Valve cover gasket can leak oil into the spark plug wells
VANOS piston seal O-rings can harden and shrink, causing loss of power below 3000rpm, surging around 3000rpm, a louder idle, and rough running – O-rings and Teflon piston seal rings must be replaced
Cylinder liners could fail due to fuels with high sulphur content – this was only a problem in the 90s, as fuel today has much less sulphur in it. Generally speaking, any car that is running today should be fine.
Oil Separator Valve (OSV) / Crank Case Ventilation (CCV) – symptoms are high oil consumption, poor running, and sometimes a whistling noise; located under throttle body, 4-8 hours of labor to replace. This is often caused by a lot of short drives. Sometimes, it can be fixed by a lot of long drives and high revs.
Camshaft sensor (easy to replace) – get an OEM part, not an aftermarket part
TSB – in extremely cold climates, moisture can accumulate and freeze in the engine oil separator or dipstick guide tube. This may cause high crankcase pressures which result in valve cover leakage/breakage, or an oil hydrolock condition which can result in catastrophic engine damage. This can be fixed by replacing the crankcase ventilation valve, hoses, and dipstick guide hose.
TSB – Deposits in the injection and induction system:
Deposits at the fuel injector’s tip – symptoms include hesitation or stumble during acceleration, loss of power, poor fuel efficiency, increased emissions, and/or CEL due to misfires
Carbon deposits at the valves or intake manifold ports – symptoms include loss of power, unstable and/or rough idle, increased emissions, and/or CEL due to misfires
Combustion Chamber Deposit Interference (CCDI) – pinging or knocking which may be mistaken for engine knock that first occurs as a cold start noise that fades as the engine reaches temperature; symptoms may also include increased emissions, poor acceleration, and engine idle speed surges