2001 Cavalier 2.2 5-speed manual. 210k miles.

So had the fuel pump and filter replaced and from just before this was done to right now - 2 months later - the engine has been running REALLY badly, So far, it hasn't stalled, but it is getting close. Never had anything like this before, so don't know where to start. I am hoping that before I take it in and have someone hook up a diagnostic computer to point me in the right direction, I might get some help from people on here with the same or similar model cars who have experienced the same thing because with my luck - if I just start replacing the 20 or so things that it might be one at a time, it will be the very last thing that was the problem.

Symptoms: from the moment it starts, runs REALLY badly with extreme rpm fluctuations and just sounds like it is going to die. Happens in neutral, idle, all gears, all speeds, all rpm ranges but is most noticeable in the morning after it has sat overnight, and then when driving in 2nd and 3rd gears while accelerating. For a couple of days, it threw a p0300 (general misfire) code but that is currently gone - or at least the CEL has gone out.
I have gone through the gas that was in there before and when the pump and filter were replaced, and the new full tank of gas has not made any difference. I replaced the air filter and cleaned the throttle body - again no difference.

So my question is - what is the list of things that could cause my problem - from most likely to least likely? I know that in addition to the injectors that there are a bunch of sensors and regulators that are on that list but without a diagnostic - that would be about $100 - I just don't know what to do.

Thanks for any suggestions.

One other thing I forgot to mention which is very unusual and has only happened in connection with this problem over the last few weeks......is that the engine cooling fan has come on at times when it normally hasn't, and shouldn't. By that, I mean not in connection with the engine temperature or the AC compressor being turned on (which are the only times it is supposed to turn on). This morning, for example, it came on just a minute or so after starting while I was horrified at how badly the engine was running and it was not even warmed up.

Does this provide any clues?

I have the same cavalier but I got an auto out of a 98'... I have the same issues. I did a full tuneup that includes air filter, fuel filter, spark plugs, spark plug wires, oil change almost every 3k. I do not see any cracked hoses or leaks. However it improved when I replaced my Positive crankcase ventilation hose that was cracked... That is the hose that goes from the valve cover to the air box above the valve cover... However nobody makes a replacement hose so I had some awesome guys at Auto Zone help me fab up a hose with some hose clamps. I cant figure out what it is. I idle at a low 600rpm. It barely produces enough to keep my headlights running. The radiator fans are supposed to come on a bit after 195.

I have the 2.4L, not the 2.2L, so this advice may be useless, but based on previous problems I've solved on my 2.4L...

It's possible that the engine coolant temperature-sensor is out-of-calibration and therefore is feeding bad temperature-data to the computer, thereby causing the computer to provide an improper richness of fuel. (Based on that sensor's data, the computer feeds richer fuel to a cold engine, like an old-style "choke".)

You also might want to replace the MAP sensor's vacuum tube, if it has one. That's cheap and easy.

Misfires on the 2.4L often mean that the ignition coil housing (which essentially has the spark plug wires) has developed invisible cracks which cause misfires. It could also be failing ignition coils.

Due to you saying its running bad & the fan is being goofy I would say RazzMaTazz is on the right track. But you did not mention any coolant temp codes which I would think you would have one if the sensor is out. Unplug it & see how it runs. You should be able to find to resistance on the Internet or go buy a coolant temp sensor from the auto parts store, let your engine cool down, set the new sensor near the one in the engine & leave it for 10-15 minutes so the temperatures are the same & measure the resistance of both. The sensor is on the passenger side just in front of the iron motor mount. Since its up high you can replace it quickly without losing much coolant.

Since you mention this started happening after a fuel pump replacement I would check the fuel pressure. Unfortunately these engines dont have a test port for the fuel system that I see.My thinking is there might be a line loose on the new pump or cracked line. If that were the case I would think at some point you would also get a P0171 lean code.

Does it smoke? Are you getting really bad gas mileage? Do you smell raw fuel from either under the gas tank or the engine compartment?

I had the same problem last year. I had a misfire code and I changed one of the fuel injectors and it didn't solve my misfire. After changing the coil pack that was on the misfiring cylinder the problem went away. Question does your car act like it is going to stall out after it rains? Mine did. Check out YouTube there is a video on there somewhere on how to change the coil pack. It's not hard at all, it will just take you a few good minutes to do it.