Mercedes M139 Rough Running or Hard Start? Compare Compression Before Teardown

Find The Outlier Before Teardown

A rough-running or hard-starting Mercedes-AMG M139 can have a broken valve spring, but Mercedes does not begin by removing the cylinder-head cover. LI01.30-P-075439 begins with a Pico relative-compression comparison across all four cylinders. The next branch depends on whether an outlier exists and whether the engine number falls before or after the model-family clean point.

A threaded compression gauge can later add a different kind of cylinder comparison when the current diagnostic plan calls for it. The AUTOOL PT610 cylinder pressure gauge is one possible manual route on compatible gasoline engines. It is not the Pico relative-compression test named by Mercedes, cannot identify a broken spring, and cannot replace the clean-point or TIPS decision.

The article’s centerpiece is a four-column chart. One cylinder must stand apart before the case gets physically deeper.

Preserve which cylinder complained

Save the complete scan before clearing codes or moving ignition parts. LI01.30-P-075439 lists P030100 through P030400, and customers may report a CEL, rough running, trouble starting, or ECO stop/start trouble. Record current/history status, occurrence count, freeze frame, misfire counters, crank time, battery voltage, engine temperature, and whether the event is cold, hot, idle, load, or restart related.

Stop repeated operation if the MIL flashes, mechanical noise appears, oil pressure warns, or the engine shakes severely. A sustained misfire can overheat the catalyst, while a valve-train fault can compound contact damage.

Record the engine number before touching hardware

The document gives separate clean points: November 17, 2022 with a stated engine number for C43/SL43, and November 19, 2022 with another engine number for CLA45/GLA45. Confirm full VIN, exact M139 variant, engine serial, build data, current software, and latest WIS/TIPS direction.

Do not infer before/after status from registration year. An engine may have been replaced, and model production dates do not map perfectly to engine sequence. Photograph the identification and preserve dealer confirmation. A clean point changes the diagnostic escalation; it does not guarantee every later spring is good.

Run the specified relative-compression comparison first

Mercedes specifies a Pico compression test before disassembly. Relative compression observes the starter’s changing load as cylinders pass compression, allowing all four contributions to be compared without first opening every plug bore. Current WIS controls battery support, fuel/ignition disablement, connection, cranking duration, and analysis.

Build a chart with cylinders 1–4, code association, relative peak/area, repeat-run consistency, and test conditions. A weak battery, changing cranking speed, starter fault, decompression event, or poor connection can distort the pattern. Repeat an outlier before interpreting it.

Pattern	Mercedes-directed implication	What is not proved
No discrepancy between coded and non-coded cylinders	Stop and open TIPS case	That ignition/fuel/software is normal
Discrepancy, engine after clean point	Stop and open TIPS case with results	Broken spring
Discrepancy, engine before clean point	Direct spring/lifter inspection branch	Which component or damage extent
No discrepancy after clean point	Guided tests for relevant codes	That no intermittent mechanical issue exists

Read symmetry before absolute pressure

The first question is whether one cylinder differs under the same cranking event. That comparison is often more robust than importing a single pressure number from another engine, altitude, gauge, temperature, or throttle state. Label the coded cylinder, but do not normalize a poor trace simply because the DTC points elsewhere.

If all four are similar, follow guided misfire tests. Mercedes notes injector performance data, spark-plug inspection for cracking/damage, and smoke testing for air leaks in the applicable later-engine branch. Repair actual findings; do not open the head because the original symptom sounded mechanical.

Use manual compression only as a different test

When current WIS calls for manual compression, warm/cold state, battery support, throttle position, fuel/ignition disablement, adapter, cranking pulses, and cylinder order must be consistent. Index all plugs and prevent debris entering the bores. Compare repeat readings and the spread between cylinders rather than chasing a universal internet target.

Manual pressure can support a cylinder-sealing concern, but a broken spring may behave intermittently, and pressure loss may also involve rings, a valve, timing, or test setup. Leak-down, borescope, waveform, and direct movement evidence answer different questions.

Open the cover only on the earned branch

For a before-clean-point engine with a confirmed relative-compression discrepancy, the document directs technicians to remove the cylinder-head cover and inspect springs and lifters. This is professional work involving timing components, precise fasteners, oil control, clean assembly, and risk of further damage.

Photograph spring height and position before moving parts. Compare the suspect cylinder with the other three. A broken coil, displaced piece, abnormal lifter, retainer, or rocker evidence should be preserved rather than immediately removed from context.

Inspect collision evidence beyond the spring

When a broken spring is found, Mercedes calls for inspection of the valve, rocker, retainer, keepers, camshaft, and lobe, plus a borescope view of the piston face for valve contact. Once the valve can seat, a leak test checks valve/seat integrity. Piston or valve-seat damage moves the case back to TIPS with the complete record.

Replacing only the obvious spring before examining contact partners can leave the cause or consequence behind. Conversely, a piston mark in a distorted image should not become an engine verdict without orientation, repeat views, and the guided repair decision.

Verify contribution, starting, and stop/start together

After the approved repair or guided-test correction, restore every component and perform the specified adaptations, oil checks, and scans. Repeat the original cold or hot start, idle, controlled load, and ECO stop/start condition. Compare all four contribution traces or misfire counters under the same state.

Close the case only when cranking and starting normalize, the four-cylinder comparison is stable, roughness and relevant codes do not return, and no new mechanical noise appears. One cleared P030x after a warm idle does not verify the original hard-start window.

Official references

• Mercedes-Benz LI01.30-P-075439: M139 poor performance and broken valve springs
• Mercedes-Benz VIN recall lookup