Couple years ago I brought 4.0 head to machine shop. It had burnt valves and this is what shop owner told me.
Gasket leak or crack in exhaust manifold ruin valves.
I don't know if that is true or not.
Is it?
Same thing about partially clogged fuel injector.

 

Burned exhaust valves are usually from running too lean, not seating correctly, bad head rebuilds, swapping cams without paying attention, weak springs/valve float..that sort of thing IMHO.
I can see a clogged injector causing problems, a cracked exhaust manifold or leaking gasket I'm not so sure.
Just from personal experience running open headers and owning a motorhome with a Ford 460 that's had a leaky exhaust manifold for 10+ years (don't they all?)...no problems.
I suppose it's possible but can't come up with a logical theory to support it.

 

engine guy said oxygen goes in through crack

 

and gets pushed out through the exhaust?

 

I would tend to think, a modern vehicle's electronic controls would compensate to some degree.
However, it is possible:
The crack would create a venturi.
Which would pull in air from the atmosphere;
creating a increase in back pressure.
Which could over heat the exhaust valve.
This could lead to a lean burn condition, causing even more severe damage.

 

It is hard to believe, but air gets sucked in exhaust manifold crack and gasket hole.
It is also decreases O2 sensor voltage

 

How is air, entering the exhaust system after the valve, going to affect the valve(s)? How does air enter an exhaust system.....isn't it called an exhaust leak (exhaust exits), not a air leak?

 

The exhaust pulses - goes from pressure to vacuum. That's how headers work. Air can get sucked in during the vacuum portion.

A leak after the valve - gasket etc - can suck in fresh air, it may get on the O2 sensor, making it run rich, not lean. Mileage drops drastically.

If it's cracked enough, or a piece is missing - when you shut the engine off cold air gets into the valve, - cold air - hot valve - it may cause the valve to bend or warp a little. Over time it'll burn from not seating properly.
That's why you see parked race cars with tennis ***** in their pipes - not rain, cold air!

A partially plugged injector can make it run lean on that cylinder - too hot, eventually burning a valve. But usually the computer richens them all up - result is one is slightly lean, the others are too rich. Mileage will drastically drop.

 

Great... thanks for bumping the hairline crack in my exhaust manifold higher in the list........

 

Don't all multi-cylinder motors have valves at different phases of open when the motor is turned off?

 

Perhaps engine guy is ON crack?

If cracked exhaust manifolds on 4.0s caused valves to burn, we'd almost all have burned valves, wouldn't we?

 

My engine guy was repairing engines for 40 years.
He also has 1911 in his desk.

 

Then almost all 4.0s should be suffering from burned valves then if the machinist is correct because cracked exhaust manifolds are extremely common and the Jeeps are regularly driven for years with cracked manifolds.

In actuality you will hear very little ever about 4.0s burning valves. That's because it is uncommon. Explain that please. Or would you rather defend the opinion of one person versus the proven facts stated above?

 

If you read my first post, I said I didn't know if it was true or not.

 

But in Post #12, you appeared to defend his opinion when you stated he's been in the business for 40 years. I just wanted to set the record straight.