Recently there has been a dramatic drop in my mpgs. It's a '85 firebird 2.8 5 speed with 135,000 on the clock. All stock except for custom intake and all new catback exhaust. I'm averaging about 14mpg right now, which SUCKS, compared to what I was getting a month ago (about 19mpg which still SUCKS).

My cat has gone to hell a little over a month ago, it rattles like crazy. I plan on replacing it with a straight pipe tomorrow. The car has had a recent tune-up (cap, rotor, plugs, wires ect within 15,000 miles). I also plan on changing the oil tomorrow as it is beginning to go black after 1,500 miles.

MY question is, why is my mpg so bad??? Can a bad cat cause a 5+ drop in mileage? Is my O2 sensor shot? The car runs great, really lacks power below 2000rpm though, just really really bad mileage.

 

Have you done a tune up on it TOM P tuen up gudie is great. I bad Cat rattles and also could be cloged could be your problem. I would replaced your cat with a Cat and not s straight pipe.

MPFI ?? could be leaking injectors. Got a fuel pressure tester and see if it holds pressure after shutting it down. Check your plugs seed if there fouled with carbon or oil or white from a blown head gasket.


I only got 19 miles on my 89 2.8 mix of city and country driving. So that could be all that you might get.

 

my son has an 89 rs 2.8 gets 29-32 mpg all the time runs 93 octane.a bad cat will kill gas milage,so will a bad oxygen sensor,both could really kill it.hope you get it fixed.later jimmy

 

use royal purple full synth oil... (after you fix the cat or whatever is turning your oil black so fast) it may cost more but it pays for it self with the mpg help it gives. i get about 5 more miles with it and i know it does wonders on engine life.

 

go to ur nearest lube center Jiffy lube etc...get a fuel induction cleansing... i have a v6 92 Camaro RS...before i got it i got an atonishing 100 miles to the tank... i got the fuel induction and it seemed to get me a good 3-4 mpg..since your car has many miles there is a lot of carbon build up in the throttle body and the injectors and the fuel induction cleans it out..i went to the Lube Center where i live and it cost $110...also get new spark plugs, wires, and distributor cap and rotor...after getting all of that i seem to get 140-150 miles to the tank

 

Sounds like a bad injector. Is it rough at low rpm/idle? Smelly exhaust?