Hello, new member and 1st post...here goes, my 2000 ranger 4x4 supercab with 4.0 motor was running great, than the check engine light would come on and off and it started running rough at idle, the local autoparts store put the computer on it and said its the ox. senser before the cat. so I replaced both of them and that didn't help. I noticed the water pump was leaking a little so I replaced it, also new plugs and wires, fuel filter, air filter and ran sea form through the tank to clean the injectors. It started overheating so I pulled the radiator and had it redone at the local shop, new fail-safe thermostat, 50/50 mixture and it still blows the lid off my resavoir and temp gauge pegged to the right and check engine light comes on, I also noticed white smoke coming out the exhaust but it clears up when you get around 30 mph than starts again when you come to a stop. I checked the oil for water and its good and don't see no oil in the anti-freez. I'm thinking a head gaskit is leaking but want your opinions in case I'm overlooking something. Sorry for the long post but wanted to give all the information I could cause I work 2 jobs and won't get on the forum till tomorrow, thanks in advance.

 

The 4.0L has been known to get cracks in the heads between the valves that allow coolant to enter the cylinders.

 

When it rains it pours, huh. Sounds like you have head gasket problems. Steam out the tailpipe is a definite indicator. If you stand behind it, the exhaust should smell sweet, like antifreeze does. You will need to remove the plugs and look for evidence of coolant on them.