Why is the MPG so low?

April 1, 2009 | By Richard Prince


I have a customer who has two identical cars, a 1986 Cadillac and a 1989 Cadillac. The 1986 gives barely 14 mpg on trips while the 1989 model gives at least 20 mpg on trips. Everything has been tuned up, including new plugs, cap, rotor, condenser, ignition timing, and tire pressure. The only thing we can find wrong is that the lower choke pulloff is broken and we cannot locate one. The only thing we did not do was go to synthetic oil in the servicing.

Can you think of anything we might have missed that would account for this poor gas mileage?


Different transmission and differential gearing and different chassis weights can make a dramatic difference in fuel consumption. You don’t provide any details about the cars but do state that they are “identical” so I will assume that

they have the same engines, transmissions and differentials, and weigh approximately the same. So what accounts for the significant difference in fuel economy between the two Cadillacs?

You have already addressed tune-up and tire pressure, so let’s consider other possible causes. Perform a leak down or compression test on the two cars to see if there is a significant difference. Weak compression in the ’86 may be at least partially to blame for the poor fuel economy. A bad knock sensor can hurt fuel economy by chronically retarding the timing. A bad coolant temperature sensor may induce the computer to maintain a perpetually rich mixture if it indicates that the engine is always cold. In a similar vein, if the engine is always too cool (for example, because the thermostat is broken) fuel will be wasted. An obstructed catalytic converter will decrease fuel economy and a malfunctioning oxygen sensor or bad EGR valve can do the same.

Have you tested the ignition system in the ’86? A weak spark from a bad coil or other defective part in the system will cause increased fuel consumption.

Confirm whether fuel pressure is correct as excessively low or high pressure can possibly impact fuel economy. Check the brakes for drag and make sure the air conditioning compressor is not constantly on.