smoke on deceleration

[Markus]

Hello all.  I have a perplexing issue that I haven't quite got a grip onto yet. Freshly rebuilt 71 280 sl engine, that I had sleeved to keep engine nos original. I believe the fellow at the machine shop told me he used 190d sleeves in the block, and the pistons are Mahle, with Gotze rings.  After driving at sustained speeds  (above 45 mph) with engine warmed up and then coasting to a stop, blueish, whiteish smoke is produced from about 2800 to 1500, sometimes more, sometimes less. After 980 miles I thought the rings would have seat. I retorqued the cylinder head at 800 miles, set valves again, and changed oil to straight 30 SAE weight from 10-40 multi-grade (no synthetic) Plugs are perfectly gray/tan not oily. The car runs perfectly with the exception of the mosquito dusting experience. Oil is getting sucked into the cumbustion chamber somehow.(rings, valve seals, ball studs)  Has anyone had the same story? How do you determine where the oil is getting past?  I am reluctant to try some oldtimers advice of trickling "bon ami" or "comet" into the intake to seat the rings. Yikes! Are the 190d sleeves so hard that the "gas" engine rings will never seat?  
Sleepless in Dallas

[Benz Dr.]

How much adjustment do you have left on the ball studs? Once you run out of adjustment the geometry on the rockers will be all wrong. The valve stems will push against the valve guides and ruin them very quickly - in less time than it takes to warm the engine up.
 One way to resolve this is to put shims under the cam bearings or use new valves which will lower the valve stems far enough to give you some adjusting clearance.
This problem can be severe or just a bit of smoke at idle with a puff coming out at throttle up. The plugs will be clean if it's exhaust valve guides and there will be almost no smoke during hard throttle up with the odd puff while coming to a stop.
If it's intake guides you will see more smoke as you down shift from higher speeds which creates high vacuum inside the intake manifold. This will pull oil down the intake giudes.
If one of the intake valve stem seals pops off ( more likely on a 230SL ) you will have a steady puff of smoke at idle as well as smoke all the time while driving. A steady stream of smoke at idle is more than one valve guide or several all at once.

On full rebuilds it's almost always the cylinder head that it as fault unless a ring got broken during assembly. Since everything is new in the bottom end and only some things get replaced during the cylinder head rebuild it's the most logical place to start looking for problems. So far, I've found that all smoking problems traced back to worn out cylinder heads.
Check the thickness of the head which should be more than 84 mmm. I also check the depth of the valve seats in the head using a new valve as reference. New seats can be installed. If the valve face is worn away replace the valve. Almost all intake valves will be worn out and there's not much you can grind off to begin with. The exhaust valves are thicker but have the same amount of material as the intake valves that can be removed. Pitting will usually be more pronounced on exhaust valves due to the high heat on the valve face.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Markus]

Thanks for the quick response and insight BenzDr, the ball studs are all new, and all new rocker arms, new push plates, all new exhaust valves, some seats new too. When adjusting the valves the intakes were tight and the exhaust loose, all studs seemed to have plenty of adjustment. Head was in spec with plenty of meat on it as per machinest. Would a cracked ring smoke all the time, or just on deceleration? This engine smokes only in the deceleration mode, when hot. Hard acceleration or idle, or high rpm reving and then closing the throttle does not produce any smoke.  I find that it smokes more now than, within the first 100 miles or so. Bizzare. The engine is smooth,quiet and produces plenty of power.

[ja17]

Hello,

Yes I agree with Dan, most likely in the head.  If oil is passing the rings you will see oil deposits on the spark plugw.  If the problem is the exhaust guides or seals, the oil simply drops into the exhaust port and exits through the exhaust without ever going into the combustion chambers.

Take a good light and check the valve seals through the springs to make sure none are off the guides.

Watch the oil consumption to verify the smoke is oil.

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

You would have signifigant compression loss with a broken ring. These engines only have one compression ring so any damage shows up quickly. This could only have happened during installation.
Do a dry and then wet compression test on each cylinder. Most of them should be within 5 - 15 PSI of each other. Small differences in conecting rod length can affect compression ratio but it's nothing to worry about. Small amounts of leakage past the rings and valves is pretty common on fresh engines until they seat in. I've found it can take a while before the engine will really pull hard.
Smoke only on decelleration is pretty much intake guide problems or at least oil getting into the cylinders. Is your car auto or standard? Small posibility it could be a very small crack in the modulator diaphram. ATF produces a lot of white smoke and it smells differently than engine oil. Is your trans fluid going down or your brake fuild?

Strangely, the oil consumption will be quite low on freshly rebuilt engines that smoke due to everything having relatively close tolerances from just being rebuilt. Even very small amounts of oil will produce a lot of smoke yet not hurt anything as long as nothing is major wrong. When you see a NASCAR blow up with huge volumes of smoke it might only be a small hole in one piston. Of course this happens at 8,000 RPM.......

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Markus]

Vehicle is an automatic, that was rebuilt in last few years and rececntly driven when restoration complete. No drop in ATF, no evidence of oil in vacum line from trans to manifold. No drop in brake fluid, and vacum line to manifold from booster also nice and dry.
  I will check valve seals (take a while) first its off to Kelowna to pick cherries, apricots, peaches and go camping with my daughter! A man must have priorities after all.
Cheers

[Markus]

Well I think I may have found my problem.  I noticed after recent tune up tinkering that in my zeal to achieve 2.0/3.5% CO the idle air mixture screw was only 3/4 open from fully seated.  If the throttle valve is closed/lightly seated and the idle air screw almost closed the suction created from highway coast down would suck oil past anything. I unscrewed the idle air significantly, retimed to 8 ATDC, 4 clicks clockwise on the FI pump idle adjust, and now it idles at 850 rpm, 35 degrees at 3000rpm, CO 1.3%, smoke on deceleration is now hardly noticeable, if at all. Retarding the timing to BBB value has made the accleration considerably more "doggy" though, and of course it runs hotter.  Have I damaged anything?

[hands_aus]

quote:Originally posted by Markus

Well I think I may have found my problem.  I noticed after recent tune up tinkering that in my zeal to achieve 2.0/3.5% CO the idle air mixture screw was only 3/4 open from fully seated.  If the throttle valve is closed/lightly seated and the idle air screw almost closed the suction created from highway coast down would suck oil past anything. I unscrewed the idle air significantly, retimed to 8 ATDC, 4 clicks clockwise on the FI pump idle adjust, and now it idles at 850 rpm, 35 degrees at 3000rpm, CO 1.3%, smoke on deceleration is now hardly noticeable, if at all. Retarding the timing to BBB value has made the accleration considerably more "doggy" though, and of course it runs hotter.  Have I damaged anything?

Do the test for correct mixture......
disconnect the inj pump linkage from the cross linkage.
open the throttle slowly, if rpms increase ~>100rpm mixture is too rich.
Increase fuel supply slowly, if rpms increase ~>100rpm mixture is too lean.
look at the colour of the exhaust pipes after a run.
sooty black.....too rich
white......too lean
silver gray....ideal

if the mixture is too lean the engine will be hotter.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Raymond]

Markus,
I'm a bit late on this but I had exactly the same experience after rebuilding my 280.  Identical symptoms at about the same mileage.  I even bought a new set of valve seals thinking that was the problem.  Getting the mixture, and timing right and 1,000 miles on the rebuild stopped the smoke.

We're so used to problems being major, that sometimes we miss the minor.  There is a reason so may threads on this forum deal with mixture issues.  The FI system those German elves conjured up is the essence of Pagoda mystique.  When it's wrong it can be a curse, but when its right it's magic.

Ray
'68 280SL 4-spd Coupe