G400 V8 Diesels

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Mon, 23/02/2026 - 12:16
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I'm starting a thread to collate various issues or things to look out for with the G400 V8 Diesel.

Here's what I found so far:

Common Engine Issues in the G 400 CDI (Especially After 5+ Years / Higher Mileage)

This engine is known for being powerful and torquey but complex, with tight engine bay packaging in the G-Class making repairs expensive and labor-intensive. Many owners report solid longevity if well-maintained, but neglect (e.g., poor oil changes, low-quality fuel) amplifies problems. Issues often become noticeable after 5–10 years or 80,000–150,000+ miles/km.

Key reported problems include:

  • Fuel System Issues (High-Pressure Pump and Injectors): Early models (2001–2002/2003) had notorious failures with the high-pressure fuel pump and injectors, causing starting problems, misfires, rough running, or power loss. Mercedes addressed this with revised parts by around 2003–2004. Later examples are generally more reliable in this area.
  • Turbocharger Problems: The bi-turbo setup can suffer failures (e.g., oil starvation, bearing wear, or actuator issues), leading to loss of power, excessive smoke, whining noises, or limp mode. High torque stresses the turbos over time.
  • Engine Misfires and Rough Idling: Common complaint, often tied to injectors, glow plugs, wiring, or sensors (e.g., cam/crank sensors). Can feel uneven or shaky, especially when cold.
  • Oil Leaks: Fairly typical on aging V8 diesels — from valve covers, turbo lines, oil cooler, or seals. Not always catastrophic but can worsen.
  • Timing Chain / Camshaft-Related Wear: Some reports of chain stretch, tensioner issues, or related problems (e.g., noisy on cold start). Repairs are major due to engine access.
  • Other Associated Issues: Overheating/cooling system glitches, EGR valve/carbon buildup (common on diesels), or transmission strain (the 5-speed auto can struggle with the torque, leading to vibrations or failures). Electrical gremlins and rust (especially pre-2006) are also G-Class staples but not engine-specific.

Many forum users note that post-2003/2004 models are preferable, as early teething problems were ironed out. Well-maintained examples can exceed 200,000+ miles with proper care (regular oil/filter changes using correct spec oil, timely belt/pump services).

Advice for Owners or Potential Buyers

  • After 5 years (or now 10–20+ years for these older models), expect higher maintenance costs — parts/labor for this engine are pricey due to the G-Class layout.
  • Regular diagnostics (e.g., leak-down tests, fuel pressure checks) help catch issues early.
  • If your vehicle is showing symptoms, get a specialist Mercedes/independent diesel shop inspection rather than a general mechanic.
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Mon, 23/02/2026 - 12:19
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Re: G400 V8 Diesels

Most Common Sources of Oil Leaks on the G 400 CDI / OM628

From owner forums (like Benzworld), specialist reports, and common Mercedes diesel patterns:

  • Around the Turbochargers: This is one of the most mentioned spots for the G400 CDI. Oil can leak from turbo oil feed/return lines, gaskets, seals, or drain hoses/connections (especially on the left side). The bi-turbo setup runs very hot under load, accelerating wear. Symptoms include oil on the turbo housings, down the engine sides, or pooling underneath. Minor leaks are common; major ones may require turbo removal for proper repair (replacing seals, lines, and sometimes the turbo itself if damaged).
  • Valve Cover Gaskets: A frequent culprit across many Mercedes V8/V6 engines, including diesels. The rubber gaskets harden, crack, or shrink from age and heat, allowing oil to seep out and drip down the engine block or onto exhaust components (causing a burning oil smell or light smoke). This is often visible on the sides or top of the engine.
  • Oil Cooler Seals/Gaskets: The oil cooler (often in the engine "V" area) can develop degraded seals, leading to external leaks. This is highlighted as a common issue in similar Mercedes diesels (like the related OM642 family), and while less specifically documented for OM628, heat exposure makes it plausible. Leaks here can be trickier to spot initially as oil may travel before dripping.
  • Other Seals and Gaskets:
    • Oil pan gasket: Degrades over time, leading to seepage from the bottom.
    • Turbo-related lines or actuators.
    • Less commonly: Crankcase seals, front/rear main seals, or oil filter housing (though more typical in other engines).

General causes include:

  • Natural aging/hardening of rubber seals and gaskets due to prolonged heat exposure.
  • Engine vibrations loosening or wearing components.
  • Infrequent or improper maintenance (wrong oil spec, overdue changes) accelerating breakdown.

