2003 impala gas mileage

had intake manifold(plenum kit) replace by my mechanic due to an antifreeze leak into the old one. 3.8l 231 engine-- before replacement was getting 26-27 mpg average- the day after repair i am getting 19-20 mpg!!! mechanic does not know what problem is if engine idles and there are no warning lites,etc.

Welcome to AF, what is "if engine idles"?, second some systems will relearn after the battery was disconnected, give it some time.

Welcome to AF, what is "if engine idles"?, second some systems will relearn after the battery was disconnected, give it some time.

Sorry about the error----Mechanic says everything is normal--engines idle and timing were set properly after replacing upper plenum kit!
It's just amazing to me in the decrease in gas mileage after what he said was a simple repair procedure. Is there something he is missing????

Any drastic drop in fuel mileage has to come from an error in a sensor input or commands from the pcm, usually this will set a code, one other possibility is the cat is beginning to fail, a back pressure test at an O2 port would reveal this issue, but it is usually accompanied by a loss of high speed power.