Chevrolet Repair: 1995 astro van 4.3 L AWD CPI, o2 sensor, tool check

Question
Hello Robert, I had my van tuned up,The mileage went up to 18mpg,it dropped off 1 month later to 10mpg,back to the mechanic who said it was a bad cpi unit because he heard the sound of gas draining somewhere? and said there was gas in the oil.Cost would be $1000... Now it starts and runs perfectly every day. 2nd opinion I had another mechanic find a bad O2 sensor and the t-stat was stuck open.Changed those parts,but the other place is still insisting its a bad cpi unit. What should I do? How can I be sure its not a bad cpi? Thankyou  Chris.

Answer
Do You have axcess to a scan tool.... check for codes... look at the data stream and see if any of the sensors are out of wack. IE: ign referance, inj pulse etc, ect, if all ok the cpi is reading properly and should be ok....if any data shows something not right then test that problem to see if it is sensor or CPI.... fuel in the oil could be inj leaking down and contaminated oil will cause the CPI to read wrong... hope this helps