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I assume you still have to pop the valve cover to determine chain stretch. What's the key test to determine if the engine has overheated - compression? And why is the six so prone to overheating?
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Yep, still pop the cover to check stretch. But on the 603, it's easier & cleaner, IMO - no linkage to mess with either.
As to how to test for past overheating, there's no test that I know of. There is a rudimentary test for the result of overheating - the dreaded cracked head. To test, open the radiator cap with the engine COLD to release any pressure and equalize the system. Then fire it up, warm it up, and drive the car hard - numerous full throttle trips to the redline. Run it as long and as hard as you want. Then park it overnight. The next morning when COLD (same ambient temp as previous day, approximately), pop the hood and squeeze the upper radiator hose. It should be soft and squishy, as if the cap were removed. If so, the head is probably fine (assuming there is ZERO coolant loss). If it's rock hard and tight as a banjo string - can't pinch at all - there's an excellent chance the head is cracked. Figure $900 for a used head, $1800 for a new one (bare), and either a couple hundred bucks in tools as DIY, or ~$1000 labor at a shop. I like these engines so much, I think it's worth it, if the problem happens during ownership just eat the cost and fix it. The new heads are stronger and shouldn't crack in the future.
Why do they overheat? Well, the cooling systems are the common fault, often from not using MB coolant and not changing it frequently enough. Also the fan clutch has a finite life span as they slowly spew the silicone fluid out and stop working properly. You can re-fill the original fan clutch (with metal blade), the new replacement clutch (from the OM606, with plastic blade) may not be refillable. Well, nobody has figured out how to refill the new one - yet. Keeping the fins clean to ensure good airflow is required, and testing operation of the electric fan & it's triggering senders is a good idea as well. When shopping for a car with a 603, pay attention to the temp gauge. They should stay under 95C in most driving, maybe up to 100C on a hot day, idling with the AC on. They should rarely if ever get to 105C (full load up a steep hill with the AC on or something), and it should cool down (under 100C) as soon as load is removed. Mine are very stable in the 90-95C range unless under load, but always drop right back.
HTH,