Symptoms to Watch For

  • Oil spots or puddles under the vehicle (especially after parking overnight).
  • Oily residue on engine sides, turbo areas, or undertray.
  • Burning oil smell from the engine bay (oil hitting hot exhaust).
  • Low oil level warnings over time (if leak is moderate).
  • No major performance drop unless oil level gets critically low.

Fixes and Costs

  • DIY/Independent Shop: Cleaning the area, replacing gaskets/seals (e.g., valve covers or turbo lines) is often straightforward but labor-intensive due to G-Class access. Turbo work can be more involved (removal required).
  • Dealer/Specialist: Expect higher costs (€500–€2000+ depending on the leak source; turbo-related fixes push toward the upper end).
  • Prevention: Use Mercedes-spec oil (e.g., 229.51 or equivalent for diesels), change every 10–15k km, and inspect during services. Address small leaks early to avoid oil starvation or contamination issues.
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Mon, 23/02/2026 - 12:22
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Re: G400 V8 Diesels

Common ECU-Related Issues

The ECU (often a Temic unit in these models) controls fuel injection, turbo boost, glow plugs, EGR, and more. Failures or faults can cause intermittent or sudden symptoms.

  • Capacitor/Resistor Failures in Temic ECUs: A recurring theme in older Mercedes diesels, including OM628-equipped vehicles. Internal components degrade over time due to heat/age, leading to communication errors, no-start conditions, limp mode, or random faults. Owners report repairing (rather than replacing) these for a few hundred euros, with shops in Germany/Europe specializing in it. Symptoms include difficulty communicating with diagnostic tools, multiple unrelated DTCs (e.g., glow plug module, transfer case, SRS), or the engine dying suddenly with EPC ("Electronic Power Control") warnings.
  • ECU Communication/Supply Faults: Codes like P2001 (controller voltage supply fault) or no-start/no-communication issues appear in threads. This can stem from internal ECU faults, poor power/ground connections, or related modules failing. In some cases, the vehicle enters limp mode, loses power, or stalls intermittently.
  • Other Symptoms Tied to ECU Problems:
    • Engine dying suddenly (with EPC light).
    • Rough running, misfires, or black smoke under load (sometimes linked to ECU miscontrolling injectors/EGR/turbos).
    • Multiple "random" electrical faults (e.g., non-working diff locks, seats, heater plugs light on).
    • Difficulty scanning codes or ECU not "speaking" to OBD tools.

ECU issues often worsen in hot weather or after hot starts. Many owners note that post-2003 models had refinements, but age affects all.

Wiring Harness and Related Electrical Issues

Wiring problems are more about general aging than a single massive recall (unlike 1990s biodegradable harnesses in other Mercedes models).

  • Engine Bay Wiring Degradation: Heat cycles cause insulation to harden/crack, leading to shorts, open circuits, or intermittent faults. This affects sensors (e.g., cam/crank position, MAF, NOx), actuators (turbo vanes, EGR), or harness sections near hot turbos/exhaust. Symptoms: Erratic behavior, blown fuses (e.g., engine management fuse), limp mode, or sensor-related codes.
  • Oil Ingress into Wiring: While more documented in later Mercedes engines (e.g., via leaking cam sensors), some OM628/G400 CDI owners report oil from turbo lines or seals wicking into harnesses, potentially reaching the ECU and causing corrosion/short issues. This can mimic ECU failure but traces to contaminated wiring.
  • Sensor and Connector Failures: Common culprits include faulty glow plug module wiring, EGR valve circuits, or throttle body/actuator connections. These trigger EPC lights or power loss. General Mercedes electrical woes (corrosion, loose grounds) amplify in off-road G-Class use.

Diagnosis and Fixes

  • Start with Diagnostics: Use a proper Mercedes scanner (Star/Xentry) for live data and codes—generic OBD may miss details. Look for communication errors, voltage supply issues, or sensor plausibility faults.
  • Common Repairs:
    • ECU repair (resolder/replace caps) — often cheaper than new (~€300–600 + programming).
    • Wiring inspection/repair: Clean connectors, check for oil/corrosion, repair chafed sections.
    • Sensor replacements (e.g., crank/cam sensors, boost pressure) if codes point there.
    • Ground/power checks: Clean battery terminals, grounds, and fuses/relays.
  • Costs: Independent specialists are key—dealers charge premium for labor in the G-Class. Prevention: Keep the engine bay clean/dry, use quality parts, and address oil leaks early (as they can migrate to wiring).

These issues are fixable and don't always mean major engine work, but they explain why many G400 CDI owners describe "mostly electrical/electronic" troubles over mechanical ones after years of use.

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