📋 Quick Facts
Time
30-45 minutes
Difficulty
Moderate
Tools
4 tools
Cost to DIY
$30-60
Compression is the pressure your cylinders build during the compression stroke. A healthy engine reads close to spec on every cylinder, with no more than 15% variation between the highest and lowest. The wet test (squirt oil in, retest) tells you whether a low reading is rings or valves.
🛠 What You'll Need
- Compression tester with assorted spark plug adapters (shop compression tester on Amazon)
- Spark plug socket and ratchet (shop spark plug sockets on Amazon)
- Remote starter switch or trusted helper to crank engine
- Engine oil (a few teaspoons for wet test) (shop engine oil on Amazon)
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⚠ When NOT to DIY thisDisable fuel and spark before cranking with plugs out. Raw fuel can spray from injectors and ignite. Pull the fuel pump fuse AND the ignition coil/MSD power if applicable. Ground each ignition wire to prevent ECU damage.
🎯 Expected Readings (Pass/Fail Reference)
| Healthy gasoline engine | 130 - 180 psi per cylinder (check shop manual) |
| Variation between cylinders | Max 15% (e.g., if highest is 160, lowest must be 136+) |
| Dry vs wet test | Wet test (oil added) much higher than dry = bad rings; same as dry = bad valve or head gasket |
| First crank stroke | Should jump to 60-100 psi on the first puff |
| Pressure should build steadily over 4-5 strokes | No rise after 2-3 strokes = severe valve or ring leak |
| Diesel compression spec | 400 - 600 psi typical (different tester required) |
Numbers are typical. Always cross-check against your factory service manual for the exact spec.
📝 Step-by-Step Test Procedure
- Warm the engine firstCold compression readings are 20-30 psi lower than warm. Run the engine to operating temp, then shut off.
- Disable fuel and sparkPull the fuel pump fuse OR fuel pump relay. Disconnect the primary ignition (coil pack or distributor). Some cars need to be disconnected via OBD2 service mode to prevent the ECU from logging a no-start fault.
- Pull ALL spark plugsEasier to crank the engine with no compression load. Keep plugs in order if you want to inspect each cylinder's color afterward.
- Block the throttle wide openWedge a stick or your foot on the gas pedal (or zip-tie on cable types). Wide-open throttle ensures each cylinder can fill with air on each intake stroke.
- Thread the compression tester into cylinder #1Use the correct adapter for your plug thread size. Hand-tight only - over-tightening damages the plug threads.
- Crank for 4-5 compression strokes (puffs)Have a helper crank, or use a remote starter switch. Watch the gauge - it should jump on the first puff, then climb with each stroke, plateauing by 4-5 puffs. Write down the peak reading.
- Release pressure and move to the next cylinderPress the bleed valve. Move to cylinder #2. Repeat for every cylinder. Compare readings - all should be within 15% of each other.
- Wet test for low cylindersIf a cylinder reads low (say 100 psi when others are 150), squirt 1 teaspoon of engine oil into that cylinder via the spark plug hole. Retest. If reading jumps significantly (50+ psi higher), the rings are worn (oil temporarily sealed the gap). If reading does not change, the leak is at the valves or head gasket. shop compression testers on Amazon.
✅ Pass / Fail Criteria
✅ PASS
All cylinders within 15% of each other, all near spec (130-180 psi), pressure builds steadily over 4-5 puffs, wet test minimal change
❌ FAIL
20+% variation, any cylinder below 100 psi (severe), no pressure build after 2 strokes (huge leak), wet test jumps 50+ psi (worn rings), or any 0 psi cylinder (likely bent valve or head gasket)
🔧 If It Fails - What To Do Next
A failed compression test usually means major engine work: head gasket ($1,500-3,000), valve job ($1,500-2,500), or rebuild/replace engine ($3,000-8,000+). Confirm with a leak-down test before committing - it pinpoints WHERE the leak is (exhaust pipe = exhaust valve, intake = intake valve, oil filler = rings, radiator bubbles = head gasket). See Why is my car burning oil? and How long does an engine last?
🔗 Related Guides
❓ Frequently Asked Questions
What is good compression for a gas engine?
130-180 psi per cylinder for most modern gas engines. Check your service manual for exact spec.
How much variation between cylinders is OK?
Max 15% spread between highest and lowest. More than that suggests a single bad cylinder.
Should I test compression cold or warm?
Warm engine, around operating temperature. Cold readings can be 20-30 psi lower and misleading.
What does a wet compression test tell me?
It isolates ring leaks from valve/head-gasket leaks. Oil seals worn rings temporarily; valves and head gaskets are unaffected.
Can a bad head gasket pass a compression test?
Sometimes - a small head gasket leak only shows up under heat and pressure. Combustion-leak (block) tester at the radiator is the gold standard for head gasket diagnosis.
How long does the test take?
30-45 minutes for a 4-cylinder, up to an hour for V6/V8.