Question: I have a 1995 Ford F150 (5 liter V8) and I just did my first computer diagnostic test. When I did the “engine running” test I got a code 311. I looked this up on the ‘net and found that it was the code for AIR system inoperative. My Haynes manual has nothing on this code or the air system. Is this the air intake system (air cleaner housing, etc) or the oxygen sensing on the exhaust manifold. Hopefully, either alternative might explain the poor gas mileage that I am getting lately (12 MPG). I just purchased this truck used and I am trying to do a little bit of a tune-up on it myself to get familiar with the engine. This is my first fuel-injection vehicle, so I am delving into unfamiliar territory here even though I have been doing most of my own repair on other my cars throughout the years. Any help would be appreciated.

I’m at home and the only reference I have here says, “311 (R) AIR system not working – Single, Right or Rear HO2S – Air Injection”. The AIR system that code refers to is the air injection system. Your truck has an air pump (commonly referred to as a smog pump) that injects air into exhaust and the catalytic converter to promote oxidization of hydrocarbon, carbon monoxide and oxides of nitrogen into more environmentally friendly gasses. There are times when you don’t want air injected into the exhaust, such as during deceleration, so part of that system is what’s called a diverter valve that can divert the air from the pump to the atmosphere. HO2S stands for Heated Oxygen (O2) Sensor (oxygen sensors have to be very hot to work and later model vehicles’ sensors are heated electrically as well as by the exhaust to light ‘em off quicker). My reference seems to indicate that the oxygen sensor is a player in the enable criteria for this code to be set so I’m guessin’ that if the computer sees a lean oxygen sensor all the time it may surmise that there’s too much oxygen in the exhaust and decide the diverter valve must not be workin’. That’s pure conjecture on my part, I don’t know if that’s really how it works, but if it is, or if it’s close, then anything that would put or allow extra air into the exhaust could set that code- like a cracked exhaust manifold- which is why I even mention all of this. Ford V-8 trucks, and it seems particularly 5.0L’s(including my own) are prone to cracking exhaust manifolds. I’ve replaced the right exhaust manifold on my truck twice and neither time did I know it was cracked till I saw the crack during some other maintenance- it didn’t get loud like ya might expect. Before I got all wigged out about this though, I’d clear the code and drive the truck for a few days and then re-check to see it returned. If it doesn’t it was a false code (not uncommon).
My truck is a ‘90 F150 with a 5.0L and an E4OD tranny. I bought it new and at first it was gettin’ about 12-14 MPG. It’s now 10 years old with 234,000 miles on it and it seems to have improved a little with age- last time I checked, which was probably 3 or more years ago, I was gettin’ 15 MPG.

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