250 SL Oil Consumption

[Atazman]

I would appreciate some information and guidance on what I might be facing.......

I installed a new MB shortblock in my 250 SL, and I had the head overhauled at a machine shop.  The engine now has about 60,000 miles on it.  Not long after the above mechanical work was done, I noticed some smoke in the exhaust and an increase in oil consumption. It is now using about 1 qt in 500 miles.  The car runs great, and it is not fouling spark plugs. I don't see any difference on the spark plugs between cylinders, and compression is good and even.

I replaced the valve stem seals and there was no significant difference in oil consumption afterward.  However, I did find an intake valve guide loose in the head.  Basis this finding, I'm thinking that there may be other valve guides loose, and possibly some ball studs loose where oil might be escaping directly into the exhaust, thus explaining why my combustion chambers look clean.

My plan is to remove the head and do whatever work is necesary to insure no oil leakage.  But first, I would like to know if there is an effective way of tightening the valve guides into the head without removing the head.  I know.....I'm probably just dreaming here.  When I found the loose intake guide, I tried "peening" the top of the cylinder head around the valve guide to anchor it, but I have no way of knowing how effective this was.  

And next question, ....Is there quite an assortment of oversize valve guides available?  I'm assuming that I will need guides larger in outside diameter than what I now have installed.  

And lastly....... what kind of an interference fit should the new guides have when pressed into the head?  I'm thinking this is where my machine shop made an error.

Your thoughts are appreciated.

[al_lieffring]

Don

If a valve guide is loose, it doesnt matter if you change the seals or not, the guide moving in the cylinder head when the valve opens and closed can actualy work like a pump and inject oil into the valve port. If the guide is for the exaust, it will not show up on the spark plugs because the oil is leaking directly into the tail pipe.

There were all kinds of ways people have tried to repair loose guides without removing the head, and from what I have seen they were all a complete waste of time.

Back when I was rebuilding heads there were guides in several repair sizes and then several oversizes. The repair guides were slightly larger to fit tighter than the original and the oversize were for guide bores that needed to be reamed oversize before the new guide was installed.

I often got the largest oversize guides and would cut them down on a lathe to the size I needed to fit into the bore.

It's not uncommon for these engines to need the valve guides redone at 60k miles. So this may not be any fault of the machine shop that built your head.

Al

[Benz Dr.]

I've had to get valve guides made if the hole in the head was too large or chewed up. I once saw a head where the valve guides were knurled to make them bigger. Boy did that thing ever drink oil!
I think a fit of about .001'' is kind of where you want the guide to fit. We heat the head in an oven and then fit the guides after leaving them in the freezer. Never use a mandrel to knock them in as it will distort the end of the valve guide. Perfectly installed guides will hold the valve with only a small amount of play and yet the valve will move freely and drop into place.
I've found that most smoking or oil consuption problems can be traced back to the head. Only a very worn bottom end will have high oil use and smoke.

[ja17]

Hello Don,

Dan and Al have a good grip on this situation, their experiences bring back many similar  experiences that I have dealt with also. Yes, the problem is most likely in your cylinder head only. Normally if an intake guide is loose some oil will leak into the combustion chamber and show up on fouled spark plugs and be burned off and turned to smoke out the exhaust.  If the problem is a loose exhaust guide, (which is more common), the engine oil drops into the exhaust port and is blown out the exhaust after turning to smoke. In this case the spark plugs do not foul.

Replacement vlave guides are available in three or four oversizes. New valve guide metal alloys are even beter than the original materials and should last longer. Find a good shop to do the machine work and you should be able to get many many more years and miles out your engine.

[Atazman]

Thanks very much......Al, Dan, and Joe, for sharing your knowledge.  I find your comments very informative, as well as comforting.  I was not looking forward to tearing into the bottom end of the engine to investigate my rings.  On the other hand, I'm eager to get the cylinder head off for repairs.

Hopefully new valve guides will be available in the size I need.

One further question would be the fit of the guide into the cylinder head.  I believe a fit of 0.001" was suggested.  Just to clarify....would this mean that the guide outside diameter should measure 0.001" larger than the hole diameter in the head when the head is hot and the guide is cold?  Or is this 0.001" fit measured with both the head and guide at room temperature?  If the latter is the fit I'm looking for, the cold guide should slide right into the warm head without using a mandrel. I'm guessing this is the intended procedure, but I want to double check.

Thanks again.....

[ja17]

Hello Don,

This is just physics and not unique to Mercedes. A good experienced manchine shop should have the procedure of installing guides in an aluminum cylinder head down pat.   However I realize your concerns. I always supply my machine shop with the correct specs from factory literature just for insurance.

