3.4 Dry, Wet and Running Compression

Cranking (Dry) and Wet Compression Testing (ASE T1 Task A.8)

Once power balance or Mode \$06 misfire data points to one or more weak cylinders, a cranking compression test isolates whether the fault is mechanical cylinder sealing.

Mandatory Five-Step Test Preparation

Skipping any preparation step skews pressure readings across the bank and can cause catalyst damage or engine bay fires:

1. Normal Operating Temperature (190°F–210°F): Warm the engine completely so pistons, rings, cylinder walls, and valvetrain clearances are at normal thermal expansion.
2. Battery Fully Charged (>= 12.6 V): Verify open-circuit battery voltage and connect a shop battery charger/maintainer. If cranking speed slows down from 220 RPM on Cylinder 1 to 140 RPM on Cylinder 8, later cylinders will read falsely low.
3. Disable Fuel Injection and Ignition Systems: Pull the fuel pump/injector relay or fuse (and relieve rail pressure) and disconnect primary power to the ignition coils. Why: Unburned fuel sprayed during cranking washes the lubricating oil film off the cylinder walls—artificially lowering ring sealing and compression readings—and dumps raw fuel into the exhaust catalyst.
4. Remove All Spark Plugs: Label each plug by cylinder number for visual inspection. Removing all plugs eliminates compression drag from the other cylinders so the starter cranks at maximum, uniform RPM.
5. Secure the Throttle Plate Wide Open (WOT): The cylinders cannot compress air they cannot inhale. On Electronic Throttle Control (ETC / drive-by-wire) medium/heavy-duty trucks, pressing the accelerator pedal to the floor with the key on often does not command the throttle plate to 100% WOT (unless the PCM's clear-flood mode explicitly opens the blade fully during cranking). Always remove the air intake boot to visually verify the throttle plate is open, use a scan tool bidirectional throttle-open routine, or use the manufacturer's approved mechanical test procedure; never force or wedge an electronic throttle blade.

First-Puff vs. Final-Puff (4–6 Strokes) Analysis

Thread the compression gauge tightly into the spark plug port and crank the engine through 4 to 6 compression strokes (counting each upward surge of the gauge needle), recording both the 1st puff and the final stabilized puff.

Minimum Specification & The 75%–80% Variance Rule

Aside from meeting the OEM minimum pressure specification (typically 130–150 psi minimum on modern gasoline truck V8s), the lowest-reading cylinder must be at least 75% to 80% of the highest-reading cylinder (a maximum reduction of 20% to 25% from the highest cylinder, respectively; do not equate percentage difference to a fixed psi difference).

Performing the Wet Compression Test Safely

When a cylinder reads below specification on the dry test, squirt approximately a metered amount (a small manufacturer-specified quantity, or 3–4 pump squirts) of clean SAE 30 engine oil into the spark plug hole, rotate by hand as prescribed to distribute the oil and check that no liquid lock occurs before starter cranking, and repeat the compression test:

• Pressure rises by +30 to +50 psi: The heavy oil film temporarily seals the gap between the piston rings and cylinder wall, supporting worn piston rings or cylinder bore wear.
• Pressure changes by < 5–10 psi: Oil at the piston ring belt cannot seal the top of the chamber, supporting leakage past an intake valve, exhaust valve, or cylinder head gasket.

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Running (Dynamic) Compression Testing (ASE T1 Task A.8)

A running compression test (also called a dynamic compression test) is performed with the engine idling at operating temperature by threading the compression gauge into one cylinder at a time:

1. Curb Idle Reading (Schrader Relief Valve Depressed): Use a gauge or pressure transducer approved for running-compression testing and follow its capture/reset procedure. Disable fuel and ignition to the tested cylinder, protect against hot gas and moving parts, and retain a secure hose connection. Because manifold vacuum restricts the incoming air mass at closed throttle, normal running compression at curb idle is 60 to 90 psi.
2. Snap-WOT Dynamic Peak: Quickly snap the throttle to Wide Open Throttle (2,000–2,500 RPM) and release. A healthy cylinder's running WOT peak reaches approximately 80% of its cranking compression (e.g., 130 to 145 psi on an engine with 170 psi cranking compression).
3. Catching a Worn Camshaft Lobe: At slow starter cranking speeds (~200 RPM), even a severely worn, rounded camshaft lobe that opens the intake valve only 0.080 in. instead of 0.450 in. allows enough time for atmospheric air to trickle into the cylinder and produce a normal dry cranking compression reading (165 psi). At running speeds (1,500–2,500 RPM), the piston descends so fast that the barely-opening intake valve starves the cylinder of air, causing running compression to stay abnormally low (35–45 psi on WOT snap) and pinpointing reduced valve lift.

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Worked Example: 8-Cylinder Compression Chart & Diagnosis

A fleet technician tests a 6.6L V8 medium-duty truck engine with a dead misfire on Bank 1 (Cylinders 1, 3, 5, 7 on Left Bank; 2, 4, 6, 8 on Right Bank):

• Dry Cranking Finals: Cyl 1 = 180 psi, Cyl 2 = 175 psi, Cyl 3 = 72 psi, Cyl 4 = 178 psi, Cyl 5 = 75 psi, Cyl 6 = 172 psi, Cyl 7 = 176 psi, Cyl 8 = 174 psi.
• Minimum Spec Calculation: Highest cylinder is Cyl 1 (180 psi). Using the 80% rule (180 psi × 0.80 = 144 psi), any cylinder below 144 psi fails.
• Wet Test on Cyl 3 & Cyl 5: Adding a small manufacturer-specified quantity of SAE 30 oil yields Cyl 3 = 74 psi and Cyl 5 = 76 psi.
• Diagnosis: Cylinders 3 and 5 are physically adjacent on Bank 1, read nearly identical low pressures (72 and 75 psi), and do not improve with oil—suggesting a shared sealing fault; confirm communication between the cylinders with leak-down testing before removing the head.