I have a 1987 5.0L 305 V8 IROC-Z28 with 2.73 gears. The car has 116k on the engine. Recently I've been driving it around and I've been averaging 4mpg. I don't beat on the car, I drive the speed limit and have ran fuel injector cleaner and I run full synthetic fluids.

Anyone have any ideas on how to improve this or diagnose possible problems? I'm not running any codes. I plan on replacing the fuel filter, the plugs/wires/cap/rotor, and do full fluid flushes as well as sea foam. Any other options?

 

I'd do a full tune up, and also CHECK FOR VACUUM LEAKS!

The number one cause for bad mpg is vacuum leaks.

I get 28mpg highway and 18mpg in town in my 5.0 tbi.

 

BTW....Don't feel bad...... Tractor Trailers loaded @ 80,000lbs get 8mpg....oh wait....feel bad

Just ribbin ya

 

Damn does it have a hole in the gas tank...Never heard of that kinda mileage!

 

Sounds almost like an injector is leaking with that kind of fuel consumption

 

Thanks for the suggestions, I've had some other plans for it along the way. I overlooked the vacuum leak at the intake manifold. Causes a high idle when in park but drops down when in gear but I'm sure that has a bit to do with it. I already have the gaskets so we'll see if anything changes after they are installed.

 

At that rate, LEAKING ain't the word!!!

 

A BIG ONE!!!

 

 

A bad coolant temp sensor will give you much worse fuel mileage then a vacuum leak.
Check the timing, make sure the ign. timing advances when you rev it.