Can anyone help me figure out y i am getting bad gas milage?

[jr. borch]

2001/pontiac/grand prix/3.8 liter V6/car 205,000 miles, engine 38,000 miles.

ok i recently had the engine replaced i had in my grand prix it overheated and locked up so i bought a 05 monte carlo 3.8 liter V6 to put into my grand prix. everything switched over fine except the sensor wires were different so we had to use the old sensors on the newer engine. my gas mileage was 23 mpg on average. now im barely able to get 20 mpg and i put a new k&n air intake on and a chip that controls air intake. before i put those in i was barely getting 18 mpg. wandering if anyone had any idea's on what i could do to help improve or help me find out y the miles per gallon dropped so much. any idea's would be helpful. im probably gonna have the exhaust looked at as i think it might be leaking but besides that what other idea's do u think it would be?

well if it helps with figuring it out my car is also making a chugging noise out of the exhaust. its like a chug pause chug paus chug. instead of a constant stream of exhaust. its a dual exhaust also

thanx

[Big Buck Buster]

I would check for a dirty MAF sensor, leaking fuel pressure reg, fuel filter,clean air intake system, clean injectors.

Be very careful not to over spray the oil on the K&N filter. They have bulletins out on MAF sensor contamination with those filters that can cause driveability problems.

[jr. borch]

thanks will look into those things

[vister]

could it be a leak in the gasket under the plentum? I've had issues like that before. car would run good, but get bad mileage, and chug like that at idle.

[Jeremy airjer W]

I wonder what the O2's are doing? They may still be switching within the calculated tolerance but may be responding slow enough to cause a fuel issue. It would also be worth seeing if there is any external fuel leaks like the fuel pressure regulator. If its leaking it will effect fuel economy!

[Shane May]

Are you geting any check engine or gauges lights on your dash? Sounds like it could be many diffrent things. You can always take your car to Autozone or Oreilly Auto parts also Checker Auto Parts and have them stick their computer on it to give you the codes for trouble shooting. I tend to do this when it comes to cars 1997 and newer.