Sudden drop in gas mileage

Hi guys,

I have a 1988 Honda Civic Wagovan (4wd, 5sp, ~145k). In my 11 gallon tank I’m normally getting 300-320 miles around town. Suddenly though, over the last few weeks I’m getting between 230 and 260 miles per tank. As far as I can tell, the car isn’t running any differently, I’m not driving differently, and nothing has broken or failed (as far as I know). I had a recent tune up a few months ago and a safety inspection last week - neither hinted of any problems that may be accounting for the mileage loss.

Any idea what may be causing this? Is there anything I can check or verify or am I shooting in the dark on this one?



I should also say that the car has had 4 new tires, an alignment front and rear, new air filter, new distributor button, and new plugs/wires within the last few months. Tire pressure is good and the oil is full. Thanks!

First, check the neighbor kid’s breath to see if it smells like gasoline.

If the Honda has a fuel pressure regulator on the fuel rail that is vacuum actuated, pull off the vacuum hose and see if it has gasoline in it, indicating a failed diaphragm.

Then see if the Honda has a function to preheat air on cold mornings, and make sure it is not feeding hot air into the engine all the time.

Air Conditioner working properly? Compressor is not running all the time is it?

Put your hand by the exhaust opening, can you feel the individual bursts of exhaust? If it is just a steady blow, then suspect obstruction in exhaust.

How are you checking the mileage? By the dashboard fuel gauge movement or the fill and check against the tripmeter method?

If it’s the latter and the mileage has really dropped off that much I’d consider one of the following.
Someone has changed the distributor position during this tune-up and retarded the ignition timing. The timing should only be checked and adjusted when the test connector has been jumped. Failure to do this will throw the timing off and possibly cause this problem.

The possible onset of a clogged catalytic converter. A partially clogged converter may not cause any apparent performance problems but can cause the system to run a bit rich while not causing the Check Engine Light to illuminate.

A ruptured fuel pressure regulator diaphragm which allows gasoline to seep (or be pulled rather) into the intake manifold when the engine is running. This often shows up as a problem on a hot engine restart though.
Hope some of that helps.

Changed brakes? Dragging brakes? Dirty air filter? Radiator fans running constantly? Engine temp gauage in normal range? Another driver, or one who is driving your car more?

If your cooling system thermostat is stuck in the open position, this can cause your engine to take a longer than usual time to warm up. When an engine runs too cold for too long a period of time, it will run on a “richer” gas/air mixture, resulting in a drop in gas mileage. Since the thermostat is one of the cheapest parts to replace, and since that thermostat is now likely to be 10 years old, it might be worthwhile to replace it and see if that makes a difference.