I've been reading the forums for the better part of a year now. It has been really helpful. But a subject I haven't seen on here yet is o2 sensor replacement. I had the chance to do it this past weekend...........eeeeeek!!!

If you have lost 2-3 mpg in a short amount of time, this is where to look. I have a 98' Expy with a 5.4 , and I went from having 17mpg on highway, to a scary 13.5mpg. I figured after 83k miles, the o2 sensors were at fault. There are a total of 4, 2 before the cats, 2 after the cats. The 2 after the cats aren't too bad, the plug is the only thing that is hard to reach on those. The 2 before the cats are interesting.....drivers side with alittle tricky hand work can be done, but who ever decided to put the plug for it between the firewall and engine, must have been on crack......now the one one on the passenger side before the cat, lets say is damn near impossible. You cannot get a o2 sensor socket on it at all. I had to use a wrench, and thankfully it wasn't in that tight. Ford decided to put it between the pipe and the firewall. To make matters worse, the tranny dipstick tube is in the way. Then to top it all off, the plug was even further behind the engine, if I had a large hand, I'd never reached it. The sad part is there is an awesome place for that o2 sensor just on the opposite side of where it is located now......grrrrr!

If you do replace your o2 sensors, make sure after you are done you take off battery cable for a min or so. I didn't at first and man did i get alot of creepy smoke coming out of my tailpipes. I reset the computer (take off battery cables) for a min, then hooked up and started it. No smoke. Took for a ride, some more performance out of it, and best of all, my gas milage came back. It would be better if I kept my foot out of it!! Just wanted to put my experience with replacing o2 sensors on here for the records!!

 

FWIW, the two downstream O2 sensors have nothing to do with fuel economy. They are there only to monitor the performance of the cats. The two upstream sensors are the ones used to monitor/adjust fuel trim.

You're right, they're a real PITA to replace!

Steev

 

I replaced all 4 of mine on my 98 F150 w/ a 5.4L. The upstream passenger was hard until I finally used an open end wrench. After that it took me maybe 10 min.

 

22mm wrench and they pop right off. Don't forget to use the anti-seize on the new ones.

JMC

 

The passenger side is pretty easy to get to if you remove the tire and plastic.

 

removing the inner fender wells is the only way to fly, takes two minutes to remove the plastic.


This trick also works for the starter. Don't know why more guys don't do the obvious.