Hello All -
The CEL on my 1998 F150 (4.2L, 2WD, AOD, 85K) came on about 3 weeks ago. On very cold days (below 20º F) the truck would barely crank over and when it did crank over, it took a very long time to finally start.
I drove the truck to AutoZone and they checked the codes and informed me that I had 2 bad O2 sensors (1 on each bank).
I replaced all 4 O2 sensors, fuel filter, spark plugs, cleaned my K&N, changed the oil, and replaced the pressure/return fuel line assembly (the last part listed was my fault, don't ask). While I was doing the repairs, I decided to disconnect the battery and charge it as well. I suspected that my battery may have been getting weak. However, my gauges have never indicated low voltage.
After the repairs were completed, the truck ran fine with no annoying CEL. However, I still felt that the battery wasn't able to crank the engine effectively on very cold days.
2 days after I completed the repairs, my CEL came back on. In conjunction with this, I noticed that the truck now had a very rough idle. That was a week ago. I suspected the rough idle was a direct result of a bad IAC.
Last night, after driving about 10 miles, I depressed the accelerator to pass another vehicle while I was traveling 60MPH. Instead of a downshift followed by rapid acceleration, the truck began to misfire. The truck was shaking and I heard a backfire or two in conjunction with the misfire. As soon as this happened, my CEL began to blink/flash steadily.
The truck continued to run like this until I returned home (5 minutes).
Upon arriving home, I turned the truck off and let the truck sit for 10 minutes and then started it again. When it started, the check engine light was on as before, but no longer blinking.
The next day (today), I started the truck up and began my drive to work. Again, the truck ran as before with the check engine light on, but it had to misfire. 10 miles into my commute to work, the truck began to misfire again (more noticeable under acceleration) accompanied by the blinking CEL. I shut the truck off when I got to work. 10 minutes later, I started the truck and it was just as it was before the misfire (CEL on continuous, truck a little sluggish to crank over and fire, slightly rough idle).
My thinking is this ... because of the rough idle characteristic, I think the IAC needs serviced/replaced.
My question is this ... could a faulty IAC cause my truck to misfire at highway speeds?

 

An IAC will not cause misfire at high way speeds. Get codes checked, this will tell which cylinder is misfiring. Could be plug wires. Also would check fuel pressure,unlikely both O2 sensor went bad at same time. Was your codes for a lean running concern?

 

What kind of plugs and what kind of wires? A lot of poeple have problems with aftermarket wires of poor quality and aftermarket plugs.