I have a 1991, 4.9l F-150 pickup with 170000miles. The oil has been changed at 4000 mime intervals since it came off the floor .It does not use any oil and runsgreat at interstate speeds. From about 30-50 it is rough. It doesn't seem tomake any difference if it's accelerating or deaccelerating.Have changed TSP,O2, anti knock sensor and cleaned throttle body and ERGvalve. The only remainingcode is 181 which i don't understand what it is tellingme- Fuel at rich adaptive limit at part throttle: system lean. Would appreciate any help anyone can give me. Thanks Ol Bill

 

The code means that the computer cannot richen up the mixture any more, but the engine is still running lean. I would check fuel pressure to start with, and also check for vacuum leaks. Another thing to check would be the O2 sensor ground.

 

Could still have an egr flow problem. The egr sometimes richens the mixture at part throttle.

Not sure about this one, verify yourself. An ignition problem would put raw oxygen in your exhaust and the oxygen sensor may interpret that as a lean mixture.

Let us know what you find.

 

Thanks to EPNCSU2006 and HardScrabble for your inputs. It is about daylight here in Baton Rouge and I plan to do some testing this AM. Will keep you both posted. Thanks, Ol Bill

 

Did you find anything?

 

I summarized yesterdays tests this morning but somehow the computer locked up on me. Spent most of today getting back on line. It is currently Miller time here. After dinner I will try to recreate yesterdays test results. Apologize for delay in responding. Ol Bill

 

EPNCSU2006 and HardScrabble- I found four ground connections attached to the frame. One has a#4 copper wire connected to the battery negative post. All of them were tight and did not show any signs of corrosion. I cleaned them and their connection points. The O2 sensorwires(4) enter a harness near the starter and this combination of wires wind back to the starting relay on the right front fender. Just before the relay terminal, their is a connectorwhich breaks out and enters another bundle which utimately winds around to the firewall. I assume this connection was the O2. Did not verify- poor thinking at the time!!.
The EGR valve has some vacuum to it under idle conditions. Removing the hose does not seem to effect engine. I measured 4-5 inches of Hg vacuum at half to three quarter throttle.
Next cleared all codes and went for a ride. The 181 code was set without the check engine light coming on. Cleared the code and plugged the vacuum line to the EGR valve. This time the the check engine light came on and stayed on. Code 181 and 332 were set.
Today I plan on some Cajun engineering to rig up a pressure gauge to check the back pressure controller.
Will keep you posted.