1998 Ford F-250 check engine light

1998 Ford F250 mileage: 120,000. I purchased the truck about a month ago. The "check engine" light comes on after being driven for five or ten miles from a cold start. EEC-IV codes were checked: Key On Engine Off test gives a code of 11 (system pass). Continuous code (stored memory) issues 41 (system indicates lean) and a 34 (EVP voltage above closed limit). I have recently replaced the EGR valve, the EVP sensor, and the oxygen sensor in an effort to correct this problem with no success. Sometime before that, the MAP sensor was replaced. The condition of the spark plugs and the engine temperature gauge do not indicate that the mixture is lean, at least not severely. I set the timing (SPOUT shorted) to 10 degrees BTDC. Except for this warning light, the motor seems to be running pretty smooth. It has an automatic transmission (no overdrive) and air conditioning. Any help would be greatly appreciated.