I have a 98 F-150 w/a 4.2 V-6 and 5 speed trans. Truck runs fine and drives good w 211,000 miles on it , however for the last 2-3 months the idle is very high when you stop for traffice lights or stop signs, usully runs up to 2k rpms. What's the problem? Also CEL is on and will not clear. Codes are PO171 and P0174. About a tear ago I had evap leak problems w/ a cracked fuel filler neck I replaced that and it cleared for a while then I get the same codes as above. I replaced all 4 O2 sensors and it ran about 3 days and showed those same codes again. Now it's idling high.

 

Look for a vacuum leak, maybe a hose off which would cause the high idle. If the vacuum leak is bad enough it will set those codes. Let me guess, a parts store got the codes and sold you sensors? Those codes do not mean the sensors are bad!

 

Look right under the throttle body where the pcv hose goes into it. The elbow probably has a hole in it. That causes unmetered air to enter the intake and the o2s see a lean condition. It can also cause a high idle. You can replace just the elbow or the complete pcv hose assembly.