flow that it should be getting. The seven cylinders with the plugged tubes are running fine.
The EGR system can still cause misfires on other engines if a piece of carbon gets stuck in the valve, preventing it from closing fully. EGR gas must never enter the cylinders at idle or low engine speeds precisely because it will cause a misfire.
These are some of the clues and observations engine performance specialists use to cut their diagnostic time. The rest of us have to fall back on what we learned years ago, and that often no longer applies to fuel injected engines.
Aug 11, 2020 at 4:48 PM
