BMW E36 high HC and CO readings in emissions test
E36 fails the emissions test, HC reading is high. Where do I start?
Start with air leaks on the intake side: the bellows hose between the mass air flow sensor and the intake manifold, the idle control valve hose and the fuel tank vent valve. The leak is found by spraying brake cleaner on the joints with the engine running: the engine speed rises at the leak.
Can spark plugs or the valve cover gasket raise emission readings?
Yes. A leaking valve cover gasket lets oil into the spark plug wells, which raises emission readings. After replacing the plugs and seals the car has passed. Warm the engine up before the test, because the HC reading can be higher when cold.
Do an exhaust leak or the lambda sensor affect emission readings?
Yes. Even a small leak in the exhaust (e.g. in the silencer) affects the readings. A leak before the lambda sensor gives the sensor a false reading, which enriches the mixture and raises HC and CO. The mass air flow sensor can also be the cause; check these.