Failed Emission Test
My Ranger failed the emission test with high hydrocarbon (284) on the low/idle RPM. The upper limit is 200. It passed on the high RPM. I had the catalitic converter tested and it is working. I tuned it up including: new cap, new rotor, new wires, new plugs, new air filter, new fuel filter, new O2 sensor, and an oil change. I checked the timing and it is off 1* from the specification sticker on the radiator support so I did not change this. I ran a tank of fuel thru it with an injector treatment additive. I don't know if this is related or not but it has been idleing intermittingly on the high but acceptable range (1100 to 1200 RPM's). Other than that, it seems to run fine with a smooth idle, even if somewhat high, and it seems to have plenty of power. Does anyone have any idea what I could do or check to correct this issue and make it pass the emission test?
1988 XLT Ext Cab 2WD 2.3L Manual