BMW E30 failed emissions: causes of high HC and CO
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What causes high HC emissions on an E30?
HC comes from incomplete combustion. The most common cause is an intake leak, the second most common is sensor faults. First check the vacuum hoses running under and between the intake manifold sections.
What does high residual oxygen in an emissions test tell you?
High residual oxygen points to an exhaust leak that lets air in with the exhaust gas. Fix the exhaust leak and the intake leaks before the next test.
Do valve clearances affect M20 emissions?
Yes. Valve clearances that are too tight leave combustion incomplete and raise emissions. Check and adjust the clearances cold to the factory value of 0.25 mm before the next test.
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