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Here are a few engine problems that hurt fuel economy:
- missing or defective thermostat, causing low operating temperature
- clogged intake manifold exhaust crossover passage (for heating intake manifold)
- engine misfire caused by bad spark plug, ignition wire, coil, dist. cap or rotor
- late ignition timing
- bad vacuum advance unit (common in old cars)
- stuck centrifugal advance mechanism in distributor (needs oil occasionally)
- bad tension adjustment of centrifugal advance springs
- defective oxygen sensor (causes rich mixture)
- defective coolant temp. sensor for engine computer (causes rich mixture)
- worn timing belt (may cause rich fuel mixture in fuel injected cars with a MAP sensor)
- incorrect timing belt installation (timing marks should line up)
- sticking or poorly adjusted carburetor choke (causes rich mixture)
- dirty air filter on an engine with a carburetor (causes rich mixture at high power)
- clogged or defective PCV valve
- incorrect valve clearance (especially if too tight)
- high performance camshaft installed (not efficient at low power)
- worn valve guides (makes fuel mixture too lean at low power)
- vacuum leak from bad vacuum hoses or intake system gaskets (causes lean mixture)
- dirty fuel injectors
- bad fuel pressure regulator
- bad MAP sensor for fuel injection
- leaking intake duct between air flow sensor and throttle body
- too much alcohol in gasoline with old car (requires carb. modification for correct mixture)
- rusted or missing exhaust heat stove or pipe for heated air intake
- loss of compression pressure due to worn rings or leaking valves
- sticking EGR valve (causes rough idling and rich mixture on some engines)
- exhaust backpressure caused by clogged catalytic converter, muffler or crushed pipe
- stuck exhaust manifold valve for some V type engines. Intended for fast warm up.
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