Compression Testing (Low Compression Diagnosis)

Symptoms

• Unexpected stalling during cold starts or when pulling away under load, even after the engine has been pre-warmed [0]
• Engine restarts immediately after stalling but stalls again under load; idling for several minutes before moving prevents a repeat stall [0]
• No fault codes stored in the ECU despite the underlying mechanical problem — the system may show clean even when compression is significantly low on one cylinder [0]
• No visible external signs: no exhaust smoke, no oil consumption, no coolant loss, and no wet spots around injectors [0]
• A faint smell of diesel noticeable in the engine bay around the injector area despite no visible leaks [0]
• All other running metrics appear normal — good fuel economy, smooth idle, no abnormal engine noises [0]

Causes

• Worn or damaged piston rings, allowing combustion gases to bypass into the crankcase and reducing cylinder pressure [0]
• Damaged or burnt valves that fail to seat properly, causing compression leak-down past the valve seat [0]
• A failing or blown head gasket on the affected cylinder, allowing compression to escape — though the owner in this case noted no coolant loss or smoke [0]
• Worn cylinder bore or scored cylinder walls reducing the sealing ability of the piston rings [0]

Diagnosis

• Perform a mechanical compression test using a diesel-rated compression tester (a gasoline engine compression tester will not fit or read correctly on the T1N diesel) — compare all cylinder readings against each other; a reading of roughly 50% of the other cylinders on one cylinder indicates a significant problem [0]
• Run the test multiple times on the suspect cylinder to confirm the reading is consistent and not a testing artifact [0]
• Scan for fault codes using a Xentry/DAS or compatible system — note that the ECU may store no fault codes at all even when compression is markedly low on one cylinder [0]
• Inspect the injector area for signs of 'black death' (carbon buildup and diesel leakage) around each injector; a clean injector area does not rule out a compression problem but does rule out injector seal failure as the cause [0]
• After confirming low compression mechanically, perform a leak-down test to determine whether the loss is past the rings (crankcase pressure), the valves (intake or exhaust), or the head gasket (coolant contamination) — the chunks do not detail this step explicitly, so consult a shop for the full leak-down procedure
• Note whether stalling only occurs under load during cold starts — this pattern is consistent with marginal compression that becomes adequate once the engine and cylinder walls warm and expand [0]

Repair

Confirming low compression is a DIY-friendly diagnostic step requiring only a diesel compression tester, but the repair that follows depends entirely on the root cause. A single low cylinder with no smoke, no coolant loss, and no oil consumption (as described in the primary case [0]) makes the cause ambiguous and requires further leak-down testing to narrow down. Repairs can range from a head gasket replacement (significant but DIY-with-skill) to full engine rebuild or replacement work (professional territory). Continuing to drive with confirmed severely low compression risks further internal damage and should be avoided until the cause is understood [0].

Parts

Plain part names — affiliate links and pricing are coming in a later update.

• Diesel compression tester with T1N-compatible adapter (tool, not a replacement part)
• Glow plug(s) — replace any damaged during removal
• Injector copper sealing washer(s) — replace if injectors are removed during testing
• Head gasket set (if leak-down test confirms head gasket failure as the cause)
• Piston ring set (if leak-down test confirms ring blow-by as the cause)

Related forum threads

• 16 ↳Low compression on cylinder #3

Related videos

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From the manuals

• Mercedes fault-code reference

  "The now limiter has been activated. camshaft sensor (B108) Synchronization error between P1354 002 crankshaft sensor (B73) and Camshaft sensor (B108) is faulty. Frequency of camshaft signal is too high. camshaft sensor (B108) Synchronization error between ~I P1354 016 crankshaft sensor (B73) and Faulty sensors or cables. No crankshaft signal from 873. camshaft sensor (8108) Synchronization error between Faulty sensors or cables. Plausibility error between crankshaft and camshaft P1354 032 crankshaft sensor (B73) and position signals. camshaft sensor (B108) Synchronization error between f P1354"

• Workshop manual (2004–2006)

  "OPERATION Intake air is drawn through the air cleaner and into the turbocharger compressor housing. Pressurized air from the turbocharger then flows forward through the charge air cooler located in front of the radiator. From the charge air cooler the air flows back into the intake manifold. DIAGNOSIS AND TESTING- CHARGEAIR COOLER SYSTEM - LEAKS NOTE: Slight engine oil pooling in the charge air inlet hose IS NOT premature turbocharger failure. Slight pooling is the normal result of the breather system. Test the air breather tube for normal operation by referring to the appropriate diagnostic m"

• Workshop manual (2004–2006)

  "The cap is satisfactory when the pressure holds steady. It is also good if it holds pressure within the 124-145 kPa (18-21 psi) range for 30 seconds or more. If the pointer drops quickly, replace the cap. CAUTION: Radiator pressure testing tools are very sensitive to small air leaks, which will not cause cooling system problems. A pressure cap that does not have a history of coolant loss should not be replaced just because it leaks slowly when tested with this tool. Add water to tool. Turn tool upside down and recheck pressure cap to confirm that cap needs replacement. WATER PUMP REMOVAL WARNI"

• Mercedes fault-code reference

  "(A80) P1187 001 Rail pressure monitoring The maximum pressure has been exceeded. P1187 002 Rail pressure monitoring The rail pressure is too low. P1187 004 Rail pressure monitoring Fuel rail cannot pressurize. P1187 008 Rail pressure monitoring The pressure control valve jams in the closed position. P1187 016 Rail pressure monitoring Leakage detected P1187 032 Rail pressure monitoring Leakage detected P1187 064 Rail pressure monitoring Control variation is greater than 1500 rpm P1188 004 Element shut off or high pressure Cable has a short circuit to voltage{+) or short circuit to ground(-). pu"

• Mercedes fault-code reference

  "P0180 002 Fuel temperature sensor (830) The signal voltage is too high. P0190 001 Rail pressure sensor (8113) The signal voltage is too low. P0190 002 Rail pressure sensor (8113) The signal voltage is too high. P0190 004 Rail pressure sensor {B 113) The voltage supply value Is too high or too low P0190 128 Rail pressure sensor (8113) Plausibility of signals between rail pressure sensor (8113) and pressure control valve P0201 001 Injector cylinder 1 (Y16) Excess current on control cable P0201 004 Injector cylinder 1 (Y16) Excess current on common cable P0201 008 Injector cylinder 1 (Y16) Cable"

• Mercedes fault-code reference

  "2625 8 Fuel temperature sensor (830) The signal from the fuel temperature sensor (830) is faulty 2633 1 Mass air flow sensor (8101) The signal from the mass air flow sensor (8101) is faulty 2634 1 Rail pressure monitoring via volume Low fuel pressure or a leak in low pressure side has been reported. control valve 2635 1 Rail pressure monitoring via volume Low fuel pressure or a leak in low pressure side has been reported. control valve 2636 1 Rail pressure monitoring via volume Low fuel pressure or a leak in low pressure side has been reported. control valve 2637 1 Rail pressure monitoring via"

Sources