What compression and leak-down tests reveal before you buy
A compression test and a leak-down test each check different things on a used engine. Here's what the numbers mean and when to ask for both.
# What compression and leak-down tests reveal before you buy
AI-generated illustration for this story; not a photograph.
**Quick answer.** A compression test measures how well each cylinder holds pressure on the compression stroke; a leak-down test pressurizes a cylinder at top dead center and shows exactly where air escapes — rings, valves, or the head gasket. Together they tell you whether a used engine's internals are sound before you commit to the purchase.
What does a compression test actually measure?
A compression tester threads into a spark plug or injector hole and records the peak pressure each cylinder produces as the engine is cranked over. It's a fast, low-cost way to spot a cylinder that's clearly underperforming. What it doesn't tell you is *why* a cylinder is low — a compression test alone can't distinguish worn rings from a burnt valve from a blown head gasket.
What does a leak-down test add that compression testing doesn't?
A leak-down test pressurizes a single cylinder with shop air while the piston sits at top dead center on its compression stroke, then measures how much of that pressure escapes over a fixed time. Where the air is heard escaping tells you the source: air at the intake points to a leaking intake valve, air at the exhaust points to an exhaust valve, air at the oil fill points to worn rings, and bubbles in the coolant reservoir point to a head gasket. That diagnostic detail is what a compression number alone can't give you.
What readings should worry you?
There's no single universal pass/fail number — acceptable compression varies by engine design and displacement. What matters more is consistency between cylinders.
| Signal | What it usually means | |---|---| | All cylinders within about 10% of each other | Normal spread, no immediate red flag | | One cylinder noticeably lower than the rest | Worth a leak-down test to isolate the cause | | Leak-down air escaping at the intake or exhaust | Points to a valve, not the rings | | Leak-down air escaping into the crankcase | Points to worn rings or cylinder wall | | Bubbles in the coolant during a leak-down test | Points to a head gasket issue |
Can you run these tests on an engine that's already pulled?
Yes, though it takes a bit more rigging than testing an engine still in a vehicle. The engine needs to be turned over by hand or with a breaker bar on the crankshaft pulley to get each cylinder to top dead center for a leak-down test, and a compression test needs the engine crankable by some method even off the mounts. Sellers who run these tests before listing an engine are usually doing exactly this.
What should you ask the seller before you test?
Ask whether compression or leak-down numbers were recorded at the time the engine was pulled, and if so, ask for the actual numbers per cylinder rather than a summary like "tested good." Ask whether the engine was warm or cold when tested, since that affects compression readings. If no numbers exist, ask whether you'll have the opportunity to test it yourself before the return window closes.
How do these tests fit into a broader inspection?
Compression and leak-down testing check the engine's internal mechanical condition, but they don't cover everything. Pair them with a check of the mileage the seller is representing, a look at the interchange or casting numbers to confirm it's the right engine for your vehicle, and a read of the warranty terms in case something turns up after installation that these tests couldn't catch in advance.
Frequently asked questions
**Can I trust a seller's compression numbers without retesting?** Treat them as a starting point, not a guarantee. Numbers can shift between when a seller records them and when you take delivery, and testing method (wet versus dry, engine temperature) affects results. Retest after you receive the engine if the numbers matter to your decision.
**What's a normal spread between cylinders?** Most healthy engines keep cylinder-to-cylinder readings within about 10 percent of each other. A bigger spread points at an issue in the low cylinder even if every reading is technically "in range."
**Do I need both tests, or is one enough?** Compression testing is the faster first pass and catches most problems. Leak-down testing is worth the extra time when a cylinder reads low and you need to know whether it's rings, a valve, or the head gasket before you decide whether the engine is still a good buy.
**Is a leak-down test something I can do myself?** Yes, with a leak-down tester, an air compressor, and the engine set at top dead center on the cylinder you're testing. It takes more setup than a compression test but doesn't require special training.
