Driveway Mechanic:
Diesel Engine Tune-Up


Owners of diesel-powered Mercedes-Benz cars know about their legendary reliability and economy. These engines are among the best, but owners frequently overlook long-term maintenance simply because the cars demand so little. This article, which will help you to restore a well-maintained engine to peak performance, includes tips that Mercedes-Benz mechanics use while servicing the car. Get your disposable gloves, read up, and have fun.


The Basics

An engine is basically an air pump. A property known as the heat of compression-the fact that air is heated as it is compressed - is the basis for making any internal combustion engine run successfully. A diesel engine uses no spark plugs to ignite the air/fuel mixture; instead, it relies on the heat of compression alone to ignite its fuel. If you have trouble starting a diesel, make sure the engine is turning over fast enough while cranking, particularly in the winter. Heat of compression drops rapidly in the winter when the surrounding metal is cold. If you have starting problems, have your mechanic check the cranking rpm

To insure that the engine is a good air pump, you must also check compression pressure. To do so, a glow plug is removed from a cylinder and a compressiong guage is inserted. Most mechanics run a static (cranking) compressiong test, but a running compression test can be made at idle, as well. A static test is made by holding the stop lever down on the fuel pump while an assistant cranks the engine on the starter motor. Compression pressures for cars before 1989 are in Table 1; 1989 and later models are in Table 2. These specification are for normal operating tempreature and normal cranking rpm. The "new" value is for a new engine with little or no wear

The wear limit shows the minimum values from a worn engine. Anything below this wear limit calls for repair so, a glow plug is removed from a cylinder and a compression gauge is inserted. Most mechanics run a static (cranking) compression test, but a running compression test can be made at idle, as well. A static test is made by holding the stop lever down on the fuel pump while an assistant cranks the engine on the starter motor. Compression pressures for cars built before 1989 are in Table 1; 1989 and later models are in Table 2. These specifications are for normal operating temperature and normal cranking rpm. The "new" value is for a new engine with little or no wear or adjustment. A running compression test at idle helps pinpoint problems such as a worn intake cam lobes, bad valve seating, or incorrectly adjusted valves. Have your mechanic perform these tests for you using a special diesel compression gauge.

If the engine fails the compression test, the next step is a cylinder leakage test, done with the engine at normal temperature, not running, and the piston at Top Dead Center on the compression stroke. During this test, compressed shop air is pumped into the cylinder, and a cylinder leakage gauge gives a reading of air leaking out of the cylinder in percent. Listen for escaping air at the oil fill cap, the exhaust pipe, the intake manifold, and the cooling system (for safety reasons, let the system cool first). This helps pinpoint the problem area. Total cylinder leakage should be no more than 25 percent; leakage from the cylinder head and valves should not exceed 10 percent; leakage from pistons and piston rings should not exceed 20 percent.

Valve Adjustment

On engines with adjustable valves, be sure the valves are adjusted to the correct clearance. A mechanic can do this in about an hour using special wrenches. Adjustment specifications are in your owner's manual. If you have cold weather starting problems, always begin by checking valve clearances. Remember, heat of compression needs good airflow to happen.

Mercedes-Benz diesels from about 1988-on have hydraulic valve lifters and require no adjustment. Check your Owner's manual in the specification section. If you see a specification for valve clearance, your engine should be checked.


Air Filter

A diesel engine is an "air" engine, i.e., it runs unthrottled, so it needs copious amounts of air to run correctly. Unclip the air filter housing clips or unscrew the top housing nut and inspect the air filter. If you have doubts, replace it with a new one. Always clean the filter housing before installing a new filter. On metal can air filters, check the rubber housing mounts; replace broken ones.

Cleaning the Pump and Lines

The easiest way for a do-it-yourselfer to intensively clean the pump, lines, and injectors is to use Lubro-Moly's Diesel Purge cleaner (Photo 1), available from advertisers in The Star. This procedure shows you how to "run the car" from 100-percent Diesel Purge solution. Keep dirt from entering the fuel lines during this procedure. To do fhis properly, you will need two wooden golf tees, a Phillips-head screwdriver, and about 4 ft of 5/16-in fuel hose cut into two pieces. This hose is available at any auto parts store. Follow these steps and remember that air must not get into the fuel supply hose, so the hoses must be kept immersed in the cleaning solution.

Find the fuel supply hose going into the fuel pre-filter (Photo 2). Loosen the hose-clamp, pull the hose from the pre-filter, and plug the hose with a golf tee to prevent fuel spillage (Photo 3). Push one of the 5/16-in hoses onto the pre-filter inlet; put its other end in the bottom of the can of Diesel Purge (Photo 4). This will be the supply line from the Diesel Purge.

Disconnect the return hose from the main (can) filter housing on the engine (Photo 5). Be careful when pulling the hose off the metal fitting; the hose clamp can accidentally break off an adjacent fuel line fitting. Twist the clamp out of the way during removal. Install the other piece of 5/16-in fuel hose onto the return line fitting on the main (can) filter housing; put the opposite end of the hose into the Diesel Purge can (Photo 6). This will be the return line back to the Diesel Purge. Plug the disconnected return line in the car with a golf tee. Check that all hoses are free of leaks, start the engine, and you should immediately see the colored cleaning solution going through the clear pre-filter into the engine.

Let the engine run until the can is almost empty, which should take about 20 minutes. Prevent air from getting into the system by not letting the can empty completely. Stop the engine, then remove the two 5/1&in hoses and the can of Diesel Purge. Reinstall the supply hose and return hose to the engine. Replace the clamps with new ones, then restart the engine. A very dirty fuel system may require two treatments. When you're done, check for leaks.


