Engine Bucking Under Load, SOLVED

1999 Ranger 4.0 OHV. 286,000 miles. Original engine, only did lower intake gaskets, and replaced every sensor once or twice throughout its life.

This one drove me nuts. I knew I had failing O2 sensors, which I originally did at about 140,000 miles. About 5 months ago the engine was bucking under load and running however it felt like on a given day. It drove me nuts, so I bought an OBDlink to watch what the various components were doing. I had a code for the O2 heater circuit, and also a bad sensor at the cat.

I had to take the Y pipe out (in garage, on the ground on my back, yuck), and the o2 sensors were seized in there solid. I put the Y pipe on the bench vise, and heated the sensor up cherry red. With a long breaker bar, the sensor moved initially but then it just snapped off.

I had to drill it out and re tap it. Same with the cat, I had to take that out also and retap the O2 sensor there. What a pain, but worth it. All 3 sensors replaced.

I didnt check the obd readings with the new sensors because it ran great. But during this last month, the problem came back! Bucking under load, running like shit. Fired but OBDlink again, and it said one of the o2 sensors wasnt working! Stuck at 1.28v, WTF?

At least it came out no problem this time. Glad i took the time to re tap it properly. And guess what? Thats when I noticed the problem. I originally had to cut the connector off the new O2 sensor, and crimp on my old one. I mixed the wires up! b I remember that night being a late one, and I was rushing to get it done. A black wire was right on a white wire. Idiot.

Fixed that, put the o2 sensor back in and bam, she fired right up, sensor working!

I just drove for a half hour, zero codes! Motor runs like a sewing machine. Its really weird that this sensor wasnt working this whole time, because for the first few months the motor ran fine... But I did wonder why my mpgs were low. I was barely getting 16mpg... I just figured the engine was tired... I never done the heads on this thing, the valves cant be perfect anymore.

Anyway, cant wait to burn a tank of gas and see if I get my 20mpg like I used to! God I love my rusty old ranger. It just wont die.

Glad you found the problem and thanks for the update. I can relate to the "late night syndrome". Once I chased a no-start problem on my race truck [in the off season]. I had pulled and re-installed the motor and taped all the connectors. Because of the tape, I never checked if the connectors were making contact. After 6 months I finally found the problem. I sure had a sore ass and a kinked foot.....