I had low mileage problems for years, then I had all of the pollution equipment changed on it and now it gets incredible mileage. I figure it must have been clogged, and now it can breathe. It almost didn't run towards the end. But runs great now. Good luck!
There are a ton of reasons why an older vehicle may get bad gas mileage but I will talk about the most common-
Ignition system
Spark plugs- generally speaking, the high performance plugs don't work as well on a high mileage engine because they have a higher resistance to increase the spark size and temperature. Use AC Delco or similar.
Make sure the plugs are properly gapped. The factory will set a standard gap, but that doesn't mean it is correct for your engine. If your engine requires a 0.35 gap and the new plugs are at 0.44, then you will loose HP as well as 10% or so of your best gas mileage potential.
Plug wires- they wear out and the connections can and will corrode. Replace them. Use Dielectric grease on both ends of the plug wires to ensure you are getting all the juice to the plugs.
Cap and rotor- These wear out as well. they need to be bone dry and no air leaks. Over time the cap can develop cracks and moisture can get in there. The high energy field can be disrupted and not give the proper current to the plugs. The rotor in side turns and makes contact for each individual plug. This contact point on both parts can wear out and you will loose voltage.
Timing- Most HEI ignitions don't need to be checked because the computer controls the timing. but it is good to check the timing you are getting with what it should be to make sure it is spot on. If it isn't then their are several possible reasons, but check for vacuum leaks and O2 sensors.
Air, Oil, Gas filters- make sure your good.
Compression- Get a compression test and compare it to the factory standard. Low compression indicates a worn or broken ring, a gap in the head gasket, or a valve that doesn't close all the way. This cant be fixed with out a head rebuild or possibly engine rebuild.
Fuel- Premium, mid range and regular are not proper terms. Generally, the higher the octane the less mileage you get. Higher octane gas is used for engines with a higher compression and a higher temperature spark. This not only makes the gas burn more efficiently and gives more power but also quiets the engine noises such as pinging.
Older vehicles don't have as high a compression as newer ones, so using the wrong fuel can put up to %20 less mileage potential.
get a 5 gallon can of gas and some gas treatment to remove water from the tank. Run the tank empty then add the 5 gallons and the fuel additive. May as well add some detergent in there as well. Now run THAT fuel mixture for about 10 minutes. Drive to the gas station and fill the tank. Use the proper fuel.
Change the oil.
After that you should see a lot more power and better mileage. I have done standard tune ups on running cars and trucks that got up to %50 better gas mileage and a lot better power. Just yesterday I replaced the plugs in a used '93 G20 I recently picked up, and it not only runs smoother and quieter, but it has better acceleration, better idle, and more stable RPM at constant speeds- where before the change, the idle went up and down, there were hesitations, and the acceleration was horrible.
Hope all this helps.
