72 SL 350 US model (4.5) smoking - PeachParts Mercedes-Benz Forum

JSFmbSL · #1 09-22-2007, 08:50 AM

Here is my curiosity. Our US model 350SL had major work last summer (2006) including, new head gaskets, timing chain, chain guide, tensioner, new valve guide seals, tune up, valve adjustment, most of the things one would want done since the car turned 105,000 mi. It ran fine through this spring and summer and still does. However, in late June it suddenly started blue smoking from the tail pipe, more when cold, then frequently when taking off or idling but disappearing at about 35-40 MPH. Because it occurred suddenly, the suspected auto tranny vacuum modulator valve was inspected and it was fine. We found good Mercedes mechanics about an hour away (formerly with Mercedes dealers in the 70s) who inspected the car and felt that the valve guide seals were improperly installed last year. So they replaced them along with the spark plugs and changed the oil to 20W50. The problem is still there. They suggested driving the car for about another 500 miles until the residue cooks out. We have driven it more than that and the problem is still there. Is there anything else you suspect? Oil consumption so far seems nil to none. Car still runs fine but the smoke is annoying.

meltedpanda · #2 09-23-2007, 09:05 AM

If oil consumption is not an issue then a posibility is your dumping too much gas at idle, Did you have your injectors refurbished?
Your sure its blue smoke? Is your radiator fluid staying put?

JSFmbSL · #3 09-23-2007, 10:26 AM

When we got the car in spring of 2006. It had 98,680 miles on it. The former owner had all the injector seals replaced and they looked new when the car arrived. There was one injector however that needed replacing because it was leaking and seemed problematic. Othwise I don't know if the other 7 injectors have ever been replaced. It has been running fine this year so I don't know how to identify injector issues. Haven't kept track of gas consumption (MPGs). In early May the radiator was pulled and cleaned and reconditioned and it runs at good temps and does not seem to lose any coolant. The smoke is slightly bluish but not white. I cannot imagine that the piston rings would be an issue yet. When we were doing the oil changes we used 10W40 but the mercedes mechanic changed over to 20W50 on this recent valve guide seal servicing. Why would the smoking symptom show up all of a sudden in mid June this year when the "alleged" bad installation of valve guide seals in June of 06 never smoked the rest of last summer and into this spring?

meltedpanda · #4 09-23-2007, 01:28 PM

is it there all the time now?
If it is and your sure it's oil related then it could only be a few things, rings or, valve seals??? or a pressure issue??
Have you made sure all of the oil breather lines, and crancase vents are open and un obstructed?
Should have PCV in or around the cold start vavle and two breathers, one of each valve cover

Tomguy · #5 09-23-2007, 02:13 PM

It's possible one of your cam oiler tubes is leaking a tiny bit, and that oil is being sucked into the intake through the CCV tube. It'll produce a lot of blue smoke though not much real consumption.

ctaylor738 · #6 09-23-2007, 06:55 PM

Maybe the car really needed valve guides, and only got seals?

meltedpanda · #7 09-23-2007, 09:30 PM

At 105 K? , surely not

ctaylor738 · #8 09-23-2007, 10:38 PM

One would hope not, but who knows what happened during its first twenty years? Short trips, infrequent oil changes, run hard when cold? By your logic it should not have needed head gaskets either at 98K.

meltedpanda · #9 09-24-2007, 08:11 AM

AHH got me, but I am not know for my logic, which is part of the reason I have all these cars!

JSFmbSL · #10 09-25-2007, 09:58 PM

The mechanic service we used last year actually took us for an expensive ride. We did find later that much of the work we paid for was not likely necessary (and some of it was poorly done), but with our lack of knowledge of early 70s SLs, anything could have been padded into the "car's needs". This year's mechanics are more older Mercedes experienced (and come highly recommended) and they said the valve guides are in excellent shape. But they said that the seals were not properly installed last year and should be replaced so they did. However, the smoking issue is still around. They said put about 500-600 miles on it to see if it subsides before bringing it back. Its been almost 500 miles. I could check out the oil breather lines to see that nothing is obstructed. I just can't imagine it would be rings. This problem surfaced fairly quickly in June this year after not smoking ever since we bought it in Spring of 2006.

brazicano · #11 09-26-2007, 05:50 PM

I just fixed a bad smoking problem on a 81 380SLC that I purchsed that way. I pulled the plugs and found 5 of the eight were wet and not even firing, the other 3 were sooty black.
I checked the overvoltage relay, and it was bad- probably blown when the car was jump started ( had a battery with a dead cell) put in new spark plugs and replaced the coil after reading posts about weak ones- and the smoke and rough running went away !!!

#12 09-26-2007, 07:09 PM

IMHO,

Valve guide oil seals are a band-aid, the guides are soft bronze and I don't believe in mileage claimed on cars anymore, it's way to easy to turn back the clock. The M117 motor was prone to have valve guide wear, and using a vacuum gauge can help in the diagnostics of the issue. If the guides are shot, even new seals will let oil down the guides and into the combustion chamber.

A vacuum gauge reading would be my next step if it was my car, if the needle isn't steady, that indicates an issue with valve guides.

ctaylor738 · #13 09-26-2007, 10:30 PM

I know that I sound like a broken record, but your car has all the symptoms of worn guides.

I can come up with one other suggestion, however: have the head bolts re-torqued. If the other mechs were inept enough to screw up the seals, they may have not put the heads back on right

Earlybird · #14 09-28-2007, 03:45 PM

That car runs an EFI system that may be running rich. Smell your oil for gas and check for oil on the EFI cams and points under the ignition breaker plate. Oil on the contacts acts as if the points fire for a longer period of time and leaves the injectors open longer. Result is black smoke. Don't know if that's the problem but they all need cleaning sooner or later and yours seems to be creaping at the correct rate.

Cazz · #15 06-22-2011, 04:23 PM

My 350SL shows a similiar smoking problem as the one you discribed . No one seems to have any ideas as to cause . I've changed rings, had two valve jobs done within 1000 miles checked all the obvious but no luck. If you've had sucess fixing yours please let me know what you did.