engine misfire & map fault

[bertelli_1]

A slightly theoretical question posed recently - you have a modern 4 cyl petrol engine, misfire on cyl 2, fault codes show manifold pressure too high & a lean mixture.

Your ideas?

[anonymous-user]

Due to the misfire, the specific crankshaft torque for a given air consumption will be lower, hence, to maintain idle speed, the throttle will open to allow more air to flow, and the non misfiring cylinders to produce more torque and stabilise idle speed. This extra air increases manifold pressure compared to the normal idle value. The misfire also passes large "chunks" of oxygen through the exhaust, hence the lambda sensor sees a lean value during misfiring.