Causes of burnt valves? A couple questions. - PeachParts Mercedes-Benz Forum

MarkM · #1 10-21-2009, 06:34 AM

I know that not adjusting the valve clearances will eventually result in premature wear and burnt valves (requiring a valve job sooner or later).

But what are the other contributing factors? If the valve stem seals leak, this will result in oil consumption, I assume by having oil leak into the combustion chamber...but will this condition also contribute to early burnt valves...by "fouling" valves (build-up of carbon in the combustion chamber and on tops of valves, preventing the valves from seating properly)?

If there has be excess buildup of carbon in the combustion chamber and on valves, do any of the fuel additives do what they say...the ones which show pictures of a seriously "carbonized" valve (before) and a nicely cleaned valve (after), after use of the fuel additive...products like "Ventle Sauber" or "Techron", or "Diesel Purge" etc.....

Thanks,

Mark

oldsinner111 · #2 10-21-2009, 06:45 AM

Yes purge works to clean up gummed injectors.I have heard that to much Cetane boosters,can damage conecting rods,and engine.That goes for all diesels.

leathermang · #3 10-21-2009, 09:41 AM

MarkM, I think you are being a little less serious about the valve adjustment than may be good for your engine...
A too tight clearance on our engines can result in a burned valve in twenty miles of driving. HALF THE COOLING YOUR VALVE GETS COMES FROM BEING IN CONTACT WITH THE HEAD AT THE RIM OF THE VALVE.
So if sufficient clearance slowly gets taken away from whatever, OR you set it wrong... a burned valve can be a very quick tragedy.
The other factor which you may can change for the better easily and cheaply is to make sure your valve rotators are working and in good shape.... on some of our engines only the exhaust had them... but they can be installed on intake and exhaust easily when doing valve stem seals for instance.. no machining... just buy and install when putting it all back together.
They help by keeping deposits of carbon from potentially blocking the valve from good contact with the head...and help keep everything Round and matched to each other.

tangofox007 · #4 10-21-2009, 10:05 AM

Quote:

Originally Posted by MarkM

If there has be excess buildup of carbon in the combustion chamber and on valves, do any of the fuel additives do what they say...the ones which show pictures of a seriously "carbonized" valve (before) and a nicely cleaned valve (after), after use of the fuel additive...products like "Ventle Sauber" or "Techron", or "Diesel Purge" etc.....

Ventil Sauber is not intended for use in diesel engines.

oldsinner111 · #5 10-21-2009, 06:08 PM

Water injection after turbo,will blast everything clean.

MarkM · #6 10-21-2009, 08:07 PM

Quote:

Originally Posted by leathermang

MarkM, I think you are being a little less serious about the valve adjustment than may be good for your engine...
A too tight clearance on our engines can result in a burned valve in twenty miles of driving. HALF THE COOLING YOUR VALVE GETS COMES FROM BEING IN CONTACT WITH THE HEAD AT THE RIM OF THE VALVE.
So if sufficient clearance slowly gets taken away from whatever, OR you set it wrong... a burned valve can be a very quick tragedy.
The other factor which you may can change for the better easily and cheaply is to make sure your valve rotators are working and in good shape.... on some of our engines only the exhaust had them... but they can be installed on intake and exhaust easily when doing valve stem seals for instance.. no machining... just buy and install when putting it all back together.
They help by keeping deposits of carbon from potentially blocking the valve from good contact with the head...and help keep everything Round and matched to each other.

If I sounded like I discounted the importance of valve clearance adjustment, that is probably because I assume everyone does this necessary maintenance task regularly...I do it about twice each year (every 8K miles or so?).

Question....what is a valve rotator...are they on123 diesels? From what I recall, the valves on our 123 diesel engines have a stem, stem seal, spring, and lock/cap nuts..isn't that it? Or are rotators something that can be installed as after market?

Mark

Diesel911 · #7 10-21-2009, 09:35 PM

My year 123 has the Valve Rotators only on the Exhaust Valves.

Getting back to the Oil consumption and burnt valve issue.

If the Valve stems and Guides are worn the Valve will not go up and down straight. The wears the Valve Sealts crooked and does the same to the Valve Stem seals.
It also allows clearance for Oil to get through more easily.

Carbon deposits on the Valve Seats will hold the Valves open to some degree.

