An engine "sucks" a valve?

[ivymike]

Hello everyone

I've heard talk time and again from old mechanic types that various things will cause an engine to "suck a valve," and I've often wondered what they might mean when they say that, and whether there is any truth to the claims. Also I'm wondering what stories may have been true in the past and have since changed. Typical stories that I've heard:

"you shouldn't drive with a leaky manifold gasket, 'cause you'll suck a valve"
"you shouldn't install a muffler that flows too freely, 'cause..."

My questions to you:
* what (in your opinion) phenomena are referred to as "sucking a valve?"

* what are the commonly cited causes for these phenomena?

* how plausible do you think those theories sound?

* what (historical) engine configurations may have led to the modern tales about "sucking valves?"


My initial answers to the above:
"sucking a valve" could be:
1) valve breaks and parts fall into cylinder
2) valve comes loose and falls into cylinder
3) valve "tulips" and deforms such that it won't seat

the one "folk" explanation I've heard for 1 & 2:
"low pressure in the exhaust manifold allows cold (fresh) air in, resulting in thermal shock to the valve and fracture thereof"

Plausibility: I suppose that I can't rule this explanation out for the case where you have an extremely leaky manifold gasket (or valves exposed to fresh air), but it sounds like a completely implausible scenario for the "muffler that flows too well" case.

reasons why I think the above failures might happen:
1 & 2) corrosion of valve, broken retainer, etc.
3) excessive seating velocity (while hot) results in downward deformation of valve body

 

[evelrod]

Well ivymike, where should I start???

You already have all the answers. It makes sense to ask a question you already have the answers too I guess. It's a test , right?

Leaky manifold gasket---intake, lean mixture and probably BAD results---exhaust, noise and probably power loss.

freeflowing muffler(or no muffler) noise and probably constabulary problems.

Sucking a valve---I have heard it and used the phrase a time or two. Had exhaust overheat and tulip (Lotus Twincam) , and intake break at the head/stem weld (Nissan L-18 racing)

Most common causes---poor metalurgy, poor design, lightening valves too much, too high revs (valve bounce), too high spring pressure, failed retainer, broken or loosened seat insert, ad infinitum.

Plausable? Urban legends come to mind.

I can think of a couple of engines that gave me fits. The above mentioned Lotus until I changed to SS valves (Jaguar in origin). A very expensive set of racing intake valves (from a very promanent purveyor of racing stuff) that pulled apart. Several early Y-Block Ford V-8's that dropped valves because I overrevved them!

The rest you have down pat! Except that a leaky intake could possibly lean the engine out to the point of meltdowm, maybe??? A leaky exhaust gasket most likely will do nothing (had an early Chrysler Hemi on a water pump, running on natural gas with one of it's exhaust manifolds completely gone. It made it's own unique noise, but it ran that way for several days until we could fix it and then it continued to run 24/7 until I left that part of the world, about 5 years and with service only when the low oil sensor would shut it down!).


How'd I do, Mike?


Rod (An OLD mechanic type)

 

[ivymike]

evelrod,

Thanks for the reply. It's not a test. Don't ever let me make the claim that I have all the answers. The reason I posted was to see if anyone could poke some holes in my preconceptions, or if anyone could augment the knowledge that I had. I threw my answers out there because I wanted to get right to the stuff I didn't know (or the problems with what I did know). I've heard that the first step toward becoming an expert is admitting that you're not one.

thanks for the help,
Isaac (a fairly inexperienced engr type)