I have a non-turbo 1978 300CD with only 145k miles. It has been running on vegetable oil for the last 8k (entire time I've owned it). I do not remember this problem from when I bought it, but in general the engine is operating better now than any time since I go it. Plenty of highway uphill power (for a nonturbo), quick starts, etc.

I do not get excessive smoking at any time other than this:

When I am idling at a light for 2 or more minutes (Bay Area traffic makes this very frequent), and then accelerate moderately, the car is smoking much more than usual. I believe it's bluish. After a minute or less the smoking goes back to where it's not noticeable.

Cold startup has a slight smoking, but not enough for me to worry about, as it quickly stops. I do burn about a quart every 1000-1500 miles, depending on driving conditions.

What I would like to know is the most likely areas of concern. I would love to fix what's causing this, but not if I have to rebuild the whole motor.

One person thought it might be a valve guide problem, and that if it were the rings I wouldn't get good highway uphill power.

I haven't had the valves adjusted since I bought the car (and don't really have any knowledge of maintenance done in the past). Would that possibly make any difference?

I would be helpful if you told us what type of "vegetable oil" you are running. Are you talking about WVO or SVO? Blended? Heated?

congrats on running veggie oil... all i run is veggie oil...

my veggie oil smoke is white... and it smokes very little...
the blueish smoke is oil.... diesel smokes black....

In the old diesels the valves tighten up against the cam shaft which cause them to be open more... resulting in some oil seepage...So when you adjust them you will probably not be able to get the feeler guage in.

here is the link showing you how to adjust them..

http://www.dieselgiant.com/valveadjustment.htm

Now if your valve guides are bad then that calls for a machine shop and head job... probably your valve guide seals are bad... they can be replaced without removing the head.... there inexpensive... and would definately cause oil to seep into the cylinder....

It's waste vegetable oil (WVO). Mostly soy, some canola. It's heated with a standard Neoteric/Plantdrive setup with a Vegtherm. The temperature at the IP varies in the 120-160F range. I also have heated hard lines with the Fattywagon heater before it hits the injectors. As far as I can tell, it's working very well. As I said, it's operating better than when I got it and did the conversion. I am also not sure that this smoking problem didn't exist from before the conversion (I converted immediately after purchase. In fact, I've never put anything in it except WVO since I got it.).

congratulations,
Your a pioneer and its a wonderful thing your doing...

adjust your valves, and then check to see if it smokes on start up..
after it sits all night long... that is a good sign of valve guide seals...

On a mercedes the valve guide is a brass tube pressed into the head.
on top of that tube is a rubber seal. So you there are some folks who
remove the valve spring and hold the valve in place then slide the new valve guide seal in place....I have never done this I always had the head totally reworked....but i here this is possible and i might try it..

for my car they cost 27 bucks.... I have an 83 300sd
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1IZ1F9XRI1U20P2JO9&year=1983&make=MB&model=300-SD-002&category=A&part=Valve+Stem+Seal+Kit

if you decide to do the valve guides... in the photo there is a clear
white plastic thing... that thing goes over the top of your valve..
then you slide the valve guide over it... that is designed to protect your
valve guide from rips and tears as you start it over the valve...
you reuse it over and over...

The cold start smoke is white, I think.

According to that article, I should have the valves adjusted every year or so.

So I will have the valves adjusted (I don't do that kind of work myself, but I have an old timer mechanic who is very good).

Can the seals be replaced at the same time, with just the valve cover needing a new gasket or is it deeper inside the engine requiring further disassembly than what is required to adjust the valves?

yes they can be done at the same time...
in fact doing the valve guides will requrie the valve adjustment..
so you would be costing yourself if you did it any other way.

The standard recommendation is to adjust the valves every 15,000 miles, which is probably about once a year for most people. You do want to do it according to the recommendation because the valves do tend to get tighter over time. I agree, that is a good time to do the valve seals, since you are in there anyway. It will cost less in labor to do both.

1984 300d Trbo 139
Order The Valve Seal Kit $78.50 From **************.com, 10 Seals For The Turbo Eng, Push On Tool, Slip On Covers, Extra Lock Nuts,Guide Book, Order The 27 Mm Socket To Turn Engigine Performanceproducts4benz.com 3-6 Hrs Diy THEY HAVE THE 2 14MM VALVE ADJUST TOOLS FOR THE ADJUST NUTS AND LOCK NUT

why not order the valve seals right here on this site...
and just pay about 28 bucks.... take them to the mechanic yourself
and go armed with the knowledge you gained right her..