79 300D develops "miss" as it warms up

[Markus300D]

Hello fellow Mercedes fans! I picked up this beautiful 300D about six months ago. One owner, 345,000miles on the odometer. I figured I could I could do a little work on it and re-sell it. But since I've been driving it and have worked most of the bugs out of it now I dont want to let it go. This thing is lots of fun to drive and quite a step up in comfort and fuel milage from my '82 Toyota 4X4 pickup. I am a mechanic and have worked on diesels occasionally but most of my experiance has been with gasoline burners. This engine seems to have a bit of an uneven idle, especially when it is good and warmed up. The previous owner told me he replaced the engine about 100,000miles ago and it is only burning about a quart of oil every 3500miles so the engine seems pretty sound. When I first got it, I found a couple of tight intake valves (I noticed a puffing sound coming out of the air cleaner). I adjusted the valves and the sound went away. I also replaced a couple of seemingly leaking injectors, the other three tested OK at the time. Fuel milage is about 21-25 miles per gallon (I've been told they can do 25-30 MPG when run right, is this true?) Also, it seems kind of smoky and smelly at idle. The engine starts right upwhen it is cold and the "miss" seems to get more and more pronounced as the engine warms up, which is something I am not used to seeing. If this engine was not a diesel I would suspect a fouled spark plug or an intake manifold leak the way it idles. The "miss" carries on noticeably when the car is accelerated slowly and smoothly from stopped but as the engine speeds up the miss fades away into the general diesel rattle and I am fairly certain that it is firing on all five cylinders and it gets around nicely. It seems to have "enough" power (for a 300D) but it has been my experience that a smooth as possible running engine in original configuration not only has maximum power but maximum fuel economy too. Am I being too picky? This is such a sweet running car otherwise. And in general I find it a pleasure to work on (unlike those crappy American made cars that my repair shop is always full of.) The only machines I have found of comparible quality and easiness to work on are Massey-Fergusens, John-Deers, Allis-Chalmers, Ford-Fergusons, and Kubotas. But all those crappy cars keep the bills paid while the farmers mostly fix thier tractors themselves. Ah but I digress. Any help, ideas or references would be greatly appreciated. Cheers!

 

[television]

I do not think that you have much wrong, you must have the injector cleaner over there that you add to the tank, I would try that first, it does sound a bit like sticky injectors. Other than that you do have the EGR valve, this effects idle and light running, they are not worth trying to clean as the spring gets weak with age, if you take it out it should be closed, but they coke up inside.

Malcolm

 

[Markus300D]

Are you sure that a 1979 300D has an EGR valve? Where is it located? No I have not yet tried injector cleaner but I shall. I have seen postings where people make up a mini fuel tank and run a couple of bottles of cleaner through the fuel system undiluted by fuel. Is that a good idea or should I just pour it in the fuel tank? I have not had any other noticeable fuel problems in the 4000 or so miles I've put on it since I got ahold of this car but it did sit around outside unused for several years. Could Injector cleaner break loose any crud that may that may have built up in the fuel tank? Thanks for any thoughts as I am alone in the wilderness when it comes to people around here knowledgable about Mercedes much less diesels. Cheers!