My gas milage sucks!!!!

[efriedrich]

My daily driver is a 86 5th ave. I get about 10 miles to the gallon. Its a completely stock 318. It runs, starts up and drives really good with maybe a slight stumble. How do I get better gas mileage and does the little stumble seem like a symptom of the whatever is contributing to the poor gas mileage. I bought the car off of the original owner who always had it serviced at the dealership so its been maintained pretty well. Where do I start checking for problems. I don't want to just throw money at it, I want to diagnose the best I can first.

 

[bill55az]

That is poor mileage, even if only town driving....what is the rear axle ratio, type of carb, does it have the lean burn computer on the air cleaner? If so, consider replacing it with the older regular electronic ignition. Is the inside of the tail pipe black and sooty? that would be a carb issue, burning rich, otherwise inside the tail pipe should be gray and not sooty. If not a lean burn and assuming there is nothing broken, try bumping the timing up until it just starts to ping, then back it off a few degrees.
Lots of possiblities to consider....

 

[MagnumMaster]

I was going to suggest dropping the leanburn if it has it. Sometimes those computer still "work" or should I say function, but poorly, enough to make it run.

I had an 84 with high mileage, smoothest running car I've owned, it got close to 20 highway. 318 btw.

 

[bremereric]

Make sure your vacuum advance is working properly. A hole in the diaphram can kill your gas mileage.

 

[77kickinaspen]

seeing ur post made me start wondering since ive done these tune ups and stuff after a year of working on this thing checking the milage. i looked at the odomimeter it was 3 miles away from being 100miles of a full tank i went and filled up it took 7.6 gallons whitch i beleive gets me having about 13-14miles a gallon dosnt sound great but i think this thing was getting 10 when i brought it home last june

 

[efriedrich]

I haven't started trying to figure it out yet but I will begin with some of the suggestions I have gotten here, but like I was saying it runs so good I really didn't know where to start but 10 MPG might be generous, this thing is a pig.

 

[77kickinaspen]

heres evreything i did maybe it'll help
pvc,breather,new valve cover gaskets ,ran a oil detergent through,new water pump,replaced evrey hose in the engine compartment i could find,good fuel filter,new thermostat,airfilter,new plugs wires cap rotor,super stock coil,new battery cables tranny cooler,muffler,and i took the carb off and cleaned it good with spray i didnt take apart just sprayed the shiz out of it,and evrey time i change my oil i put a good oil additive (stp)lucas w/ei also usa stp fuel treatment evrey or evreyother fill up that and always have my tire presure perfect and i got a few mpg more hope this helps.

 

[Cordoba1]

Lean Burn Computer

I would bet good money your Lean Burn computer is in fixed timing mode, which the car more or less runs correctly, but the components can't "do their thing" to maximize fuel economy and driveability. Can a dealer test these boxes anymore?

 

[rumblefish360]

Do you need to pass a tail pipe emissions test in any way?

IF not, the lean burn computer takes care of all timing and you do not have a vacuum advance distributor. That would be the first item I would get. Drop in a new distributor from MP or Accel and hook a MSD up to it. (Or equal like ignition) Then open that plug gap up to .050.

The next thing holding you back is;
Actual timing
Tune of the carb
exhaust
induction starting from the air cleaner to the carb to the intake

I did the like mods above to my '79 Magnum w/a 360. I was able to get 20 mpg's from a identical 10 mpg when I first got it with a working lean burn. The exhaust was dual off the manifolds @ 2-1/4 w/a "H" pipe into twin cats (Needed at the time) into twin mufflers (Thrust turbo's) with the pipe to the bumper.

Carb (New) of choice was a 625 Carter on a OE iron 4bbl intake with a '73 trap door air cleaner and a K&N. Gear ratio was a 2.76 with 26 inch tires.

Have at it!

 

[bill55az]

I would bet good money your Lean Burn computer is in fixed timing mode, which the car more or less runs correctly, but the components can't "do their thing" to maximize fuel economy and driveability. Can a dealer test these boxes anymore?

not likely, altho they may say they can just to get your money....
Emissions laws generally allow after market ignition systems when original parts get discontinued....

 

[bremereric]

You can install a MSD ready to run distributor...I did for my M body and totally did away with the air cleaner and lean burn crap. It is a very easy install. Nothing more than the distributor and plug wires.

 

[DobaDaddy]

With above suggestions being taken in to serious consideration. When the heck was the last tune up done? I.e. Plugs wires cap rotor fuel filter ect.

 

[crayzcuda]

Sounds like the exact same problem I had with my '85 Diplomat. When the computer went, it went into what some call a flop mode, which gives you poor performance and gas mileage.

At the time, it was $400+ for a new computer or 300 to convert to electronic ignition.
So, I bought the Mopar conversion kit and a 1974 318 2 bbl carb and I can tell ya that the performance and gas mileage had never been that good, not even on the day I bought it.
They told me that the computers only last around four years.