The factory workshop manual lists all the clearances, and oversides guides and also goes into detail of installing them.  The factory "Technical Data Manuals" also contain all specifications.

Most machind shops have modified the procedure to suite themselves. For example, I believe the factory freezes the guides in liquid nitrogen before installing them. Other alternatives are heating the heads to operating temperature and chilling the guides in a freezer or dry ice, which is not as cold as the liquid nitrogen but heating the head makes up for most of the difference. The machine shop may allow a bit more of an interference fit to make up for it also. I have seen some shops center punch the guides on the outside and use a bit of locktite to make sure they stay put. Just another shops method. The secret is to find a good shop experienced with aluminum heads.

Don, if you do not have the factory specs let me know and I will list most of the information.

[Benz Dr.]

I believe you would be looking at .001'' with both the guide and the head at the same temp.

I've been asked about engine building costs and there's a basic cost for almost everything if the repair is 'basic.' However, it rarely is. Every engine wears out but they tend to wear differently due to driving and maintanance conditions. Some engines show little wear at 100,000 miles while others are junk.
 
The parts for a basic rebuild tend to stay the same. Oil pump, chain, rails, guides, pistons, bearings, gaskets, water pump, chain tensioner and a few other pieces are the parts in a regular rebuild.
Once you start getting into machining it can be a totally different thing. Align boring, crank straightening/ballancing, sleeving cyinders, straightening cyinder heads, machining top of head, installing new valve seats, making oversided valve guides, new valves, ball studs with rockers can increase that cost considerably.
I've had to do all of these things but never on one rebuild. Each build is a custom job. Some are 'basic' and some are very difficult.

[Atazman]

Getting back to you guys.....Joe and Dan.  As before, thank you for taking the time to respond to my questions.

You folks have triggered my thought processes, and I realize that I have "jumped the gun" in asking these detailed questions at this time.  It would have been much more appropriate for me to get the head off...see what my problems are....and then seek advice for the best way to get the pagoda back on the road.

All along, I was thinking that newer valve guide metallurgy is available and should be used when I refurbish the head.  And again....I thought that installation procedures might vary, depending upon which guide is selected.  Maybe not??  Whatever I end up doing, I want to know the thoughts from this forum before I go to the machine shop and end up at their mercy.

So.........I'll be getting back with you when I get the head off and understand the magnitude of my problem.  I'm afraid this will not happen for another couple of months.  Sorry for my poor timing with the previous questions.

[Benz Dr.]

Don, it's not poor timing - it's normal thoughts about what you might find and how it will be fixed. Never hurry when it comes to this work. I've been bit enough times to know that you can't hurry this type of work and that it will always come back to get you if you do.

[Atazman]

Hi gentlemen…….  I’m following up with this old thread regarding oil consumption in my 250 SL.  I have just removed the head, and found several things that cause questions.  I need some advice before proceeding.  But let me provide some history first………….

A new MB short block along with a rebuilt head was done in 1984.  The valve stem seals used in that head assembly were white Teflon on the intakes and black Viton on the exhausts.  The exhaust valve guides had the groove cut to hold the seal in place.  These exhaust guides are consistent with the MB Service Manual which says that 250 SL engines after 000618 should have the exhaust valve guide with the groove cut in the top, and use exhaust sealing rings manufactured by Freudenberg. My engine number is 129982-10-001704.

Oil consumption since 1984 (after 60,000 miles) has dropped to something less than 500 miles per quart.

In 2003, I made an attempt to solve the oil consumption problem.  My focus was on the head (not bad rings), so I removed the Cam Cover to make a head inspection.  As stated above, the valve stem seals were white Teflon on the intake guides and black Viton on the exhaust valves.  These seals actually looked good.  The only problem I found (again….this is back in 2003) was that the cyl # 3 intake valve guide was loose in the head.  I temporarily coated the outside of the guide with JB Weld, staked the head around the guide with a center punch, installed MB valve stem seal kit # 108-586-00-05, and ran the engine until today (2/12/09).  Obviously, I should have questioned this back in 2003, but that was before finding this forum and I just trusted MB to give me the correct seals for my engine.  So the question………. Is the MB valve stem seal kit # 108-586-00-05 intended for the newer 250 SL engine?   I ask this because both the intake and exhaust seals are all white Teflon type.  I cannot tell whether the exhaust seals have the internal ridge for the grooved guide.

