The diesel engine itself is burning more fuel than it should due to maintenance neglect or maladjustment. Dirty filters, incorrect injection timing, worn injectors, or incorrect valve clearances all increase fuel consumption.

What you'll see

Fuel consumption is noticeably higher than it used to be, or higher than another identical machine on the same job. The engine may produce more smoke than normal. It may run rough or not quite reach full power. Performance has degraded gradually over time due to accumulated neglect. Often it is a combination of several small issues: partially clogged filters, slightly worn injectors, slightly out-of-spec valve clearances -- each adding a few percent to fuel consumption.
Before you assume this is the problem

Make sure the excessive fuel use is from the engine and not from the compressor running at excessive load (leaks, too-high pressure, stuck loaded). See: Air Leaks, Load Control Not Working, Demand Exceeds Sizing.

See all causes of high fuel use / poor efficiency →

How to diagnose

  1. Review the maintenance history

    Check when fuel filters, air filters, engine oil, and valve clearances were last serviced. A machine running on overdue maintenance will consume more fuel. The cumulative effect of dirty filters, old oil, and loose valve clearances is significant.
    Result: Overdue maintenance items identified.

  2. Check for exhaust smoke

    Black smoke = too much fuel for the amount of air (dirty air filter, faulty injectors, late timing). Blue smoke = oil burning (worn engine). White smoke = incomplete combustion (cold engine, timing issue). Any persistent smoke indicates the engine is not running efficiently.
    Result: Smoke color indicates the type of engine issue.

How to fix it

  1. Perform a full engine service

    Replace fuel filters, engine air filter, engine oil and filter. Adjust valve clearances to specification. Check and bleed the fuel system. Clean the exhaust system if carbon buildup is present. This standard maintenance package often restores 5-15% fuel efficiency on a neglected machine.

  2. Have injectors and timing checked

    If a full service does not resolve the excess fuel consumption, have the fuel injectors tested and serviced by a diesel injection specialist. Check injection timing. On high-hour engines (8,000+ hours), injector wear and timing drift are common and can add 10-20% to fuel consumption.

Common mistakes

The most common mistake is not tracking fuel consumption at all. If you do not measure it, you do not know when it increases. Keeping a simple log of hours vs fuel consumed gives you a baseline to detect problems early. A sudden increase means something changed. A gradual increase means maintenance is overdue.

Parts & tools

Engine oil and filter. Fuel filters. Engine air filter. Feeler gauges for valve clearance. Torque wrench. This is standard maintenance -- no specialty tools needed.

Review safety precautions before starting →

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