2002 Ford Ranger runs rough at idle but no OBS code?

Try replacing your fuel filter. An old fuel filter can get clogged with particles and sludge after use, so a new filter will allow more fuel to pass. At idle the fuel pump isn't running at full pressure, so this is when the engine can be noticeably bogged down by a bad filter. It sounds like that's your problem, but if it's something else then you've only spent ~$15 replacing something that should be replaced anyway. You might even unleash a couple of lost horsepowers and MPGs.

If it's the IAC, you should be able to disconnect it and unbolt it from the TB. Get some carb or TB cleaner and spray the spring in the IAC to remove the carbon build up. It's a good idea to clean out the throttle body too, but just spray a rag with the carb or TB cleaner and wipe it clean. Do NOT spray it directly into the TB. Although it is "ok" to do so, you have to do that with the engine running to prevent the liquid from settling and entering the combustion chamber in full liquid (not atomized/gaseous form). You run the risk of stalling or, at worst, hydrolocking the engine.

# Clean mass airflow sensor.

# Fill up with premium and add a can of SeaFoam to fuel.

# Make sure you have a good deal at the gas cap and the oil fill cap.

# Make sure your air filter cover and entire assembly is fitted properly.

# Check all vaccum lines for leaks and repair any questionable areas. You can find parts to cut and splice these hard lines.

# If you have access to an OBD2 check the Data Stream screen for the TP (absolute throttle position) make sure reading is in spec.

1. Good luck