2003 impala gas mileage

[bert5]

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.

[maxwedge]

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

[bert5]

Originally Posted by maxwedge

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????

[maxwedge]

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.