EVERY CLOUD?
QEven wellmaintained cars like my Rover P4 seem to smoke on the overrun. Is there ‘smokefree’ oil, or do ‘stop smoke’ additives work? What else can be done?
Simon Piant (sic), Alford.
Matt says:
AOn over-run, the pistons are driven by the motion of the car and are pulled down the cylinders – creating an intense suction, since the throttle is closed. This draws in a certain amount of oil, principally down the valve stems. It burns, leaving blue smoke behind it. It’s very noticeable if the car has an over-run fuel cutoff valve – the oil builds up until the throttle’s depressed and the fuel turned back on, leaving a dense cloud at the bottom of every hill. Engines from the Seventies on usually have rubber valve stem oil seals that harden and become useless, and simply replacing these often cures the problem. Otherwise, the solution is to replace the valves and guides – at which point you might as well fit unleaded valve seats too. It shouldn’t be prohibitively expensive, and it’s a once-only job. We wouldn’t recommend a stop-smoke additive as they alter the carefully designed liquid characteristics of the oil.