Horrible Gas Milage

[Danny-Boi]

I have a 1990 gt almost completly stock except for headers and flowmasters and the previous onwer had some other K&N filter (without the plastic thing that goes around it and connects to the engine bay). If I am lucky I will get about 12 mpg...does this sound right to anyone? What could be the problem here?
Thanks, Danny

 

[ECU5.0]

dead cylinders is my guess. drive it around for a little bit, take it back to the house let it cool down and pull your spark plugs, if any of them are covered with gas then its not getting spark. what you could also do it drive your car enough to heat it up and than open the hood and spray some water on the headers and see which ones dont boil or steam as much. i had a dead cylinder and the header was significantly colder than the other ones even though i had just turned the car off. i could actually put my hand on it for a good 10 seconds before it hurt, i couldnt even touch the other ones.

 

[92Patrol5.0]

Wouldn't you be able to tell if you had a dead cylinder? Wouldn't the engine run rough and idle rough? And it would be missing also wouldn't it?

 

[wisconsinboy]

not neccesarily. in a v8 if your used to the way the car runs you wont notice one cylender not firing specially cause you got 7 others to balence it out along with the harmonic balencers and motor mounts ect.

 

[ECU5.0]

my #3 cylinder wasnt firing at all, because of a bad plug wire. it would run fine at low rpms just drivin around town and i couldnt really tell because im used to how the car sounded. i went to see some friends up at work and they noticed that it was missing. you could really feel it at high rpms, it would kind of hesitate and buck a little bit. changed the wires and it runs great.

im just guessing that youve got a bad cylinder, i dont know what else would cause that bad of gas mileage. maybe o2 sensors are bad, mass air is bad, tps is off, lots of things could be bad

 

[Danny-Boi]

Thanks, I will try the things you have suggested!
Danny

 

[cevtv]

, i dont know what else would cause that bad of gas mileage.

A heavy right foot............................

 

[Mustangless]

I got the same problem, 10mpg