1992 3.1 Liter Z24 Cavalier

[tigfarm]

Car has over 200k miles. Symptoms began with a rough idle which developed, over a few months (approx. 3-5k miles) into intermittant major loss of power. Still starts just fine and will never completely quit, but the power loss is significant enough to limit max speed to 20 to 25 mph. Along the way I got codes 34 (MAF or MAP), 44 (lean), 45 (rich) all at once. After spending a lot of time on the car, had to let it sit for last 3 months, resumed troubleshooting and now I'm consistently getting code 45 (rich). Rough idle remains, rich smell remains, and started to notice the power loss interestingly only occurs with the engine temperature between approximately 120-135 deg F. Above that temperature the power is fine, but with the rough idle. While the ohms check on the oxygen sensor says it is working....is it possible this sensor could be causing this problem? Read mulitple threads on oxygen sensors and they indicate this sensor only has influence during idle. It is just $20 for a new one, but thats $20 on top of several $$ I've put into this project. To explain further, which may/may not aid in troubleshooting this thing....

Have performed a long list of checks/replacements, mostly with the aid of a Haynes manual:

Started with an overdue tune up - plugs, wires, filters. No change.
Suspected misfire because of rough idle and rich exhaust so I started ignition checks.
Only check that seemed off was the crank position sensor. Replaced, No Change.
Pulled plug wires one at a time and spark removed from two cylinders (1,5) seemed to have no effect. Two cylinders did not correspond to coil pair, but swapped the coils around anyway, No Change, but noticed one coil had a small crack in the outer shell. Replaced this coil but No Change.
Checked compression on all 6 cylinders and all measured between 165 to 170 psi.
Also had been noticing, up to this problem, catalytic converter was starting to rattle, especially with the rough idle. During one of the test drives, I rapped on the converter with the car running. Car instantly ran better and did so for the few blocks home. Replaced the cat converter. No Change.
Technician that replaced the converter also said car sounded like misfiring, and suggested I check the firing order. Did so and order was correct.
Next checked the emmision control sensors and fuel delivery.
MAT, MAF, Oxygen checked fine.
Don't recall checking the TPS.
Fuel pressure 44 psi.
Electrical check at each injector with noid light was good.
Pulled both rails and swapped injectors, No Change.
Pulled EGR and cleaned off carbon build up, No Change.

[manicmechanix]

How do you know the MAP sensor and the O2 sensor checked fine? That's what the OBD was pointing towards. Other than that I think you would need to check the TPS and coolant tempature sensor reads properly and smoothly with a scanner or multimeter. You might also want to check the pressure regulator vaccum hose and make sure it's not leaking fuel into the intake. Also does your fuel rails hold pressure? The injectors on this model are known to short out. It's hard to get to them individually but you can check the total injector resistance at the harness connecter.

[J-Ri]

The O2 sensors will affect the mix at all engine speeds, not only at idle. Infact many non-heated O2 sensors will stop responding during extended idling. I don't believe the O2 sensor can be confirmed good by checking the resistance. Since you have both rich and lean codes, I'd guess the O2 sensor is working. Every one I've seen fail has read one or the other. Definitely check the MAP sensor, it can cause all your problems, including the O2 codes.

I'm guessing you checked fuel pressure at idle or KOEO? If one of the sensors above isn't bad, reconnect the fuel pressure gauge and drive the car (tape the gauge to the windshield) and see if the pressure drops off. You could have a plugged fuel filter that isn't flowing enough to keep up with demand, or even a bad pump that is causing the max speed of 25 MPH and something else causing the rough idle. Usually a "top speed" issue is either a lack of fuel or a lack of air (or the PCM reading the wrong ammount of air/load)