O95376 Dubai · Mercedes C200 · Black

# DIAGNOSTIC PLAN DRAFT — Mercedes C200
**VIN:** WDDWF8AB8KR479292 | **Mileage:** 160,128 km | **Comeback:** No

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## 1) SYMPTOM SUMMARY
Customer reports water and oil mixing ("خلط مياه وزيت") — this typically presents as a milky/mayonnaise-like residue on the oil filler cap or dipstick, oil in the coolant expansion tank, or coolant loss with no external leak. On the C200 (M274 engine family), the oil cooler housing/seal and cylinder head gasket are the two most common internal cross-contamination points at this mileage. This needs to be confirmed visually and isolated to a specific component before any parts are ordered.
**ملخص:** خلط بين الزيت والمياه — الاشتباه الأول بمبرد الزيت أو جوان الكفر، يجب تأكيد المصدر بالفحص قبل الطلب.

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## 2) LIKELY CAUSES (ranked)
1. **Oil cooler housing / oil cooler seal (heat exchanger) failure** — known wear point on M274 engines at high mileage; sits between coolant jacket and oil galleries, direct path for cross-contamination.
2. **Cylinder head gasket breach (coolant-to-oil passage)** — possible at 160k km, especially if there's also coolant loss, white exhaust smoke, or overheating history.
3. **Oil cooler seal on the oil filter housing / thermostat housing o-ring** — secondary sealing points that can leak internally without external drips.
4. **Cracked cylinder head or block (rare)** — only considered if head gasket test is inconclusive and pressure/compression findings point that way.
**الأسباب المحتملة:** مبرد الزيت أولاً، ثم جوان الكفر، ثم أورينجات الهاوسنج، وأخيراً تشقق بالكلتة (نادر).

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## 3) TEST SEQUENCE (cheapest first)

1. **Visual check — oil cap, dipstick, coolant tank**
   Do: Remove oil filler cap, check dipstick, open coolant expansion tank (engine cold).
   Expected: Clean oil, clear/correct-color coolant.
   If fail → confirms contamination is real; proceed to step 2. مقارنة بصرية أولية.

2. **Coolant condition & level history check**
   Do: Ask advisor/customer about recent coolant top-ups, overheating warnings, oil changes. Check coolant level trend if any prior service records exist.
   Expected: Establish if this is new or recurring.
   If prior oil-cooler or head gasket work exists → this becomes a comeback, restart from that repair record. تأكيد تاريخ الصيانة السابقة.

3. **Autel MaxiSys Ultra S2 — Auto Scan / Topology**
   Do: Run Auto Scan to pull all fault codes across modules (engine, transmission), check for coolant temp sensor faults, overheat codes, or oil level/quality sensor faults.
   Expected: No codes = proceed with mechanical checks; codes present = correlate with cooling/lubrication circuits.
   If fail (relevant codes found) → note codes, proceed to live data. سكان شامل لكل الوحدات.

4. **Live Data PIDs — coolant temp, oil temp, oil level sensor (if equipped)**
   Do: Using Ultra S2 live data, monitor coolant temperature sensor, oil level/quality sensor (if fitted), and check for erratic readings suggesting sensor exposure to wrong fluid.
   Expected: Normal, stable readings.
   If abnormal → correlates with internal leak location; proceed to step 5. مراقبة القراءات الحية.

5. **Cooling system pressure test (cold engine, no dismantling)**
   Do: Pressure test the cooling system to the spec pressure — get the spec from TOPIx/ALLDATA for this VIN. Watch for pressure drop with no external leak visible.
   Expected: System holds pressure steady.
   If fail (pressure drops, no external leak) → strongly suggests internal breach (head gasket or oil cooler); proceed to step 6.
   فحص ضغط نظام التبريد.

6. **Combustion leak / block test on coolant (chemical test kit — cheap, non-invasive)**
   Do: Perform a block/combustion leak test on the coolant expansion tank.
   Expected: Negative (no combustion gases in coolant).
   If positive → indicates cylinder head gasket breach into coolant; go to step 8 (head gasket path). If negative → oil cooler is more likely; go to step 7.
   اختبار غازات العادم داخل المبرد.

7. **Oil cooler housing inspection (moderate dismantling)**
   Do: Remove oil filter housing / oil cooler assembly, inspect housing seal, o-rings, and cooler element for cracks or seal degradation. Pressure-test the oil cooler element off-vehicle if shop equipment allows (soak in water bath under air pressure — get target test pressure from TOPIx/ALLDATA).
   Expected: Cooler holds pressure, no bubbles, seal intact.
   If fail (leaks/bubbles or damaged seal) → confirmed root cause: replace oil cooler seal/housing. If passes → proceed to step 8.
   فحص هاوسنج مبرد الزيت.

8. **Cylinder head gasket / head flatness check (highest cost — only after 6 & 7 point here)**
   Do: If block test was positive or oil cooler tested clean, remove cylinder head cover area as needed, inspect head gasket, check head flatness/warpage against spec — get the spec from TOPIx/ALLDATA for this VIN. Run compression test or leak-down test (Autel Ultra S2 can log cranking compression via relevant PIDs/scope if using a pressure transducer accessory) to cross-check.
   Expected: Gasket intact, head flat within spec, even compression across cylinders.
   If fail → confirmed head gasket/head damage — proceed to repair with technician sign-off on parts.
   فحص جوان الكفر واستقامة الكلتة كخطوة أخيرة.

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## 4) DATA TO COLLECT
- Photos of oil (dipstick + cap) and coolant tank contents (before touching anything)
- Any fault codes from Auto Scan (module, DTC, freeze frame if available)
- Live data log: coolant temp, oil temp/level sensor readings (screenshot or saved session)
- Cooling system pressure test result (hold/drop, time, spec used)
- Block/combustion leak test result (positive/negative)
- Oil cooler pressure test result (if performed) — pass/fail, visible damage description
- Head flatness measurement and compression/leak-down values per cylinder, compared to spec
- Service history notes (any prior coolant/oil related work)
**البيانات المطلوبة:** توثيق الصور، الأكواد، قراءات الضغط، ونتائج فحص مبرد الزيت والكلتة.

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## 5) SAFETY
- **Hot cooling system:** Never open the coolant expansion tank cap on a hot/pressurized engine — risk of scald injury. Allow full cool-down before step 1 and step 5.
- **Lifting:** Use approved lift points per TOPIx for any under-vehicle access during oil cooler removal (step 7).
- **Combustion leak test kit:** Handle test fluid per manufacturer's chemical safety sheet — avoid skin/eye contact.
**السلامة:** لا تفتح خزان التبريد وهو ساخن، والتزم بنقاط الرفع المعتمدة.

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DRAFT — the technician must verify before any part is ordered. مسودة — القرار النهائي للفني.