FAQ
TL;DR: Boost pressure of 1 800 hPa—about 38 % above the 1 300 hPa spec—was logged on a limp-mode ML 320 CDI; “first CHECK THE TURBINE SETTING” advised an experienced owner [Elektroda, Mateusz1560, #11358641; tolson, #18080578]. Flashing the ECU with factory software restored normal power. Why it matters: Ignoring overboost and bad software can destroy the turbo and injectors within 10 000 km.
Quick Facts
• Boost target: ≈1 300 hPa under load, overboost >1 600 hPa triggers limp-mode [Elektroda, Mateusz1560, post #11358641]
• Fuel-rail pressure: idle 250 bar, WOT 700 bar (spec: 250–1 350 bar) [Elektroda, Mateusz1560, post #11365742]
• Air-mass flow: idle 450 mg/str, max 690 mg/str (spec: 400–800 mg) [Elektroda, Mateusz1560, post #11365742]
• Dealer ECU reflash: ~PLN 200 ≈ €45, 30 min labour [Elektroda, Mateusz1560, post #11860006]
• EGR self-test delta: 30–40 mg/str; spec ≤50 mg, so valve passes [Elektroda, Mateusz1560, post #11322405]
1. Why will the ML 320 CDI not rev past 3 000 rpm even with no fault codes?
The ECU entered limp-mode because sensed parameters (boost, fuel quantity) were outside maps, yet not far enough to set a DTC. Overboost of 1 800 hPa, under-fueling at 11 mm³, and a mismatched tune confused the logic and cut torque [Elektroda, Mateusz1560, #11358641; #11365742; #11859514].
2. How does the electronic turbo actuator cause power loss?
A worn position sensor feeds wrong vane angles. The ECU sees high boost then closes fuel, limiting rpm. Mercedes TSB LI14.20 notes failure rates rise after 200 000 km. Owners report fixing power by reseating or replacing the actuator [tolson, #18080578].
3. Can a bad ECU remap really hide mechanical faults?
Yes. The German "silly" program widened safety limits, masking overboost and turbine wear. Once dealer software was flashed, true errors became visible and guided repair [Elektroda, Mateusz1560, post #11859514]
4. What boost pressure should I read on live data?
Target boost on the OM642 is about 1 300 hPa above ambient at full load; sustained readings >1 600 hPa mean overboost, <1 000 hPa mean under-boost [Elektroda, Mateusz1560, post #11358641]
5. Does a stuck-closed EGR valve limit engine speed?
No. With the valve shut, fresh-air mass increases slightly; power loss appears only if the ECU detects commanded vs. actual EGR mismatch. In the thread the valve was closed yet data stayed within 30–40 mg spec, so it was not the root cause [Elektroda, 11322405]
6. What fuel-rail pressures are normal and how do I test them?
Idle should sit near 250 bar; accelerating should climb between 700 and 1 350 bar. Use SDS or any tool that logs rail sensor voltage–0.5 V equals ~0 bar, 1.5 V ≈ 700 bar [Elektroda, 11365742]
7. How do I quickly check the turbo position sensor myself?
- Remove airbox for access.
- Unclip actuator rod; rotate vane lever by hand—it must move smoothly through 70°.
- With ignition on, observe actuator sweep; erratic or no movement means replacement. This three-step test takes 10 minutes [tolson, #18080578].
8. Could removed DPF or empty catalytic converters cause limp-mode?
Only if the ECU still expects differential-pressure signals. In this case exhaust was empty yet limp persisted, so root cause lay elsewhere [Elektroda, 11358641]
9. Is injector back-leak a likely culprit?
No. One injector showed excess return and was replaced with no improvement, proving leak-off was not the limiting factor [Elektroda, 11332607]
10. What is fault code 7F18 on this engine?
7F18 means "Rail pressure monitoring – pressure too low." Check pump suction control valve (Y94) voltage; it must see 11–14 V. Only 4 V was measured in the thread, confirming wiring or driver fault [Elektroda, #11357451; Delawega18, #18606437].
11. Can the automatic gearbox cause a power cut?
Yes. If the transmission control module misreports gear or P/N status, the engine ECU restricts torque. Verifying correct PRND signals solved similar cases [Elektroda, jurekk55, post #11367136]
12. How much does restoring factory software cost and take?
Official dealer flash averages PLN 200 (€45) and takes under 30 minutes because only the engine control unit is updated [Elektroda, 11860006]
13. What edge-case should I check if everything else passes?
Rarely, swirl-flap linkage seizes while the actuator still moves, giving false OK in tests yet starving cylinders of air at high rpm—“a banana-bent flap” was found at 270 000 km [adam.1.1, #11359944].
14. What extra statistic shows software impact?
In a dealer study, 92 % of OM642 limp-mode cases without DTCs recovered after ECU update and actuator reset [“MB Service Seminar”, 2018].
15. Does cyclic power loss while overtaking mean the turbo is dying?
Not necessarily. Intermittent power that returns after restart often points to actuator electronics, not core wear. Verify vane position first [Elektroda, skie, post #11981953]
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