Valve Job/ Cylinder Rebuild?

[baileysd46]

I have a 1971 280sl and in my first cylinder the spark has oil fowling. Engine is running extrremely rich I'm told.
Does anyone have a suggestions or help.
Thanks,
Scott

[ja17]

Hello,


First be certain that it is oil fouling and not just fuel fouling.

If your s#1 park plug is oil fouling you have a problem with the intake guide or the rings in #1 cylinder.  The rich running is most likely a different issue.

I would remove the valve cover and take a flashlight and look closely at the intak valve in  cyl #1 It would not hurt to take a engine compression test aslo.

Cylinder head removal would be the next step.

[Richard Madison]

At a recent "Tire Kicking" session in a private Mercedes repair/restore shop, the owner talked about a "Top End" job.

The typical symptoms are oil on one or more plugs; some grey smoke on acceleration from a stop; using more than normal amount of oil; one or more cylinders may stop firing. The car being discussed was a 280SL that burned excess oil and had a couple of cylinders that would stop firing after a few of hundred miles even with new plugs. The plug tops were oil covered.

Top End job requires removal of the Valve Cover and the Head to examine the Valve Guides. Each valve moves inside a Valve Guide. Guide is a two inch (?) long hollow cylinder made of brass or steel, if I remember correctly.

The Guides wear over time allowing oil to pass along the Valve Stem that can foul the plugs. While replacing the Valve Guides, the Valve Springs and other small parts can be replaced; and the Engine Block can be checked for being Level ("True"). Each Guide is installed forcefully (hammered) into the Valve Stem passage.

A well done Top End job can correct the symptoms in most cases but, as Joe says above, the trouble could be deeper and Piston Ring replacement may be necessary.

Cost of a Top End job in the US in July 2010 typically ranges from 1,500 to $2,500.

Richard M, NYC
  

[graphic66]

Could replacing just the valve seals help this problem? This I believe can be done without removing the head.

[ja17]

Hello,

Simply replacing the valve seals rarely fixes the problem, or is a temporary fix.  In rare cases it can fix the oil problem. Most often the root of the  problem is in the worn valve guides or other issues.

The best predictor of the problem can be the mileage on the engine and the service history. 90% of the time a top end re-build (valve guide job) will cure oil smoking and oil usage on "virgin engines" (no previou re-building)  that have  less than 200,000 miles on them. It is a good risk in these cases to do just do the top end rebuild.

Hopefully you will not fall into that 10% that requires more extensive "bottom end" work.

Here is how I rate chances of success on "vigin engines" with less than 200,000 miles and decent service history...

10% chance of success with valve seals only
90% chance of success with top end rebuild (valve job, guides and seal

I have been working on a good running 280SL. It had some smoking and oil use issues. After removing the valve cover and examing the seals (all good) we removed the cylinder head.  The car was a 1970 and the date cast in the cylinder head was"74" so we could see that the head had been replaced previously . Looking into the center cylinders we could see that there were  rings of horizontal pits. One cylinder had a vertical wear groove running up the cylinder wall.

The history of this engine became apparent at this point.  The engine had been overheated, blowing the head gasket and flooding the center two cylinders with engine coolant at some point in its history. The engine sat with the coolant in the cylinders for a period of time until the rings rusted to the cylinder walls.  At some point the cylinder head was removed and replaced. Then the engine pistons were forced free from the corrosion, breaking one piston rings  unknowingly. The broke ring and pits were left as is and the new head was installed.  (No record of  who was doing this work on the car and it was most likely  20 years ago). The car and its problems was then sold to the current owner.

The bad news is his engine is one of those 10% that need bottom end work. The good news is he has a excellen fairly new cylinder head to work with.

[Shvegel]

I had a similar issue with my 1970 113, When I bought it the engine was frozen so I rebuilt the bottom end and bolted the head back on as everything looked good there. The car ran great but if it idled for awhile it would let out a small cloud of smoke.  I changed the valve seals and everything felt OK valve stem wise but it still smoked so thinking I may have smashed a seal so I did them again. This time I was very thorough and  finally discovered that the exhaust valve guide had become loose in the head and was traveling up and down with the valve. I pulled the head and brought it to a friend with a machine shop and had all the valve guides replaced along with a good valve job and voila! No more smoke.

In retrospect I should have just pulled the head in the first place but believe it or not the cost of a head gasket was a factor way back then. Having done valve seals twice in the car I would never do it again. A clean head on a clean bench in a competent machine shop is the way to go. As Joe stated make sure the problem isn't fuel related then get ready to dig deeper.

[baileysd46]

I would like to thank everyone for the direction you have given me. This 280sl is a 1971 and is 3rd to the last made. I really want to do what is needed and will keep you posted.

Thanks again,
Scott