Please help: Check Engine Light
1996 Grand Cherokee 4.0L
Check engine light is always on, but car runs great. A friend scanned the codes and found a RUNNING LEAN code. Both O2 sensors jump around somewhat but generally agree with each other. What is strange is the engine light goes off when we drove the car to low altitude (we're in Denver at about 6,000 feet). When we got back to Denver, light is always on again. Which sensor is most effected by altitude (atmospheric pressure)? This engine has no EGR system (unlike the V-8). Also, does this engine have a MAF sensor? There is no mention of one whatsoever in the Dodge Shop Manual. Is it called something else?
My research points to a clogged fuel injector, clogged fuel filter, faulty fuel pump, or faulty O2 sensor. But my instincts tell me it is something else since the problem goes away at low altitude.
Coincidentally, my '88 Chevy Silverado has almost the exact same problem. Light is on only at high altitude. I've changed the EGR solendoid (EGR valve checked out ok), changed O2 sensor, and fuel filter with no improvements. I feel like the Jeep has the same problem and I want to minimize replacing good parts. Hopefully some of you have had the same problem and know exactly what is wrong?