When I removed the head today, I found that all of the exhaust valve guide seals had “popped” off the guides.  Intake valve guide seals in 3 cylinders had also “popped” off, and these 3 cylinders had copious amounts of black, oily deposits.  The other 3 cylinders had the intake valve stem seals in place and the combustion chambers were clean.  Pretty good evidence that oil is going past the intake valve guides.

The MB Service Manual specifies using valve guide seal installation tools.  So here is my 2nd question………Do you guys use these tools (one for the exhausts and one for the intakes) or are there other techniques equally effective?  Where do I get such a tool?

When I inspected the head today, the cyl # 3 intake valve guide that was loose back in 2003 and “temporarily repaired” seemed to be in place.  This was one of the cylinders where the intake valve guide seal had stayed in place too, and the combustion chamber was good and clean…….free of oily deposits.  Obviously, I will have this guide replaced before the head goes back on.  But my 3rd question is this…………how can I tell if the guides are loose in the head? They feel tight, but I did not tug on them with pliers and I did not drive on them with a mandrel/hammer to see if they moved.  I’m afraid to do that!!

Aside from the possibility of loose valve guides, it appears that my oil consumption problem may be due to the following:  (1) wrong installation technique on the valve stem seals since I did not use the MB Service Manual recommended installation tools and/or (2) the wrong seals were installed in 2003.  I see no evidence that oil leakage may be going past the ball studs.

I appreciate your comments and advice on how to get this little jewel back on the road.

[Benz Dr.]

I believe there are two types of valve stem seals for our cars. The ones that are used for 230SL's and the ones used for 250 and 280SL's. I use the valve installing tools which prevent damage to the valve seal. This tool will drive the seal all the way down where it will grip the valve guide firmly.
The 230SL seal has no groove to hold it on and it relies completely on tension or fiction. These seals MUST be installed dry. Any oil on the valve stem seal will cause it to pop off and sometimes before you have the springs in place. The later valve stem seals have groves which helped considerably in this area but I like to install them dry as well. I think the late seals have two different materials as stated earlier.
I would measure the valve stems and check for wear. You want about .0015'' clearance between the valve guide and the valve stem. This will allow for free running but still hold the valve squarely in the guide. Any more than that and the valve will be loose and wobble. Any less than that and it will bind and possibly damage the guide. A loose valve in the guide will damage the valve stem seal so putting seals on a worn guide is a waste of time.

 A good cylinder head machinists isn't easy to find and once you see the difference between done right and thrown together you'll know why. Done right they're a dream. Done wrong they're a nighmare. This is where all the magic happens in your engine and I've found the right guys after looking for a while. The first head they did for me was a 190E 16 valve. It came back looking like it just rolled out of the Cosworth factory and it was really beat up with three burned valves and seats that were smashed flat. What a mess!
The thing I like about these guys is that they're gear heads. You can ask them any question and they'll tell you how it works as long as they know you're getting it. They refuse to do it half assed or wrong because you want it that way. They'll simply hand it back to you and point you in a different direction.
Part of good work is dependant upon the skills of others. I've always maintained that a shop has to at least meet or exceed my expectations. If they mess up I give them a chance to sort it out ( everyone is human ) as long as it didn't mess me up too much. If they do it to me too much I don't complain, I simply stop doing business with them and go somewhere else. Once I find a place that I like we become friends sometimes but we put our interests first.

This is a small community of people that do my sort of work. We tend to know each other or at least know about different places within the old car comunity. The better places maintain the respect of those who know them. We tend to be colleagues and not necessarily competitors. Larger commercial operations running newer cars are very different because they move a lot of cars through each week and it can be very completive. Guys that I know in the old car biz tend to be more laid back than that.   

[Atazman]

Thanks, Dan, for sharing your thoughts.  I'm about ready to take the head to a reputable shop......and hope for the best.

[glenn]

Oil consumption: Per the 'Owner's Manual' 250SECoupe(same engine?) page 23 - 'approx 0.15-0.25 ltr./100 km'.  A quart or so/300+ miles???   Also, 23 mpg at 60 mph for gas.   Anybody get these kind of numbers?

[J. Huber]

Well, I use less oil and more fuel... usually never add more than a quart (often just 1/2) between changes which is about every 2500-3000 miles. MPG fluctuates according to highway or city but generally I am in the 15-16 range. If I take a long highway trip -- just highway -- I can get in the 20s.

[Benz Dr.]

I had a '66 250SE Coupe with a 4 speed gear box. I used to get around 23 - 24 MPG at road speed. These were imperial gallons.
I took that engine out and using only the short block and intake manifold I installed that into the rocket. It uses 10 litres per 100 KM or about 28 MPG. I'm sure the 5 speed accounts for most of it while the rest is engine work.