1990 Volvo 740 previously ran REALLY rich, fouling the gas. Replaced Throttle position sensor, Fuel Pressure Regulator, Coolant Temp Sensor, Spark Plugs, Oil, hoses. Didn't change.
Took it to an import guy in town, he found that some wires had been "chewed" through, probably by a rat. It was friggin the ECU all up. Anyway, he replaced wires, did soldering, and it runs pretty good now, though will run somewhat rich after it warms up, and HE can't figure out why.
How do you check the O2 sensor? Can it be cleaned?

 

Some people clean the O2 sensors with a torch but I don't reccomend it.
You can check the O2 sensor by using a multi meter.Put the positive lead on the single green wire on the O2 sensor.The the negative lead on ground.Start the car and watch the meter once the car warms up.It should swep between 0-1 volt.It should sweep pretty fast.

You could also have a bad coolant temp sensor.

 

I'll try that. thanks. Already replaced coolant temp sensor before. no change really, or at least, I didn't notice a change. By the way, it's got 178,000 miles on it. Are there any specific oils you'd recommend?

 

Just use the normal 10/30 or 10/40 unless you want to use synthetic.