Kyle Blackmore's Spotting a 123 Dog

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Spotting a 123 Dog

Written by Kyle Blackmore

Thinking of buying a 123? These are some tips to help out along with a normal inspection on any used vehicle. Mercedes diesels are known for longevity and reliability but what a lot of people don't realize is that they require a strict maintenance schedule to enable a long and healthy lifespan.

If you are considering buying one of these cars and expect a mechanic to complete all required maintenance it will be a very expensive endeavor. Thankfully, 123's are easy to work on yourself, all it takes is pretty much every weekend of your life. Just kidding.

The following checklist will help you in your search for the perfect 123.


Check window seals (front & rear) for cracking and brittleness. They often fail and leak into cabin and trunk. Use watering can to check.

Body drains - under hood at firewall, under battery box, in hinge area. Pour water in cowl vent, it should drain out elbows on firewall. Pour water in hood hinge areas (pockets) it should drain out wheel wells. Check trunk seal, door seals, floor drains and plugs, lift carpets on all four floors inside car. Any dampness found is an indication of leaks. Leaks lead to rust problems in areas that are designed to drain. Often dirt, leaves and other debris will plug the body drains. If equipped with sunroof, there are four drains with it as well. The sunroof isn't watertight, water seeps into a sunroof pan and drains out the four corners of the roof. The front drains down the A pillars about six inches below the windshield, the rear down the C pillars behind the trim. With sunroof open, pour water in all four corners of pan, it should drain out. Rust stains down the C pillars are a sure sign of a dog needing a new sunroof pan.


Battery should be proper size for battery tray, too small a battery is hard on the starter. Diesels need a good battery and starter to spin it fast enough for easy starts. Check operation of all electrical switches (window switches, sunroof (ask first), lights etc.) Common problems include sunroof and antenna motors, window switches (usually from spilled coffee), instrument panel dimmer, blower motor, light sockets exposed to weather.


Check for obvious oil and fuel leaks, oil leaks will be jet-black, fuel leaks will be brown or clear. Common leaks are cam cover, oil pan and the braided fuel return lines, all easy fixes. Ask to see the engine start from dead-cold...feel the top rad hose when checking under the hood to ensure that it is. When starting the engine glow-plug light should come on for ten seconds or so, then go out, push the throttle a little and turn the key. If everything is okay it should spin fast and start within a few seconds, if not there could be expensive problems. After starting, set the fast idle (little knob below oil gauge on dash) by turning it counter-clockwise while pushing the throttle. Check the exhaust for excess smoke, all diesels will smoke slightly when cold due to incomplete combustion. Clouds of blue, black or white smoke are signs of a dog. Ask the owner how often the oil is changed (should be 5000 miles), frequent changes make diesels last a long time. Listen to the engine, it will clack like crazy, that's normal, but hollow knocks and rattles aren't normal and could lead to expensive repairs.

Once the engine is warmed up (after the test drive) open the oil cap on the cam cover and check for 'blow-by', on an engine with lots of miles there should be some vapor inside the cover, on a new or rebuilt engine there should be none, on a dog the cam cover will puff smoke out like an old steam locomotive.


When the engine is cold, open the radiator cap and check the color and level of the anti-freeze. It should be bright orange, yellow or green. Murky or dirty looking anti-freeze probably means it wasn't changed at regular intervals. Check the general condition of all hoses, there are a lot of them. Radiator hoses are a common replacement but other hoses could get expensive.


Check condition of all hoses relating to the diesel injection system, most of these are easy to replace and not too expensive. Any leaking of diesel could be an indication of more serious problems. Throttle linkage is complicated to set up, but general maintenance (lubricating joints) is straightforward. There should be some play from the accelerator pedal to the injection system, this is normal but large amounts of play could require more attention.


The automatic transmissions are known for longevity if properly serviced (25,000 miles) but they are prone to strange shifting problems which may only require an adjustment. If the transmission seems to slip, an oil change may cure it but if it doesn't, a rebuild is expensive.

Driveshafts are normally trouble free, however, rear axles sometimes require replacement. They will growl or vibrate to indicate they are worn out.


The most common complaint is play in the steering box. This can be corrected with adjustment if it is not too far gone. Play of one to two inches is common in the steering wheel. Steering box replacement is expensive. Other front end parts are easily replaced and not overly expensive. Clunks and play felt on the test drive should be thoroughly inspected to determine which parts are at fault. Rubber bushings are labor intensive to replace but are very cheap.


The independent rear suspension is complicated to service, but again, the rubber bushing parts are cheap to replace. The self-leveling suspensions on station wagons can be expensive to repair, any rear-end sagging is an indication of a problem.


The exhaust systems on these cars are very simple, consisting of a muffler and a resonator. Cheap replacement mufflers are available. Left unchecked, vibrations can cause leaking.


When checking under the hood, observe the color of the brake fluid. It should be a clear bluish liquid, rusty or black brake fluid indicates a system that wasn't maintained. Brake fluid should be flushed annually, if this isn't done moisture is absorbed into the brake fluid and causes rust and corrosion in components. Brake pad replacement is a cheap job but replacing calipers and master cylinder can become very costly.


After checking under the hood you are ready to take a road test, 123 style. You want to check for all the normal things that you would on any test drive but pay particular attention to the transmission shifting and the feel of the suspension as you hit large bumps and potholes. This is when you will hear clunks associated with front or rear suspension bushings. Don't expect blistering acceleration but check for excessive smoke in the rearview mirror which could indicate injection problems. If, after all this, you still think the 123 isn't a dog spend the money to have a qualified mechanic inspect the vehicle on a hoist.