Timing Chain Tensioner Inspection (M110)
⚙Tools Needed
- 10mm/13mm sockets
- Valve cover gasket set
- Torque wrench
★Supplies Needed
- New valve cover gasket
- Fresh engine oil for top-off
Step-by-Step Instructions
- 01
Remove the Valve Cover
The M110 twin-cam engine has a single large valve cover. Remove the air cleaner assembly if it obstructs access. Remove the breather hose and any wiring clips attached to the cover. Remove all valve cover bolts using a 10mm socket—there are typically 15-20 bolts around the perimeter. Lift the cover carefully and set it aside. If the gasket comes away cleanly, inspect it for reuse, but it is best practice to install a new gasket during reassembly.
- 02
Inspect the Timing Chain Slack Visually
With the valve cover removed, look at the timing chain where it runs between the two camshaft sprockets. Press on the slack side of the chain (the side without the tensioner) with your finger. On a healthy chain, there should be minimal deflection—no more than about 10-12mm of free play. If the chain can be pushed in significantly (more than 15mm), it is stretched and the tensioner system may not be able to compensate. Also listen for chain rattle at idle, which indicates excessive slack.
- 03
Measure Tensioner Rail Wear
The M110 has plastic-faced tensioner rails (guides) that the chain rides against. Examine the rail on the tensioner side—it should have a smooth, convex surface with even wear. If the plastic facing is worn down to the metal backing, the rail must be replaced before the metal backing damages the chain. Look for grooves, chunks of missing plastic, or cracks in the rail. On high-mileage engines (over 150,000 miles), rail wear is almost guaranteed.
- 04
Check Chain Elongation Guide Marks
Mercedes-Benz M110 engines have timing chain alignment marks that indicate chain elongation. With the engine at TDC on cylinder one, check the relationship between the camshaft sprocket marks and the corresponding marks on the cylinder head or timing cover. If the marks are more than one tooth off from the reference, the chain has stretched significantly and should be replaced. Even a half-tooth deviation indicates substantial wear that will continue to worsen.
- 05
Evaluate Tensioner Spring Tension
The chain tensioner uses a spring-loaded plunger to maintain pressure on the tensioner rail. With the valve cover off, observe the tensioner arm position. If the tensioner is fully extended (plunger pushed out to its limit), it has reached maximum compensation and can no longer take up additional chain slack. Compare the tensioner extension to the workshop manual specifications. A fully extended tensioner combined with significant chain slack means the chain and tensioner should be replaced together.
- 06
Assess Guide Rail Condition on Both Sides
In addition to the tensioner-side rail, inspect the fixed guide rail on the opposite side of the chain. This rail guides the chain but does not move. Look for the same wear patterns: worn plastic facing, grooves, cracks, or exposed metal backing. Both rails experience wear over time, and replacing only one during a chain job is false economy. If either rail shows significant wear, plan to replace both rails along with the chain and tensioner.
- 07
Determine If Replacement Is Needed
Based on your inspection, categorize the timing chain system condition: Good (minimal slack, intact rail faces, tensioner not fully extended—continue to monitor), Marginal (moderate slack, visible rail wear, tensioner near full extension—plan replacement within 10,000 miles), or Replace Now (excessive slack, worn-through rails, tensioner at maximum extension, chain rattle at idle). Document your findings for future reference. Reinstall the valve cover with a new gasket, torquing bolts to 10 Nm in a crisscross pattern, and top off engine oil to account for any that was lost during the inspection.
⚠Warnings & Safety Notes
- 01Do not rotate the engine by hand while the valve cover is removed unless you are specifically checking timing marks. The chain can jump a tooth if the tensioner is weak.
- 02Pieces of broken timing chain guide rail can fall into the oil pan and block the oil pickup screen, causing catastrophic oil starvation. If you see pieces of rail material, inspect the oil pan.
- 03A failed timing chain on the M110 can cause the valves to contact the pistons (it is an interference engine), resulting in severe engine damage. Do not delay replacement if the chain is significantly stretched.
- 04When reinstalling the valve cover, tighten bolts gradually in a crisscross pattern. Over-tightening will crack the valve cover or crush the gasket, causing oil leaks.
Frequently Asked Questions
How often should the timing chain be replaced on an M110 engine?+
Mercedes-Benz did not specify a fixed replacement interval for the timing chain, as it was designed to last the life of the engine under ideal conditions. However, in practice, M110 timing chains and guides typically need replacement between 150,000 and 200,000 miles, or sooner if the engine has been run with infrequent oil changes. The duplex (double-row) chain on the M110 is more durable than single-row chains but still wears over time.
What does a failing timing chain sound like?+
A worn timing chain produces a distinctive rattling or slapping sound, most noticeable at idle and during cold starts. The sound typically comes from the front of the engine and may diminish as the engine warms and oil pressure increases. Some owners describe it as a "diesel-like" clatter. The sound may also be present during deceleration when the slack side of the chain is loaded. Any chain noise should prompt an immediate inspection.
Can I replace just the tensioner without replacing the chain?+
If the chain itself is not stretched and the guide rails are in good condition, replacing just the tensioner is acceptable. However, this situation is uncommon—usually by the time the tensioner has failed, the chain and rails have also worn significantly. Given the labor involved in accessing the timing chain, most mechanics recommend replacing the chain, both guide rails, and the tensioner as a complete kit. The parts cost is modest compared to the labor.
Is the M110 an interference engine?+
Yes, the M110 is an interference engine, meaning the valves and pistons occupy the same space at different times. If the timing chain breaks or jumps multiple teeth, the pistons will strike the open valves, causing bent valves at minimum and potentially cracked pistons, damaged cylinder head, and bent connecting rods. This is why timing chain maintenance on the M110 is critical and should never be deferred once problems are detected.
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