Where I worked we took in 4 Ford Gasoline Truck (not pick-up trucks) Engine heads from V-8s. Both sets of Heads had Valve Rotators on the Exhaust Valves. On both sets of Heads the Exhaust Valves had been worn down to Knive edges by the Rotatators

leathermang · #8 10-22-2009, 06:24 AM

Most of our engines have the rotator at least on the exhaust valve..
but it fits on the intake also... just a matter of taking those parts apart and installing them...
Some of our cars I think have them on both to start with... but they can wear and seldom get replaced or checked... and they cost very little... just like the valve stem seals or new springs... if you have it open it is a cheap great preventative maintenance measure.

MarkM · #9 10-22-2009, 06:46 AM

Quote:

Originally Posted by Diesel911

If the Valve stems and Guides are worn the Valve will not go up and down straight. The wears the Valve Sealts crooked and does the same to the Valve Stem seals.

It also allows clearance for Oil to get through more easily.

Carbon deposits on the Valve Seats will hold the Valves open to some degree.

Yes...what you describe addresses the challenges of running and maintaining a motor with 320,000 miles on it....there will be wear which results in various characteristics that need to be "managed" prior to reaching the point where you finally decide that it is time to remove the head and do a valve job. With my car, it has always been maintained very well....I have all records since first owner. So, while I have low compression (240 psi) in a couple of cylinders, the car does run well. I know that the valve guides are a little worn, but there is almost no blowby (no smoke from oil filler opening when engine is running), so valve guide wear is very minimal. Also, there is very little smoke from tailpipe (unless really stomping on the accelerator), so engine performance is good.

What I would really like to do is get another 100K miles out of the car without doing a valve job. My strategy is this...let me know your thoughts...

1) Worn valve guides will result in wobble at the valve stem and the valve stem seal, allowing oil to pass by the seal into the combustion chamber. But also consider that the current valve stem seals are original and probably brittle, affording a poor seal as the valve stem wobbles. I will install new valve stem seals to counter this and minimize oil leakage into combustion chamber. Valve guide wear is very minimal as evidenced by very little or no blowby, so new seals will probably minimize oil leakage. I have a new set of valve springs...I will might as well put these in when I install valve stem seals.
2) New valve stem seals will minimize leakage of oil into combustion chamber and will therefore minimize carbonizing of the valve seats. Since I assume valves and valve seats are a little worn (due to high mileage), I should adjust valve clearances more frequently than normal (three times per year, every 5K miles?), and I should adjust valve clearances a little more open than spec. to allow the valves to seat as well as they can (they will be allowed to bang themselves in if clearances are kept open)...new valve springs may help the valves bang in and seat as well as they can, but this benefit will probably be very minimal (old springs are probably just fine).
3) Because oil changes have always been done religiously (every 3 to 5k miles), I don't think there are major problems with lower end (rings are probably good), but as a preventative maintenance, I will use an oil additive with every oil change....NOT those products with some kind of leak stop crap, but something like Lucas products that say it is for freeing rings, upper cylinder lubricant, etc.

Thats it...I will just monitor compression each year, and hope for the best. If compression goes down too much, it will probably be the result of bad valves. If it gets to the point where I can't reliably start the car during cold winters, then I will need to do a valve job. (I would first do a leak down test to make sure lower end is good....if not, then I would have a real decision to make...I don't thing I want to get into replacing the rings...I have never done that, and I assume it is a big job...especially with no garage...I am a driveway mechanic).

Thanks,

Mark

leathermang · #10 10-22-2009, 09:41 AM

You have EVERYTHING apart....so ALL the labor is already invested when you have it down to where you can install the valve stem seals....

You have the valve springs sitting out on the table... they cost something like $5-7 each... have you done the math on how many times they are used per mile ?

If I did not put new springs into one of our diesels at this point , for whatever reason, I would NOT admit it to anyone. Same with making sure the valve rotators ... which keep the valve and valve contact are ROUND ... thus making seating better and even...

At least make sure you understand exactly what ' taken apart' place you are at when you get to the valve stem seals...

AND examine the cost of new springs and valve rotators ( and valve cap nut and locking nut as these are mentioned in the FSM as ' replace on principle'....because they affect how close you can adjust your clearances and how well they hold that setting ) before you put the engine back together..

moon161 · #11 10-22-2009, 11:54 AM

Quote:

Originally Posted by leathermang

At least make sure you understand exactly what ' taken apart' place you are at when you get to the valve stem seals...

Seals only can be done with the cam in place by pulling the tappets and minding TDC (this is critical) when removing valve springs. You could also put a touch of air on the cylinder through a leakdown adapter if you're nervous. An afternoon, evening and 3 are you done yet?'s for the first time. I have documentation to lend that fleshes out the FSM procedure considerably. Of course if the guides feel loose, that's a job to postpone until you really need or want to pull the head.

