Is replacing the head gasket in a 1993 Mercedes 300ce, an expensive repair? I've been told it is better to just replace the engine all together. Is it a tough job for a backyard mechanic or a diy person?
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The Mercedes M103 engine in your 300CE is a proven design, old enough to demonstrate its longevity—lots of them are running around still with 300,000+ miles on them. The head gaskets are pretty much the only serious weak point. If the engine was not burning excessive oil (more than a quart every 5,000 miles) or showing other signs of extreme wear before the head gasket blew, it is definitely worth repairing. In other words, there is no need to buy an entire new engine.
If you buy a head gasket from the Mercedes dealer rather than an aftermarket part, the problem won't come back; the new gaskets are an improved design. Buy new head bolts from the dealer as well; they are designed to stretch when installed and cannot be reused.
The cylinder head is very heavy and will take at least two people to remove it safely from the engine. Overheating it probably warped the head enough that it won't seal correctly unless planed on the bottom (this is true of all aluminum heads, not just Mercedes), so you will want to take it to an automotive machine shop to do that.
With the head off on the M103, you have much easier access to the timing chain cover, which tends to leak on these engines, so you can change the gasket there. Also, you will have to drain the coolant before removing the head, so during the process, use a hose to flush the entire cooling system, and consider replacing any hoses that look old.
When you put the whole thing back together, it's crucial to follow the procedure given in the factory service manual or the Haynes manual for 124 chassis cars to correctly tighten the head bolts down; incorrect tightening will just warp the head and make it leak again. You will need a good-quality torque wrench.
All in all, it's not too bad a job, but I wouldn't recommend it for a novice mechanic unless assisted by someone more knowledgeable. Mercedes 124s are wonderful cars when sorted out, though, and the coupes are future collectibles—good luck getting yours back on the road!
One thing to consider when replacing a head gasket is that your engine's head may have warped ever so slightly when the car overheated (caused by the blown head gasket). If that's the case, it would have to be machined by a workshop.
As for actually replacing the gasket yourself, it's a matter of removing the top part of the engine, scraping away the remains of the old gasket and sticking a new one on in its place. It's a long job, but nothing especially difficult. It would probably take you a couple of hours. Just make sure you have a good quality replacement gasket, a torque wrench and the kids are somewhere where they can't hear you swear.