I posted this before, but I guess it was in the wrong place, so I'm going to try here.

My 92 F150 5.8L (351w) E4OD has recently running very rich, black smoke with any throttle, getting about 4mpg, bucking in high gear, and getting error codes telling me that the line voltage to many sensors was high (173, 334). I found out that Ford has dis-continued the EEC-IV computer (along with many other things), and bought a used one from a junk yard (I have many Q's about the 12 different EEC comps for the same truck). The replacement EEC-IV only gave me a slight improvement, as I am now getting about 6mpg (normal was 12mpg), black smoke only under heavy throttle, and it still bucks in high gear. The Check Engine light came on again. I pulled the codes and here is what I got:

111 - System Pass
173 - Heated ExhaustGas Oxygen (HEGO) sensor - voltage signal indicates "rich" (Bank#1)
334 - EGR Valve Position (EVP) sensor - signal voltage higher than expected )closed valve position)

These are the same codes I had with the 1st computer. So either I have 2 bad computers, or there is something else wrong. Following suggestions from another thread, I probed the EVP and got in tolerance resistance at 0, and 10 in vaccum (which held for 10 min). Visually, I can see the valve opening. I checked the voltage and found proper voltage at the plug to the EVP with the key on. Started the motor, manually pulled a vaccum at idle, and the motor ran rough. I pulled the EGR, inverted it, poured break fluid in, and let it sit for 1/2 hour. The EGR showed no signs of letting the break fluid past the seal. I re-assembled everthing, and test drove the truck, and still have the same symptoms. I replaced the O2 sensor a year or two ago, so its not that old.

So now I'm at a loss as to what to look at next. My gas bills are killing me, and the exhaust smell is not that great either.

 

I have spent much time and money trying to fix the same problem with my 89 351W Bronco. I even took it for the ultimate test at a Ford dealer (see the write up in the Bronco section a few months ago). I have nothing left to change in it, I did the throttle body last weekend.
It is going to another mechanic next week for them to get a nice income from it. I really suspect there is a problem in the wiring harness somewhere. But where?
Hopefully you will find your problem, it could and most likely will be a combination of problems.
Pam

 

Check your fuel pressure and fuel pressure regulator first of all. If the pressure is too high, or the diaphragm in the regulator is leaking, it will suck all kinds of fuel into the engine. The computer is not the cause of the problems here. Also check the wiring between the O2 sensor and the computer to make sure nothing is shorted out.

 

also what degree thermostat is in there. it should be a 195*. otherwise the motor will reach proper temp and cause it to run rich .

 

Just to add to what EPNCSU2006 said;
If that FPR did fail and let lot's of fuel by, then that O2 is shot as well.....

 

I had the same problem in my 93 Bronco 5.8L. Fuel pressure regulator got changed for $18 at autozone and my problems were solved. By the way, why did ford have to put the screws for the FPR on the bottom? I said a few bad words to that one.

 

The problem did turn out to be the Fuel Pressure Regulator. I was finally able to locate a fuel pressure gage that would attach to the fuel rail and got 80psi engine running or not. Picked a new one up at Autozone ($18 and change), and cursed a bit for those upsidedown hex head screws. End result was pressure around 39psi. I finally made a highway trip today and did not use up 1/2 a tank on a 25 mile trip. After a month of driving with it as rich as it was, I am going to go ahead a replace the O2 sensor as well.

Thanks to all for your help and suggestions.