I have a 99 F-150 supercab with a 4.6. Two months ago I had Auto Zone scan this truck, I had an intermittent engine light, the cause was slow responce from an O2 sensor. Shortly after the light stayed on always with no noticable effects. Before a weekend road trip I had a machanic friend of mine scan the codes and change the O2 sensor. The code was a missfire, he cleared the codes did not change the O2 sensor the light is off. On my weekend road trip on the highway cruise on and in Over Drive the truck would chugggggg, on up hill grades the engine light would flash but would go out. Some times it would just start to chug then down shift and not chug, it didn't run like this before. Around town it dosen't happen. The light is off now. I should tell you It has 106,000 mi and had plugs at 65,000. Please help!!!!

 

OK, first was the slow response from the sensor 1 (pre cat sensor used for feedback for mixture control in closed loop) or the sensor 2 (post cat sensor used to monitor catalyst efficiency)? If the 1st one, the pre cat sensor, you either have poor mixture (rich or lean) or more likely it is running in open loop mode. If it is the post cat sensor, it has no effect on running, just verifies catalyst operation.

Little background, sorry if you already know this. Your truck runs in two modes. In open loop mode it measures air flow with the MAF and calculates the amount of fuel to inject and totally ignores the all four O2 sensors. In closed loop mode it determines the mixture from the pre cat O2 sensors and totally ignores the MAF. The three way catalytic converter requires closed loop mode. Your engine always runs in closed loop except for three cases: (1) when cold and the O2 sensors aren't warmed up yet, (2) when in wide open throttle and it runs rich to cool and protect from detonation, and (3) when a sensor failure prevents closed loop. In the last case (3), it will light the MIL (check engine light). Making lots of assumptions, this sounds likely.

Even if a pre cat O2 sensor failed preventing it from closed loop, it should run OK in open loop using the MAF sensor. One scenario would be if the MAF (normally only used first few minutes after cold start and at wide open throttle) was dirty. Did it run poorly until warmed up or under full throttle acceleration before this happened?

 

Great info, I didn't know all this! It was from the #1 sensor in the #2 bank. I think your assumption is right it might be running in open loop. The truck has run bad in cold weather just for a short time 30 seconds or so and "kicks in"and runs fine. The colder it is the longer it runs bad. This started after the truck was in the body shop for several weeks, sounds like a dirty MAF. It has not run bad even under full throttle until last weekend. Thanks for your help!