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Timing Chains Don't Last ForeverPosted 9/15/1998By Rich Cregar
Everyone in the auto service business is tuned into the need for timing belt inspection and maintenance, but too often when we see that an engine is chain driven, we walk away from the service opportunities the overhead cam chain engine presents. Mercedes-Benz pioneered the production passenger car overhead cam engine with the introduction of an OHC inline 6 back in the early '50s. The legendary 300SL Gullwing used an OHC, fuel injected inline 6! This basic layout was essentially unchanged, and widely copied, all the way into the '90s. The timing chain used by Mercedes was a double roller design tensioned by an oil-fed tensioner and was extremely durable. For whatever reasons (which I presume were weight and cost concerns), Mercedes engineers had a flirtation with using a single roller chain with the introduction of the model 230, 230-S and 230SL cars in late 1964. These engines had nothing but timing chain problems and Mercedes quickly reverted to the old double chain with the introduction of the 250S, SE and SL cars in 1966. By the way, the Diesels have always used a heavy duty double chain and that's one reason you seldom see chain problems on these cars. The introduction of the 3.5/4.5 V-8s in 1971 meant a much longer chain and problems were created by chain stretch, defective tensioners and broken guide rails. But these were nothing compared to what happened when Mercedes, in response to the energy crisis, developed the light alloy V-8 which was brought to the United States in a 3.8 litre version in 1980 (the 380SE, SEL, SL and SLC). Perhaps the engineers who designed the 230 motor were retired - or had been fired! (Have you wondered what happened to the poor guys who developed the GM 5.7 Diesel?) In any event, the decision was made to use a single roller chain and all the old problems of chains breaking and cams jumping time returned. Mercedes campaigned most of these cars with their dealers installing a double roller chain retrofit kit, and the independent shops repairing the rest. They also did a production change, installing the double chain at the factory. With the introduction of the M-102 (190E-2.3) engine in 1984 and its evolution in 6-cylinder versions of the M-103 and 104 (260E, 300E, 300E DOHC), a radical change was made in head design to a Cosworth style head with the cam bedded in the head rather than being mounted on cam towers - and a single roller timing chain! As Mercedes engines age, they are prone to developing chain problems. Remember too that these engines tend to acquire far more mileage than other cars. The use of hydraulic valve adjusters compounds the problem because the valve covers are no longer removed regularly for valve adjustment, so the chain drive doesn't get regular inspections. Diesels still required valve adjustment into the mid-1980s, so chain problems were usually found in time. Be aware of any customer complaint about engine noises. And pay attention when the car is in your shop! Symptoms that should all be investigated immediately include rattle on startup lasting only a second or two; front engine noises on acceleration or deceleration; valve noise; and excessive oil consumption. Be especially aware of unexplained changes in ignition timing on the 2.3 litre engine. On these engines, the distributor drive sprocket will wear severely and the chain will jump the sprocket, altering base timing. This symptom is usually followed VERY SHORTLY by chain failure and possible valve train damage. Also be very careful to follow the correct reset procedures for the timing chain tensioner whenever the chain is being serviced. Failure to reset the tensioner will result in chain failure! Do your customers a service by recommending periodic inspection of the timing chain and valve train on their Mercedes. Problems with the chain, tensioner, chain guides, camshaft, and valve seal, and valve guide failure are all common to many Mercedes engines. Repairing these components is very profitable and prevents your customer from spending even more money and suffering a total breakdown when the engine fails entirely.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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