What Is a Good PSI for a Compression Test?

The short version for gas engines

A healthy gasoline engine usually reads somewhere between 150 and 200 psi in each cylinder when cranked with the throttle open. Some older or low compression engines read lower and still run fine. Many modern high compression engines read higher.

No single number fits every engine. The best target is the figure in your service manual. If you can't find it, the compression ratio gives you a decent rough guide, shown in the next section.

Remember that the test is a comparison. Four cylinders that all read 160 psi are in much better shape than three at 175 and one at 120.

Rough targets by compression ratio

Higher compression ratios squeeze the air and fuel harder, so they produce higher readings. Use this table as a general rule of thumb when you don't have the factory spec. It is not a replacement for the manual.

Why the spread between cylinders matters

A single low cylinder tells you more than a slightly low average. Mechanics generally look for the weakest cylinder to be within about 10 to 15 percent of the strongest. Some turbocharged engines are a little different, so check the spec for yours.

A large gap usually means one cylinder is not sealing well. That can cause a rough idle, a misfire or a loss of power, even when the other cylinders look fine.

Readings that are low across every cylinder point in another direction. Common causes are a weak battery or starter that cranks too slowly, a gauge that is not seated properly, or general wear.

How to run the test

The process is simple, but the details change the numbers. Follow the same steps on every cylinder so the results are comparable.

• Warm the engine to normal temperature, then turn it off.
• Disable the fuel and ignition systems so the engine can't start. Your manual shows the right fuse or relay.
• Remove all spark plugs. Label the wires or coils so they go back in the right place.
• Block the throttle wide open so enough air can enter.
• Thread the gauge adapter into the first plug hole by hand. Don't overtighten it.
• Crank the engine for about four to six compression strokes, until the needle stops climbing.
• Write down the reading, release the pressure with the gauge valve and repeat for each cylinder.

Safety notes before you start

Disabling the fuel pump and ignition matters. Cranking with live ignition can send a spark into the open plug hole, and fuel vapor can ignite. Keep sparks, open flames and loose fuel lines away from the work area.

Wear safety glasses, since plugs, debris or a popped adapter can fly out of the hole. Let the engine cool enough that you can touch the exhaust manifold area without burns, and keep hands, hair and loose clothing away from belts and fans while someone cranks the engine.

On diesels, always release pressure through the gauge's relief valve before you disconnect it. The kits on our best compression tester page that list a pressure relief valve make this easier.

What low readings usually mean

If one cylinder reads low, the next step is a wet test. Squirt a small amount of engine oil into that cylinder and repeat the test. If the number jumps noticeably, worn piston rings are the likely cause, because the oil temporarily helps seal them. If it barely changes, suspect a valve that is burnt or not seating, or a head gasket problem.

Two neighboring cylinders that are both low often point to a failed head gasket between them. Very low readings across the board suggest a timing issue or widespread wear.

A leak-down tester goes one step further. It feeds air into the cylinder and shows the percentage that escapes, and you can often hear or find where the air is going: the exhaust, the intake or the coolant. Several kits on our compression tester comparison include one, and the 46-piece DUOYI and Wzyblixo kits both list a leak-down tester with a quick release valve.

What about diesel engines?

Diesel engines compress air much harder because they rely on heat from compression to ignite the fuel. Typical readings are far higher than for gas engines, so a standard 300 psi gas gauge is not suitable.

The diesel capable kits in our data read up to 700 psi (DUOYI and XSTARYE) or 725 psi (Wzyblixo), which gives headroom for high compression engines. By contrast, the BETOOLL kit is listed as gasoline only with a 300 psi gauge.

Diesel specs vary a lot by engine, and the allowed difference between cylinders is often stated in the manual. Use the factory figure rather than a gas engine rule of thumb.

Picking a gauge that gives clear numbers

Gas engine owners need a gauge that reads comfortably above their expected pressure, with a margin. A 0 to 300 psi dial covers nearly every gas engine. The BETOOLL gauge, for example, has a 3 in dial with psi and bar scales, plus a pressure release button.

Check that the adapters fit your spark plug thread and that the hose is long enough to reach each hole. A quick release valve and a clear dial save time when you are reading cylinder after cylinder.

Frequently asked questions

Is 120 psi bad for a compression test?

Not always. Some older or low compression engines spec lower pressures. Compare it with your factory spec and with the other cylinders. A reading well below the rest is a bigger warning than a number that is simply modest.

Should I test with the engine hot or cold?

Most guides suggest a warm engine, since parts have expanded to normal operating size. Cold engines can read a bit lower. Whichever you choose, use the same conditions for every cylinder.

Why does my reading keep climbing on each crank?

Compression builds over several strokes. Crank until the needle stops rising, then record that top number. If the first stroke is very low and it climbs slowly, worn rings are a common cause.

Can a bad gauge or adapter fake a low reading?

Yes. A leaking adapter, a gauge that is not seated or a damaged hose can all lose pressure. Recheck the connection, and try another adapter if one cylinder reads oddly.

How many psi can cylinders differ by?

A common guideline is that the lowest cylinder should be within about 10 to 15 percent of the highest. Your service manual may give a tighter or looser limit for your engine.