Misfire?

[superfans]

Another stupid question, hope you won't mind. Why would a sticky hydraulic tappet or tappets will cause a misfire or backfire upon acceleration? Thanks in advance.

[geza]

The cam lobes actuate rocker arms or work directly on the surface (top side) of the tappet. This means that tappets stick after the valve is opened - and if open (by being stuck) far and long enough, the piston crown will hit the valve head and force it back on the valve seat.

Misfiring and backfiring are both possible symptoms. A compression test on each cylinder may help isolate any tappet issues - but you should also get the opinions of very seasoned Porsche technicians on your specific model car.

[superfans]

Dear qeza,

I understand that if a tappet is stuck, then the valve may be slightly opened because of the shortened length. However, I am not sure why a misfire or backfire will occur when the piston crown hit the valve which force it back on the valve seat. Could you please explain a little bit more? Thanks in advance.