 | |||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
Mercedes Diesel Service NotesPosted 11/12/1998By Rich Cregar
Mercedes built the first production passenger car diesel in 1937. The overhead cam diesels were made without basic change from the mid 1950s until 1985, except in 1975 when a 5th cylinder was tacked on the back, which created the venerable 5 cylinder 3.0 liter diesel. These are the series OM616 (4 cyl) and OM617 (5 cyl). They are magnificent engines and Mercedes sold a lot of them, especially in the United States. You won't see as many of the replacement series, OM601(4 cyl), 602(5 cyl) or 603(6 cyl), because of the general decline in the diesel market, and Mercedes had problems with emissions certification in the late 1980s, but there are still quite a few out there. Mercedes denotes its diesel engines with the prefix "OM," meaning oil motor because, literally, these motors run on 'oel' (diesel fuel). You can group them into four basic types: Engines made prior to 1984 (616 & 617 series) turbo and non turbo; and engines made after 1984, turbo and non-turbo. For purposes of this article - but not to offend the purists out there - I am lumping all the older 4-cylinder diesels (such as the 200s and 220s) into the 616 category, which is analogous to the 240-D model line. The 617 motor is turbo or non-turbo. It's easy to identify any of the older engines - they all have cast iron cylinder heads! 1984 and newer diesels, 601, 602 and 603, are also grouped turbo or non-turbo, but they all share serpentine belt drive for engine accessories and a light alloy cylinder head! So by looking at whether or not it has a turbo, an iron or alloy head and number of cylinders, you can quickly identify the engine type. Also, all 617 series engines made from 1981 were turbo-charged. No 4-cylinder iron head engines were ever turbo-charged although there are a few with aftermarket turbo kits running around. All iron head diesels require valve clearance adjustment at 15,000 mile intervals or at least twice a year. Because diesel valves will go tight as they wear, failure to adjust valves will cause compression loss and hard starting or rough idle cold complaints. This is also a good time to inspect the cam lobes for "ramping" wear and to check cam timing to ensure the chain is not stretched and the chain guides and tensioner are all OK. Camshaft replacement is often needed and is easy (and profitable) on these engines. By the way, always replace the valve cover gasket! If you have the hood open on a 617 turbo diesel, always check the air cleaner mounts. These three little rubber mounts break all the time and cause lots of engine rattles. Oil leaks at the front of the motor are usually from both the front crankshaft oil seal and/or the turbo oil drain tube seals. Replacement is again easy and profitable, but don't ever expect a Mercedes turbo to be absolutely dry, especially at higher miles. Assuming these engines get proper care and maintenance, only one really bad thing happens to them: The cylinder walls taper out and they lose compression, and then they don't start cold. Other symptoms are oil consumption and lack of power, especially when cold. Oil consumption can get real severe. I've seen some diesels go through one quart every 150 miles that still start and run well in warm weather. When you evaluate a diesel, just remember it's the compression! Also remember a compression test is meaningless unless valve clearances are checked first. It takes 300 psi of compression at cranking speed for a diesel to be pronounced "fit." Alloy-head diesels have hydraulic valves, but you still need to inspect for cam wear and timing chain stretch. This also gives the engine a chance to get a new valve cover gasket once in a while. If the complaint is poor performance on a turbo, check out the trap oxidizer. Like a catalyst, it will plug up and cause severe (and often intermittent) exhaust restriction. If you encounter a bad trap oxidizer, call your local dealer or Mercedes-Benz. Mercedes may cover trap oxidizer replacement - which is serious bucks. When you change the oil filter on a Mercedes diesel, always check the two small o-rings at the bottom of the oil return tube attached to the filter housing cover. Replace them whenever they look damaged or brittle. Failure of these o-rings will cause oil pressure fluctuation, especially at idle. Order a supply of these from your dealer so you'll always have some on hand - just in case (Part No. 015-997-94-48). Finally, when a diesel shows up that starts really hard, knocks really bad, wants to die, and the problem occurred very suddenly, save yourself some diagnostic time. Take off the fuel cap, stick your nose in the filler tube and take a whiff. Odds are what you smell will be the gasoline your customer just put in the tank! Drain the tank, purge the lines, replace the filters, prime the pump and send your customer down the road happy ... but probably embarrassed!
|
||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | |||||||||||||||||||||||||||||||||
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
