Chrysler Repair: Keep getting Code 51: 2.5L 4 cyl 95, oil filler cap, digital voltmeter

Question
"We have a 1995 dodge caravan 2.5L auto. We have tried everything;replaced the O2 sensor about a year and a half, the map sensor,cleaned the TBI injector which was very dirty.Replaced and secured all vaccum lines. when we first start it; it runs so good,but after driving it for about 6 miles, the check engine light comes on.Let it sit a while and restart it and it runs great until traveling again about 4 to 6 miles and the check engine light comes on.Then after that the gas gauge needle goes down pretty fast. PLEASE HELP>!!!!!!"

Answer
Hi Vick,
The question is whether the oxygen sensor is inaccurate or if the mixture is really too lean (which is what the code 51 suggests). The fact that it gets worse as the engine warms up makes me think that the O2 sensor is reading false lean, but you have to also wonder if it may really be lean.
If it is false lean, then the controller will richen it up and that would have the effect that you observe, stumbling when warmed up. If it was truly lean, that may be something to find out why.
Take a look at the O2 sensor's signal wire voltage on the harness side of its plug. The black/dark green wire on that side of the plug is the wire in question and so take a digital voltmeter and a pin so as to probe the connector for that wire and see if it reads closer to 0.1V rather than normal mixture which would be 0.5V, doing this reading after the engine is fully warmed up.  
If it does, then I would check the egr valve to be sure it isn't stuck ajar which will cause a too lean mixture at idle/slowing down. It is located on the back side of the engine (firewall side) near the front. So take a look from the passenger side where you will see it just to the rear of the oil filler cap. It has a round vacuum operated top, and a body that is bolted into the small exhaust side pipe nearby, and a flange in between those two parts. In the flange area notice a rod with a circumferential slot. Use the tip of a screwdriver inserted in the slot to move the stem back and forth, against spring-action which tries to close the valve. See if the spring closes it firmly to a dead stop. If not, spray the stem where it enters the valve body too loosen up the 'action' with WD-40. Work the stem till it moves freely. Then see if that solves the problem.
If the signal wire reads 0V then the wire is broken, probably near the sensor wire connections to its tip. If that is the case then replace the sensor.
Roland