Good evening, all

I need help with a 1991 Mercedes 420 SEL in strong overall shape. I looked at the car today. The owner shared photos and details. I want advice before I move forward.
Here is the situation.

• Exterior and interior look excellent.
• The timing chain snapped, and the chain hit the front case.
• The housing for the chain case shows a crack.
• I need to know if a new chain and new guides will bring the engine back.
• I also need to know if the chain failure pushed the valves into the pistons or if the engine stayed safe.
• The sunroof motor needs service.
• The exhaust system is original. It needs an update soon.

Details from the owner.

• 1991 Mercedes-Benz 420 SEL
• 4.2 V8
• 171,000 miles
• Second owner
• Strong care history
I want guidance on next steps.
I would like to know what to look at first and what a full repair list should include.

Anyone with experience on the M116 engine, please let me know your thoughts.

I followed up and checked the engine specs. The M116 4.2 V8 in the 1991 420 SEL is a non-interference engine. This gives me some hope that the valves stayed safe when the chain broke. I still plan to inspect the guides, sprockets, tensioner, and the cracked timing cover once the front of the engine is open.

I welcome advice from anyone who has rebuilt this timing assembly or dealt with a broken chain on this engine.

Thank you for your time,

Ron

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A generic way to see if valves hit pistons is to remove the camshaft or shafts, remove spark plugs or glow plugs and crank the engine slowly by hand in the proper direction by hand and see if it hangs up during rotation. If you hit something there a good chance valve is at least bent and not closing.

Knowing the engine can be rotated without hitting anything sets you up for a compression test.

Hopefully someone will post a site that has a free online manual for you.

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84 300D, 82 Volvo 244Gl Diesel

Thank you for this information. I appreciate you.

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Before we go any further, we need to clarify one very important detail: did the chain snap (like broke a link and separated), or is it still in one piece but it just jumped a tooth on the sprocket? If the chain broke a link and separated, then forget about this car and move on.

I am going to assume for now that the chain just jumped a tooth but is still in one piece. That usually happens because the chain tensioner was not changed when time came to do it, or it was replaced with a cheap one (that in most cases would operate correctly 6-12 months). Regardless, now it is what it is.

By no means the camshafts should be removed, that may create a new set of problems. (which I will not go in detail now). What should be done is removing each rocker (and mark it so it will be placed back in the EXACT same location) and spark plug, and perform a compression test on each cylinder. If the valves are bent, then they will not close completely and the compression will be 0.

At 126board.com we offer assistance with FRM (Factory Repair Manual) sections to the forum members, specific for each job.

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1991 420SEL (Cal), 1991 560SEC (Fed)