COMPRESSION QUICK REFERENCE
Variance thresholds, wet test interpretation, leak-down ranges, and the diagnostic decision tree. Print-ready — save it to your toolbox lid.
Most "bad compression" comebacks are misdiagnosis, not bad rings. Get these four right or every number below is meaningless.
1 · BATTERY VOLTAGE DURING CRANK
| > 9.6V | Trust the readings |
| 8.5–9.6V | Marginal — recharge first |
| < 8.5V | Unreliable. Fix battery before testing. |
2 · ALL PLUGS OUT
Leaving plugs in the other cylinders fights cranking torque. Engine spins too slow → false-low readings across the board. Pull all four (or six, or eight) before testing any of them.
3 · WOT THROTTLE
Throttle closed = restricted intake = false-low readings on every cylinder. Wedge the throttle blade fully open with a clean rag or block before cranking.
4 · ENGINE AT NORMAL OPERATING TEMP
Cold engine = tight clearances + thick oil = misleading-high readings that hide marginal sealing. Test warm (180°F+ coolant) unless the customer's complaint is cold-only.
Absolute numbers vary by compression ratio, displacement, and cam profile. Cylinder-to-cylinder variance matters more than the absolute number.
| Engine | Typical cranking compression | Compression ratio range |
|---|---|---|
| 4-cyl gas, naturally aspirated | 140–180 psi | 9.5:1 to 11.0:1 |
| V6 gas, naturally aspirated | 150–200 psi | 10.0:1 to 11.5:1 |
| V8 gas, naturally aspirated | 150–200 psi | 10.0:1 to 11.5:1 |
| Direct injection turbo gas | 170–230 psi | 9.0:1 to 11.0:1 |
| Hybrid gas (Atkinson cycle) | 125–165 psi | 13.0:1 to 14.0:1 (effective lower) |
| Light-duty diesel | 350–500 psi | 15.0:1 to 18.0:1 |
Atkinson-cycle hybrids (Prius, Camry HV, Accord HV, etc.) read LOWER than conventional gas engines because of intake valve closing strategy. This is normal — variance is still the diagnostic signal.
Variance = (highest reading − lowest reading) ÷ highest reading × 100. This is the strongest sealing signal on a multi-cylinder engine. Customer should be paying you to think about variance, not absolute numbers.
| Variance | Status | Action |
|---|---|---|
| < 10% | Healthy | Cylinders sealing. Look elsewhere if you have a drivability complaint. |
| 10–15% | Wear | Document. Monitor at next service. Run leak-down if customer mentions oil consumption. |
| 15–20% | Early failure | Run wet test + leak-down on lowest cylinder. Quote the customer the diagnostic path. |
| > 20% | Sealing problem | Wet test + leak-down required to isolate the failed component before quoting any repair. |
After a low dry reading, squirt 1 teaspoon of clean engine oil through the spark plug hole, crank 5–6 revolutions, and re-test. The pressure rise tells you whether the leak is at the rings or somewhere else.
| Dry → Wet rise | Diagnosis | Repair path |
|---|---|---|
| < 5 psi | Valves or head gasket — oil can't seal what's leaking | Leak-down test next. Locate valve vs. gasket by ear. |
| 5–15 psi | Mild ring wear, possibly normal on high-mileage | Document. May not require teardown — discuss with customer. |
| 15–40 psi | Significant ring or cylinder wall wear | Quote teardown evaluation. Bore wear + ring replacement likely. |
| > 40 psi | Severe ring failure or hole in piston | Endoscope cylinder before pulling head. May find broken ring or melt damage. |
Wet test rules out / rules in rings. If wet test doesn't rise, you've eliminated rings → narrow to valves or gasket via leak-down.
Apply 100 psi shop air to a cylinder at TDC compression stroke. The leak-down gauge reads the percentage escaping. Combine with where you hear the air going for the diagnosis.
| Leakage | Status | Notes |
|---|---|---|
| 0–10% | Healthy | Normal across all engine types and mileages |
| 10–20% | Monitor | Acceptable on high-mileage engines (100k+). Document. |
| 20–30% | Sealing problem | Locate by ear before quoting |
| > 30% | Condemn | Verify TDC compression first — false-high readings if at TDC exhaust |
WHERE THE AIR ESCAPES = WHAT'S LEAKING
| Throttle body / intake | Intake valve not sealing |
| Exhaust tail pipe | Exhaust valve not sealing |
| Oil filler / dipstick | Rings or cylinder wall |
| Radiator coolant bubbles | Head gasket between cylinder and water jacket |
| Adjacent spark plug hole | Head gasket between cylinders |
Symptom: customer reports rough running, power loss, or you have a misfire DTC for a specific cylinder. Start here.
A cylinder reads 130 psi on a 4-cyl engine — that sounds bad until you check the other three: 132, 131, 133. The "low" cylinder is dead-on healthy. Always think variance, not floor.
Weak battery → slow crank → low readings on every cylinder → tech replaces "bad" head gasket → customer comes back. Battery load test is not optional. Verify 9.6V+ during crank before trusting any number.
Compression tells you SOMETHING is leaking. Leak-down tells you WHERE. Compression alone never justifies a teardown — you'd be guessing at intake valve vs exhaust valve vs rings vs gasket. Always leak-down before quoting major repair.
15% leak-down on a 220k-mile cylinder isn't a death sentence — it's normal wear. Customer doesn't need a $2,400 rebuild. Document, recommend monitoring, suggest oil consumption tracking. Save your teardown quotes for engines that actually need them.
This is the page worth printing and sticking inside your toolbox lid.
BEFORE CRANK
Battery > 9.6V during crank · All plugs out · WOT throttle · Engine warm
VARIANCE
| < 10% | Healthy — look elsewhere |
| 10–20% | Wear — wet test + leak-down |
| > 20% | Sealing problem — required diagnostic path |
WET TEST RISE
| < 5 psi | Valves or gasket — leak-down next |
| 5–40 psi | Rings — teardown quote |
| > 40 psi | Severe ring failure or piston hole |
LEAK-DOWN %
| 0–10% | Healthy |
| 10–20% | Wear / monitor |
| > 20% | Sealing problem — locate by ear |
LEAK LOCATION
| Intake | Intake valve |
| Exhaust | Exhaust valve |
| Oil filler | Rings or wall |
| Radiator | Head gasket → coolant |
| Adjacent plug hole | Head gasket → adjacent cyl |