I have a 1994 f150, 5.0, 4x4 with about 145,000 miles on it. It's run fine until yesterday when I first noticed it coughing a bit whenever I slowed down at an intersection, like it was running out of gas. It only did it a couple of times until I came to a stop at a light and it coughed and spit and then died. It wouldn't restart so I pushed it off the road and checked the codes. 172 , system always showing lean, showed up. I tried to start it again and it fired right up, running fine for about a mile and it did it again. It would restart after a couple of minutes of letting it sit. During the remaining drive home, this continued. It would cough and spit whenever maintaining a constant speed or on decelleration (spelling?), but seemed to run fine if I could keep the rpm's up high. So I just manually shifted gears (it's an automatic) whenever I had to slow down and that was enough to get me home. Now it's home, It starts and idles fine but it's still showing the 172 code. I need to know where to begin. I'm pretty certain it's not the fuel filter or pump due to the fact that it runs better upon acceleration. Please help. O2 sensor?

 

O2 sensor, they pretty much only read at lower RPM. Replace it and see how she runs.

 

In my experience, there has to be something drastically wrong for an O2 sensor to throw a code for malfunction on OBD-I. Once the engine has reached operating temperature, the O2 sensor reads at all rpms and all throttle positions except for full throttle. I would give the truck a good look for a vacuum leak or a fuel problem. Does it seem to be running rich or lean judging by the tail pipe or plugs? I understand these codes to mean that the lean adaptive limit has been reached and that the O2 sensor is still reading rich. It could be a vacuum leak or could possibly be a malfunctioning air diverter in the secondary air injection system.

 

Thanks for the suggestions guys. I just finished cleaning the MAF sensor and about to take it for a ride. I'll check the exhaust/plugs for signs of rich or lean running, as well as the dreaded hard to find vacuum leak. I'll let you know what happens. Again, thank you.

 

A dirty MAF will cause a lean condition, so hopefully that got it fixed.

 

Another thing to check would be the ERG valve and make sure the pipe going to the EGR vavle is not clogged.

 

Sorry about the slow response. Just an update, I cleaned the MAF sensor, started it up and haven't had a problem since. Thanks to all who helped.