Fuel Filters and Screen

Mercedes-Benz diesel engines use two fuel filters to protect the expensive fuel-injection pump from water and tiny amounts of dirt. In my opinion they should be changed every 5,000 miles. They can easily be replaced by the do-it-yourselfer. Using a golf tee or locking pliers to plug or pinch off the fuel supply hose prevents fuel from spilling while you change the pre-filter. To change it, loosen the two hose clamps with a screwdriver, pull off the hoses from the filter, and replace it. The arrow on the filter shows the direction of fuel flow, toward the engine.

Replace the main (can) filter only if you have new sealing washer(s) for the hollow-stem bolt. The main filter mounts like a spin-on oil filter.

Loosen but don't remove the two bracket bolts using a 13-mm socket, extension, and ratchet (Photo 7). Remove the center hollow bolt (Photo 8). Replace the main filter by removing the two bolts in step 1. This gives you enough clearance to move the bracket and spin off the can filter completely (Photo 9). Wet the rubber gasket with clean diesel fuel, then install the new can filter. Use a new sealing washer, then tighten the hollow bolt. Reinstall the two bracket bolts, and check for leaks.

You will have to manually bleed air from the pump and lines using the built-in hand pump or by repeatedly operating the starter motor. Typically, you may have to pump the hand pump 30 or more times or until you feel hydraulic resistance while pumping (your owner's manual tells you how to operate the pump). On later models-engines 602 and 603 without the hand pump - operate the starter for 30 seconds at a time, then wait for the starter to cool before repeating this process until the engine starts. (One mechanic's trick is to fill the filter canister with diesel fuel to expedite starting. Ed.) Check for leaks with the engine running.

Another source of fuel starvation, which can rob engines of full power, is a clogged screen in the bottom of the fuel tank. Because the fuel tank must be emptied, this needs to be serviced by a professional mechanic. The screen is at the bottom of the tank where the fuel supply hose comes out. It usually has a large hex fitting which must be removed for service. Tell your mechanic to use caution while removing the fitting. If the filter is clogged, he will get a face full of fuel when the clog is broken. Additives such as Diesel Doctor prevent screen clogging.

Intake System Cleaning

The most common cause of performance loss in late-model diesel engines is clogging of the intake manifold with oil vapor build-up (Photo 10). I've seen the air supply pipe practically closed off by oil sludge build-up. Thorough cleaning of the turbo crossover pipe and intake manifold should be done every 50,000 miles. Use caution; a diesel engine can run away if solvent, fuel, or other combustible cleaning material gets into the engine in large quantity. This means that the engine runs at a very high rpm, out of control, which will damage it.

Oil vapors on engines such as the 602 and 603 are introduced into the intake air stream by a breather hose that vents the crankcase next to the oil fill cap. Vapors build up and eventually choke off the intake air supply. The breather hose should never be pinched off or plugged, as major engine oil leaks will result. The intake valves become severely coated with an oily residue, preventing the engine from breathing. Ultimately, engine performance and fuel economy suffer.

To clean the engine, find a truck or car service center with a Sun Diesel Motorvac engine cleaning unit (Photo 11). If you want to clean the intake system yourself, the following steps guide you through a five cylinder 602 engine (1990 300D 2.5 Turbo). The six-cylinder engine (603) is similar. If you do this yourself, be prepared for a dirty mess. Have several pairs of disposable gloves plus carburetor cleaner, old toothbrushes, rags, an aluminum roasting tray, and lots of cardboard to prevent spills. Better yet, remove the intake pipe as described below and pay your local garage to soak it in parts cleaner.

With the engine turned off, remove the two hex-head bolts with a 5-mm socket (Photo 12). Remove the bolt using a 5-mm socket, and hold the nut with a 1O-mm socket (Photo 13). Be careful not to lose the nut or bolt. Twist the intake crossover pipe up toward the windshield, rotating it on the cylindrical end (near the exhaust). Remove the pipe and inspect it. Remove and discard the O-ring at the turbo end fitting. Remove and discard the gasket at the intake manifold. If necessary, use a razor blade to scrape off the old gasket. Using an aluminum-roasting pan, clean out the intake pipe with carburetor cleaner spray and a toothbrush or rag. Working at the engine, clean out the turbo end fitting with a rag and carburetor spray. Make sure the throttle plate and EGR orifice are clean as a whistle (Photo 14).

Clean out the intake manifold as much as you can, reaching in with a rag soaked in carburetor cleaner spray. Try to spray a little cleaner down each intake tube to soak the intake valves. Don't spray more than 1 or 2 ounces of spray directly into the manifold, or the engine may hydro-lock or run away when started later. Clean the intake crossover pipe mating flanges on the engine. Install a new 0-ring and gasket, reinstall the intake crossover pipe, and tighten the bolts.

When it is safe to do so, start the engine outside. The exhaust smoke is sludge being burned. Test drive the car, accelerating at full throttle several times. The above steps may have to be repeated several times to thoroughly clean the engine.

Checking the Accelerator Linkage

Checking the throttle linkage is easy and verifies that the accelerator pedal is positioning the fuel-injection pump in a maximum delivery position when you are trying to accelerate.

With the engine off, have a helper press the accelerator to the floor and hold it. In the engine compartment, try to wiggle or move the linkage attached to the fuel injection pump. Look for wear or slack. If any is found, have a qualified Mercedes Benz mechanic repair, adjust, and lubricate the linkage for you. Don't try to adjust it yourself.

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