replaced engine and now really bad fuel milage?

[jr. borch]

hi i have a 2001 grand prix. the 3.8 liter V6 engine went out when it overheated and locked up then i bought a 2005 monte carlo engine 3.8 liter V6 and had it installed by a family friend.

was wondering if there is anyone who can help me figure out y i am getting less than 20mpg with this car when before the first engine locked up i got around 25 mpg. was also wondering if something with the installation could be installed wrong and thats y. any input into this problem would be much appreciated

thanks for looking

[Scott K]

It is possible there may have been something done wrong, but I would look elsewhere first. Possible bigger inj? Were the original injectors used? Possible bad o2 sensor? Mass air flow sensor dirty? How does it run? Good?

[jr. borch]

it runs good and i put a new air intake on simular to the k&n air filter systems.

[Jeremy airjer W]

This is the second time I have heard of this happening in the last month? weird!

[Shack]

First question that comes to mind is, was it a remain/rebuild, or a used motor with miles on it?

If a reman or rebuild, there will be a break in period (rings seating) for around the first 5-10,000 miles. During this time you will see less MPG.

[Jeremy airjer W]

First question that comes to mind is, was it a remain/rebuild, or a used motor with miles on it?

[Shack]

We will "assume" it has miles on it then

Thanks Jer. After reading it they way you broke it down, it is clear. I would say it is a long shot he got a great deal on a reman that was VIN'ed for a 2005 Monte Carlo 3.8 liter V6 or the family member went threw and rebuilt the motor before putting in Could happen though

I just read the title and scanned the post for "reman", "rebuild", "used" or "1xx,xxx" miles. I have to read these posts more clearly or maybe less

[jr. borch]

ya we bought it from a parts place in brainerd and it had no modifications except i believe we had to use the sensor wires from the original engine in the grand prix

[NIBOWMAN]

Check the fuel pressure regulator, they are known to go bad. This will typically make the car run alittle rough and start slow. Also on the first engine did you check to see if the plastic upper intake was leaking. Often they will leak internally causing the engine to overheat, when you stop and shut off the engine to add coolant or let it cool down it will leak coolant into a cylander causing it to hydrolock. New spark plugs, oil, and a new upper intake and your off and running.