Mercedes M260/M264 Runs Rough? Compression Screens; Leak-Down Locates

Compression Screens, Leak-Down Locates

Preserve the symptom and codes, rule out basic ignition/fuel causes, and compare cranking compression as a screen; an abnormal cylinder still needs Mercedes’ cylinder-leakage test to locate air at the intake or exhaust before the head is removed and valve seats are judged. Compression and cylinder leak-down are often spoken of as if they were the same test. They are not. Relative cranking compression can tell a shop which cylinder deserves attention; controlled leak-down can help tell where that cylinder loses sealing. Mercedes’ valve-seat branch depends on the second question.

The useful tool category here is a gasoline-engine compression gauge used for repeatable cylinder-to-cylinder comparison, but only after the safe test point and question are clear. The AUTOOL PT610 is one practical route for that bounded measurement. PT610 can compare cranking compression among cylinders on a compatible gasoline engine with correct disabling and adapter procedures; it cannot perform cylinder leak-down, identify intake versus exhaust leakage, prove valve-seat damage, or authorize cylinder-head replacement.

This is a boundary explainer rather than a code-to-part guide. The goal is to stop a low compression number from becoming a cylinder-head order before the leakage path is located.

Describe the roughness before naming a cylinder

Record whether roughness occurs cold, hot, at idle, during acceleration, or after a soak. Note poor shifting or a jerk only when engine torque becomes uneven, and separate it from a transmission-only event. Save all mixture, injector-quantity, misfire, fuel-pressure, cam-timing, and voltage codes with freeze frame. If the MIL flashes, the engine knocks, oil pressure is low, or the engine shakes severely, stop running it to protect the catalyst and internal components. Do not repeatedly clear adaptations to chase a smoother idle.

Keep no-code cases in the investigation

Mercedes notes that one or more symptoms may occur and that no fault code may be stored. A no-code rough engine still deserves cylinder balance, ignition, fuel, air, and mechanical evidence. Conversely, an injector or mixture code does not prove the injector caused the event; poor cylinder sealing changes oxygen and combustion signals that the controller must interpret. Build the symptom timeline before moving coils, plugs, or injectors so the original cylinder pattern is not erased by several simultaneous changes.

Use compression as a relative screen

Prepare the engine consistently: correct battery support, approved fuel/ignition disabling, proper adapter, comparable cranking speed, and the throttle/air path set as the service procedure requires. Index plugs and record each cylinder in firing-order or physical order. Compare repeatable readings and cranking pulses rather than applying one universal pressure number across M260 and M264 variants. A low cylinder is a screen. Gauge calibration, sealing at the adapter, valve timing, wash-down, temperature, and cranking speed can all affect the result.

Test	Useful answer	Cannot answer alone
Scan/misfire record	When and where roughness appears	Mechanical sealing cause
Cranking compression	Which cylinder is relatively weak	Intake versus exhaust leak
Cylinder leak-down	Leakage amount and likely path	Visible valve-seat damage
Head inspection	Whether a seat is damaged	Whether earlier electrical/fuel faults coexist

Repeat any low result before escalating

Repeat an unexpected low result with the adapter seal confirmed and battery/cranking speed stable. Compare a neighboring cylinder and, if WIS allows, use additional relative-compression or running evidence. Do not pour oil into a direct-injection cylinder unless the exact procedure calls for it; excess liquid can damage a catalyst or engine. If the result changes after ignition or injector parts were moved, record which single variable changed. A credible mechanical branch begins with a stable, reproducible imbalance.

Use leak-down to locate intake or exhaust loss

Mercedes directs a cylinder leakage check under WIS group 01.00. Position the cylinder correctly, regulate test pressure with the proper equipment, restrain engine rotation, and listen/measure at approved points. Air at the intake path supports an intake-valve sealing loss; air at the exhaust supports an exhaust-valve path. Crankcase or coolant evidence suggests other directions. Leak-down percentage is equipment- and procedure-dependent, so use WIS limits rather than an internet-wide pass/fail number.

Remove the head only after leakage earns it

Only excessive leakage located at intake or exhaust valves earns cylinder-head removal and valve-seat inspection. Head work requires timing control, high-pressure fuel safety, coolant/oil handling, precise marking, and contamination protection. Mercedes warns not to interchange narrower intake rocker-arm rollers with exhaust components and requires reused parts to be inspected. The engine should be rotated through at least two revolutions after assembly checks. This is not a repair to begin from a single scan code or one hurried compression reading.

Do not rework a damaged valve seat

If valve-seat damage is found, Mercedes says the complete cylinder head must be replaced; the seats cannot be reworked. Do not substitute local seat machining for the stated branch. Protect oil ports during surface cleaning, avoid abrasive or sharp tools and unapproved cleaners, and remove debris from the coolant jacket so it cannot cause later overheating. Change engine oil and filter after the repair as directed. Other worn components remain separate findings rather than automatic parts of the valve-seat conclusion.

Verify oil, timing, compression balance, and the original drive event

After assembly, confirm timing, fluids, leak integrity, and clean fuel connections before starting. Save cold and hot compression balance where appropriate, run XENTRY checks, and verify smooth idle, acceleration, and shifting under the original condition. Watch misfire counters, mixture behavior, oil/coolant state, and code return. A smoother engine in the bay is encouraging, but the repair record should connect located leakage, visible seat damage, correct head replacement, and a controlled drive with no recurrence.

A disciplined shop can use both tests without asking either to do the other’s job. Compression identifies a cylinder that deserves attention; leak-down locates the sealing loss; only direct inspection makes the valve-seat replacement decision.

Official references

• LI01.30-P-072328 v10—Wear of Valve Seat or Valve Guide
• Mercedes-Benz Recall Lookup