FAQ
TL;DR: A 2008 Mercedes A‑Class W169 CDI with 95,000 km showed idle “humming” until a faulty thermostat was replaced; “Do a decent computer diagnostics.” [Elektroda, moto-kord, post #16865626]
Why it matters: This FAQ helps Mercedes W169 owners diagnose idle surging/hunting and avoid unnecessary parts swaps.
Quick Facts
- Vehicle in thread: Mercedes A‑Class W169 CDI, 2008, 95–97k km documented. [Elektroda, krzysztofx2, post #16904515]
- Symptom: gentle idle waves/hum; no warning lights; strong power reported. [Elektroda, krzysztofx2, post #16747141]
- Stored code noted: B2/5 HFM (MAF) signal line short/break; occasional spike to ~1000 rpm. [Elektroda, krzysztofx2, post #16904515]
- Confirmed fix: new thermostat; coolant now a steady ~90°C, idle surge gone. [Elektroda, krzysztofx2, post #17044426]
- Scan visit cost mentioned: 50 PLN for clearing the code only. [Elektroda, krzysztofx2, post #16904515]
What’s the most likely cause of idle humming on a W169 CDI like this thread?
In the documented case, a stuck‑open thermostat caused low operating temperature, unstable fueling, and idle hunting. Replacing the thermostat restored a steady ~90°C and stopped the surging. Start by checking coolant temperature behavior before buying sensors. [Elektroda, krzysztofx2, post #17044426]
How do I confirm the thermostat is the culprit?
Watch coolant temperature. If it stays below ~80°C when driving and drops at idle, suspect the thermostat. A proper warm engine should stabilize near 90°C. After replacement, the user reported stable temp and smooth idle. [Elektroda, krzysztofx2, post #17044426]
Quick How‑To: check for thermostat‑related idle hunting
- Cold start, drive gently, and monitor coolant temperature trend.
- Note if it won’t exceed ~80°C cruising and falls further at idle.
- Replace thermostat if temps never reach ~90°C and idle surges persist. [Elektroda, krzysztofx2, post #17044426]
Is unplugging the MAF (HFM) a useful test on this engine?
Yes. The owner unplugged the flow meter, and the idle stopped waving and felt livelier. Reconnecting brought the surge back, suggesting MAF involvement or masking a thermal issue. Use this as a quick A/B test, then diagnose properly. [Elektroda, krzysztofx2, post #16907194]
What does error B2/5 HFM signal line short/break mean in plain terms?
It indicates the hot‑film air‑mass meter’s signal circuit saw a short or open. Expect erratic airflow data and idle instability. The same session noted confirmed injector health and a spontaneous idle rise, supporting airflow signal suspicion. [Elektroda, krzysztofx2, post #16904515]
Which live data should I log with Star Diagnosis (SD) to find the root cause?
Log air‑mass value, EGR test status, commanded vs. actual rail pressure, and injector corrections. As one expert said, “air mass, EGR valve test, required and current injection pressure, corrections on the injectors ...”. [Elektroda, Dziarski Hank, post #16865477]
I replaced the fuel pressure regulator and nothing changed—normal?
Yes. In this thread, swapping the regulator did not affect the idle wave. This shows why data‑driven diagnosis beats part‑swapping. Consider temperature control and airflow signals before rail hardware. [Elektroda, krzysztofx2, post #16825148]
Are warning lights expected with gentle idle waves?
Not always. The owner reported no warning lights and normal starting and power despite the humming idle. Don’t rely on lamps alone; inspect temperatures and scan live data. [Elektroda, krzysztofx2, post #16747141]
Why does idle sometimes jump to ~1000 rpm on a warm engine?
The session documented sporadic rises to ~1000 rpm along with an HFM signal fault. Irregular airflow readings or thermal mismanagement can trigger unstable idle control. Capture a live trace when it happens. [Elektroda, krzysztofx2, post #16904515]
What is HFM on a Mercedes—same as MAF?
Yes. HFM means hot‑film mass air flow sensor. Mercedes diagnostics use HFM wording, as seen in the B2/5 error. It is the engine’s primary airflow sensor. [Elektroda, krzysztofx2, post #16904515]
Will cleaning the EGR valve solve idle hunting?
Not in this case. The EGR had been cleaned and the intake checked, yet the idle wave persisted until the thermostat was replaced. Prioritize temperature regulation checks. [Elektroda, krzysztofx2, post #16772306]
What did a professional SD visit actually find here?
The visit confirmed 97,000 km, good injectors, and stored B2/5 HFM signal fault. The shop cleared the code for 50 PLN, with no improvement. Use SD to record live data, not only erase faults. [Elektroda, krzysztofx2, post #16904515]
After fixing the thermostat, what should I expect at idle?
Expect a steady coolant temperature near 90°C and the idle surge to disappear. The thread author reports stable temperature and smooth operation after replacement. Verify with a warmup and short drive. [Elektroda, krzysztofx2, post #17044426]
“Do I need diagnostics if the car drives fine?”
Yes. As advised: “Do a decent computer diagnostics.” Check airflow, rail pressure, injector trims, and EGR tests to prevent blind repairs. A smooth drive can still hide control issues. [Elektroda, moto-kord, post #16865626]
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