Smokey exhausts

B

bodlan

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"If you get blue smoke coming out of the exhaust on the overrun it means worn valve guides. If it comes out whilst accelerating it's the rings."
This my old Dad told me when I was 16 and I believed him. I have just passed on this snippet to my son but guess what. He wants me to prove it. He wont believe that just because Grandad say's so it must be true.
So firstly, is it true? And how can I explain it to him in a more technical way that he might believe?
 
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1660bob

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I was brought up on the same maxim. As far as I know the reason for smoke on the overrun being indicative of valve guide wear is that, with the throttle shut (hence overrun), a partial vacuum develops on the inlet side of the engine, as the piston is trying to draw air/petrol, but cannot draw much due to the throttle being shut. Thus air, and oil, seeps into that vacuum in the inlet tract via the (worn) guide. The oil is ingested by the engine, burns in the combustion chamber, and emerges as blue smoke . Its why so many engines have rubber valve guide seals, on the inlet guide especially.
As for worn rings, the ability of the rings to seal out crankcase oil effectively diminishes with the wear until again, sufficient oil burns in the combustion chamber with the petrol to cause blue smoke, and emerges with the exhaust fumes on acceleration. Quite why crankcase oil does not burn on the overrun, I`m not sure, perhaps someone wiser than me can answer that one, Bob
 
roy the mechanic

roy the mechanic

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Crankcase oil does not burn on the overrun because there is no pressure to "blow-by" without combustion! (throttle shut not trying to make power)
 
Bill Thomas

Bill Thomas

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So what is it, After a Brisk ride to the Forest, On my poor old Comet, Sat there at tickover, Chuffing out smoke like a good un !!, Cheers Bill.
 
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