Why does my Mercedes CLS550 rattle on startup or leak oil from the timing cover?

Short answer: A rattle on startup on the M278 V8 usually points to the timing chain tensioners or low oil pressure at startup. Timing cover oil leaks are also common with age. If camshaft timing codes show up, have it checked soon.

Symptom: A rattle for a few seconds after starting, a check engine light with camshaft position or timing codes, or oil leaking from the front of the engine.

What to check first

1. Listen for a rattle on cold starts.
2. Read codes for camshaft position or timing faults.
3. Look for oil at the timing covers.
4. Check service records for tensioner or oil pump work.
5. Use the oil spec Mercedes calls for and keep up with changes.

Likely causes

1. Timing chain tensioners. Tensioners keep the timing chain tight. Owners replace them when they hear a rattle on startup.
2. Low oil pressure at startup. One owner's rattle came back after new tensioners, and the shop blamed low oil pressure at startup and replaced the oil pump.
3. Timing cover gaskets. The covers over the timing chains can leak oil as they age and need resealing.

What it usually costs

One owner priced new tensioners at about $150 in parts while the covers were already off. No full shop prices were posted.

Is it safe to keep driving?

Short trips may be OK if it runs normally, but with timing codes or a growing rattle, get it checked. A stretched chain can damage the engine.

DIY or shop?

Resealing timing covers is advanced DIY. Most owners use a Mercedes specialist.

Recalls, bulletins and complaints in plain words

• Recalls: NHTSA's recall lookup returned no data for 3 model-year queries for the Mercedes-Benz CLS-Class on 2026-11-05, and the years that did return show no recall for this problem. NHTSA's recall lookup returned no data for some model years on 2026-11-05. Check your VIN at nhtsa.gov/recalls. If your car is part of a recall, the dealer fixes it for free.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect.
• NHTSA complaints: 11 keyword-matched complaints for Mercedes-Benz CLS-Class 2012–2014 (of 39 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-05; keywords: timing, chain, rattle, camshaft, cam, oil leak, engine, check engine) (NHTSA returned no data for 3 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In M278/178 Timing Cover Leak - Change Tensioners? on Reddit r/mercedes_benz, a 2012 CLS550 owner with the early M278 V8 at 115k miles was resealing a leaking timing chain cover and asked whether to add new tensioners for about $150 in parts. Another 2012 owner had just pulled a cover to reseal it and found everything looked good inside.
• In Dealership diagnostic. Need advice on Reddit r/MechanicAdvice, a 2012 E550 owner with the same M278 V8 at 120k miles had the tensioners replaced for a startup rattle, but it kept rattling. The shop then replaced the oil pump, and later the car set camshaft timing codes. A mechanic said timing codes often come from oil pressure problems, and that a stretched chain can let a piston hit a valve, with bad misfires first.

Quick tip

If the covers are coming off for a leak, ask about tensioners at the same time.

Vehicle focus: Mercedes-Benz CLS550 (W218) with the 4.6L twin-turbo M278 V8, 2012-2014. The E550 shares the engine.

Safety / mechanic note

Timing problems can cause misfires and engine damage. Verify with a mechanic.