Norgermish · #12 10-22-2009, 03:54 PM

Quote:

Originally Posted by leathermang

Most of our engines have the rotator at least on the exhaust valve..
but it fits on the intake also... just a matter of taking those parts apart and installing them...
Some of our cars I think have them on both to start with... but they can wear and seldom get replaced or checked... and they cost very little... just like the valve stem seals or new springs... if you have it open it is a cheap great preventative maintenance measure.

You cannot just install a valve rotator on any given valve. On some engines they sit above the spring and work also as a retainer, i.e. Chevy, Ford, Chrysler and many others. Some later model Chevy's had them on both intake and exhaust, it didn't solve the intake guide wear problem in these engines. On the Mercedes' that I have built the rotators are under the spring. There is a pocket recessed in the head to allow for the extra spring tension caused by the rotator. If you put a rotator under an intake spring you will cause excessive pressure on the spring and may even bind up the spring when the valve is opened fully, causing the retainer to destroy the stem seal.

The main causes of burned valves are; improper adjustment, excessive valve guide wear, pre-ignition/detonation, lean fuel mixture. When either of the first 2 conditions exist you have a constant compression loss in that cyl. When the piston comes up and fires the fuel, the flame is compressed through the gap and causes a torch like situation....hence burned valve. Pre-ignition/detonation occurs when the piston is on the way up and the fuel is fired (pre-ignited) which usually results in holes in pistons, burned intake valves and over-all DETONATION of the motor.

moon161 · #13 10-22-2009, 04:46 PM

Quote:

Originally Posted by leathermang

You have the valve springs sitting out on the table... they cost something like $5-7 each... have you done the math on how many times they are used per mile ?

If I did not put new springs into one of our diesels at this point , for whatever reason, I would NOT admit it to anyone. Same with making sure the valve rotators ... which keep the valve and valve contact are ROUND ... thus making seating better and even...

The stress limit for infinite life exists in engineering school for some materials (steel) and is generally regarded as %50 of the elastic limit. Does this exist IRL? Someone with more cycles under the belt could chime in on this.

http://en.wikipedia.org/wiki/File:S-N_curves.PNG

A pretty severe failure on an interference engine tho. Keepers, nuts and springs can all be replaced in an afternoon with the head and cam in place.

leathermang · #14 10-22-2009, 05:06 PM

Quote:

Originally Posted by Norgermish

You cannot just install a valve rotator on any given valve. On some engines they sit above the spring and work also as a retainer, i.e. Chevy, Ford, Chrysler and many others. Some later model Chevy's had them on both intake and exhaust, it didn't solve the intake guide wear problem in these engines. On the Mercedes' that I have built the rotators are under the spring. There is a pocket recessed in the head to allow for the extra spring tension caused by the rotator. If you put a rotator under an intake spring you will cause excessive pressure on the spring and may even bind up the spring when the valve is opened fully, causing the retainer to destroy the stem seal.

The main causes of burned valves are; improper adjustment, excessive valve guide wear, pre-ignition/detonation, lean fuel mixture. When either of the first 2 conditions exist you have a constant compression loss in that cyl. When the piston comes up and fires the fuel, the flame is compressed through the gap and causes a torch like situation....hence burned valve. Pre-ignition/detonation occurs when the piston is on the way up and the fuel is fired (pre-ignited) which usually results in holes in pistons, burned intake valves and over-all DETONATION of the motor.

Do you own a copy of the paper Mercedes Factory Shop manual for the Mercedes engines I assume we are talking about.... 617,616 ?

Have you perchance called a local Mercedes dealer to inquire as to whether rotators can be installed on a 617 or 616 engine if they did not come with them ?

leathermang · #15 10-22-2009, 05:13 PM

Quote:

Originally Posted by moon161

The stress limit for infinite life exists in engineering school for some materials (steel) and is generally regarded as %50 of the elastic limit. Does this exist IRL? Someone with more cycles under the belt could chime in on this.

http://en.wikipedia.org/wiki/File:S-N_curves.PNG

A pretty severe failure on an interference engine tho. Keepers, nuts and springs can all be replaced in an afternoon with the head and cam in place.

I do not know what you are talking about...

I am saying that with an engine which has billions of rotations on it... if you have the valve train down to where you can replace the valve spring seals it makes no sense ( given how little they cost ) to put the old springs, the old valve caps and lock nut, back in putting it back together.... penny wise and pound foolish...
but not against any laws...