97 Ranger 4.0 auto bad misfire, O2 sensor code

[63A]

Looking at buying another Ranger. 4.0 engine just rebuilt, engine idles high (approx 1200 rpm) but engine sounds good- no strange rattles, etc. only code is for O2 sensor. When trans is shifted from park to D or R, engine misses and gets worse with throttle.

Any ideas appreciated- looking at truck again this afternoon.

 

[Andy D]

Tell him to call you when he finishes putting the engine back in. First guess is a vacuum leak. But it could be anything

 

[RonD]

Cold engine idle will be 1,100 and then drop down to about 750(automatic) as engine/coolant warms up
650 for manual

High warm idle and misfire under load could be a vacuum leak.
Could also be a bad cylinder seller is hiding by setting a high idle using the anti-diesel screw in the throttle linkage, looks like an idle screw, but fuel injected engines can't use an idle screw

Setting high idle hides the misfire until engine is under load

Could just be a bad spark plug as well, 4.0l OHV engines eat spark plugs.


Need specific O2 code, there are many and they all mean different things
And O2 code can be caused by a misfire, so important to know what code it is