I bought my 305 1991 z28 a week ago, and I drove it 6 hours home. I calculated my MPG at about 22mpg.

Now that I drive it in the city it seems like the mileage is horrible. I just drove the car 15 miles and went through a quarter of a tank. Unless my fuel guage is inaccurate, I calculated that my car is getting 3.75mpg, driving with my foot completly out of it.

Driving home at 1/4th of a tank I start hearing a loud whining noise from the tank, with a mild fuel smell. I guess my fuel pump is on its way out? Will this cause bad mileage?

The car drives great, however when in neautral, If I rev the engine atleast 2k, I hear a slight backfire on decceleration.

What can cause this?

 

Quite possible

Exessively noisy pump is usual indication it is on the way out; less fuel
in tank means pump may be uncovered so you can hear noise easier.

 

Ok. When I brake or accelerate the gas guage does jump above or below to 1/4 a tank from the actual fuel level. I guess I just have to fill it up to see if the guage is accurate.


Also there is nqo way for these engines to run normally if they are getting 4mpg....right?

 

When level is low in tank ,fuel can slosh around more easily moving the gauge float

You never do mpg from the gauge; too inaccurate.
You fill tank ( to a point where you can see fuel ) ; drive X hundred miles and refill tank to exact same spot.
Then you work out ACTUAL MPG

 

Man I have an 87 IROC 305 with the same exact problem! You described the problem to a T....Mine just started and was thinking it was the fuel pump...Where is it loacted?

 

ha, wow. Well hopefully we can figure out whats up with this. If you fix your problem PLEASE let me know. From my understanding the pump is in the tank, and I've heard its a major PITA to replace.

I know calcualating mpg through the guage is inaccurate, but it was enough to tell me I've got a problem.

Anyone else with ideas on what is going on with my car?

 

Well it looks like it is the vapor canister being clogged problem is I can find one for an 87 IROC VIN G.