February 25, 2015 Maurice Donovan

Excessive Fuel Consumption

Check List for Excessive Fuel Consumption

Often a customer will state that their vehicle appears to be using excessive fuel. Over the years I have created a Check List as to what could be causing such a problem. I call this my list of  Intensive List Of Possibilities.

We have different procedures for many scenarios we see in our workshop. My list is a “work in progress” as many things change in the world of diagnostics.

These procedures helps to keep us focused and uniformed when doing diagnostic investigation research on our customers’ vehicles. It also gives us something to show our customers when a car is brought into us  which allows us to justify a necessary diagnostic charge before starting a job.

Before starting, check with the customer and obtain at least the last 3 fuel consumption figures. Ensure to enquire from the customer how much driving has been city driving and how much highway driving. Look up the factory standards for fuel consumption for this vehicle and insure there is actually an excessive fuel consumption issue.


Do a visual check; see if there is a rich smell or black smoke from the exhaust tail pipe. Scan check vehicle for codes and check live data for clues, pay close attention to fuel trim readings and insure that the O2 sensor is oscillating between rich and lean and is not slow. This must be checked at all the rev ranges.

If nothing has stood out to indicate a rich mixture, set up our portable gas analyser and road test.

After this we can be sure the vehicle is not necessarily running rich and the below steps need to be examined.


1              Missing or defective thermostat, causing low operating temperature:

2              Clogged intake manifold exhaust crossover passage (for heating intake manifold):

3              Engine misfire caused by bad spark plug, ignition wire, coil, dist. cap or rotor:

4              late ignition timing:

5              Defective oxygen sensor (causes rich mixture) (should have already been checked)

6              Defective coolant temp. sensor for engine computer.

7              Worn timing belt or chain (may cause rich fuel mixture in fuel injected cars with a MAP sensor):

8              Incorrect timing belt installation (timing marks should line up):

9              Dirty air filter:

10           Clogged or defective PCV valve:

11           Incorrect valve clearance (especially if too tight):

12           Worn valve guides (makes fuel mixture too lean at low power):

13           Vacuum leak: Smoked test

14           Dirty fuel injectors or leaking fuel injectors:

15           Bad fuel pressure regulator:

16           Bad MAP sensor for fuel injection:

17           Bad or dirty MAF meter:

18           Leaking intake duct between air flow sensor and throttle body:

19           loss of compression pressure due to worn rings or leaking valves:

20           Exhaust backpressure caused by clogged catalytic converter,

21           Binding brakes:

22           Leaking evap purge valve:

23           Wrong oil velocity:

24           Low tyre pressure:

25           Have the wheel properly balance/aligned:

26           Alternator uses power from the engine to replenish the lost battery voltage, which adds up       to increased fuel consumption:

27           Driving habits and environments play a significant role in fuel economy:

Contact Us

Repco Authorised - 12 month 20,000 km National Warranty