F-150 about to purchase

[barrett86]

OK, im looking at getting a 98 F-150 with the 5.4l 4x4 green with tan interior, 3door, blah blah blah. so ok i took it for a test drive, now they told me the o2 sensor is bad and so ill tell you the story , go out has a dead battery, its been stupid cold here, so jumped it let it run, now it ildes fine, but in park if u give it gas it stays up for like 10 seconds or so then comes back down, and while driving its fine but it likes to cruz about 35 if im not on the gas but u have to give it gas, derr, but lets say i romp it and let it off it comes down to about 32-35 and stays there unless i hit the brake, now it idles fine at a stop light, park, and drive, but it just acts funny when drivin, would a bad o2 sensor cause problems like that or EGR or TPS, or MAP, my mass air flow sensor, ???? im a loss, my knowledge lies in 80-96 broncos with 302's and 351's.BTW they said they are replacing the o2 sensor as it is, but yah idk.

 

[racebreedbronco]

doughtful its a 02 sensor, suspect a vacuum leak, check the pcv tube at rear of intake manifold, very possible its leaking there

 

[scatter57]

I have the same problem with my Ford F150 1997, idle around 1500 for 10 sec and then it goes back to normal, I think it is a vaccum leak or a faulty TPS or IAC.