[BigV]

Ok, well then I guess the O2 sensor is not indicating an error.

NO readings means that the problem is coming from some source(s) in the system that don't report back to the computer. Things like ZenGum's "loose wire".

glatt, how much do you know about a 4 cylinder internal combustion engine like the one you've got? I don't want to talk down to you, I'll just think out loud.

When it's running right, two cylinders fire at the same time, like 1 and 4, then two strokes later cylinders 2 and 3 fire (a common firing sequence). Since everything is balanced mass wise and thrust wise, it feels smooth. As soon as something in that little set of sequences doesn't happen (and there's a missing piece, not an extra piece. No phantom fifth cylinder appears to unbalance the weight or the firing. No. Something's NOT happening, like one cylinder not firing unbalancing the engine, manifesting as rough idle). So, what could cause a cylinder to periodically not fire/misfire?

The holy trinity of internal combustion is fuel + air + fire. If you're missing one or more of those you're screwed. And they have to be delivered in precise amounts and precise proportions and at precise times. What could cause an interruption to any of these?

Let's subtract unlikely candidates and see what's left. It's unlikely that you're not getting air to a cylinder. A dirty air filter would restrict airflow to all the cylinders, not just to some, so... I can't think of another likely cause for "missing air" that wouldn't also have some other much more dramatic symptoms. I think your engine's getting the right amount of air.

And, that air is being delivered continuously and at pretty constant pressure and temperature, so it can be treated as a constant, putting the responsibility for fuel/air mixture proportions on the fuel side. You said the engine is fuel injected. Ok, what controls the injection? If it is mechanical injection, it's a masterful clockwork designed to figure out how much fuel to squirt, but your car almost certainly has electronic fuel injection. What electronic fuel injection is about is a system of sensors like throttle position, speed, gear, emissions, etc etc that help the computer decide how much fuel is needed to meet these conditions.

I had an old Buick LeSabre that would sometimes give me grief. It turned out there was a (gah, I hesitate to tell) design weakness that involved the air mass sensor (the MAP sensor in your car). Sometimes the car ran like shit, or wouldn't start at all. I learned to open the hood, and give the plastic intake snorkel a sharp whack, close the hood and carry on, problem solved. It turns out that the sensor was used to determine how much actual air was moving through the engine by putting two different magical detectors very close together, kind of like the rubber hoses you saw in gas stations years ago that rang the bell ding ding when you drove over them? Well the modern version in traffic surveys has a pair of these hoses across the roadway and it can calculate the speed of the cars by measuring the length of time between the tire touches on the two lines. Well this sensor worked like that but the wires were so close together that when a tiny piece of crud was sucked into the airstream, if it caught on the sensor part of the wires (strung directly through the airstream, just like the traffic sensor) it could cause erroneous readings. A smack on the housing usually dislodged the minute particle of junk, the sensor was then able to read the amount of air, and reported back to the main computer and the rest of the calculations would result in the right amount of fuel to be injected; life was good.

Back to your car, something could be interfering with a perfectly good sensor, like some crud on the sensor, and causing it to report bad values, with the downstream effect of a roughness because the cylinder's not getting the right amount of fuel. But which sensor when the computer check says none is ... really tough and expensive.

Maybe you have a tiny piece of dirt in one of the injectors. I have an old house and a very low flow showerhead (2.5 gal / min). Sometimes some rust/crap from the inside of the pipes flakes off and makes it's way to the showerhead. This clogs it and the flow is reduced even further. Not good. I take the showerhead off, clean out the crud, reassemble and carry on. Now, the tub faucet is on the same damn pipes, but I never have this trouble there. The flow/current/pressure (whatever) is just so high that if it even happens, I never notice it. Back to your car. Maybe you have a little piece of dirt in one of the injectors and at idle, low speed, low pressure the dirt can interfere with the squirt of gas, causing a misfire. At road speed, you don't notice because you're pushing a lot more fuel through (like my tub faucet), so problem isn't noticed. To me, this is plausible. Fixing something like this can be pretty hard. The injectors aren't really.... repairable. They're replaceable, but could be bux. The orifice is super tiny, imagine the size of hole that produces a vapor or fine mist, like a perfume mister or spray can. Psssshhhhht. or more like psht psht psht psht psht 800/2 times per minute. You could try commercial injector cleaners that you pour in the gas tank.

What else could cause irregular fuel delivery. It is also plausible that the whole engine is being a little starved for fuel under low pressure situations like this, as might happen if the main fuel filter was moderately dirty. Have you checked your fuel filter? This can be an easy and cheap fix with little downside. Once more, at road speed, this would be less obvious at the higher pressures.

Fuel...fuel... a physical restriction, or bad instructions to cause it to erroneously deliver the wrong amount of fuel... those are all the situations I can think of at this time for fuel. Air, check. Fuel, check. Now fire.

--to be continued--