Compression or Leak-Down Test? Find the Leak, Pick the Tester

Your engine is down on power, hard to start or using oil, and you suspect a tired cylinder. A compression test finds the weak cylinder; a leak-down test shows where its pressure goes. Here is which tester to choose and how to read it.

The Compression & Leakage Testers range at PartsForMachines has 105 products, including 19 compression test kits and 4 cylinder leakage testers from Draper, Sealey and Sykes Pickavant.

Compression Test or Leak-Down Test: Which Do I Need?

Start with a compression test to find the weak cylinder, then a leak-down test to find out why. A compression gauge fits each plug hole in turn and records the peak pressure the cylinder builds while the starter cranks the engine.

A leak-down test is done with the engine stopped. The piston sits at top dead centre on its compression stroke, so both valves are shut, and the tester feeds in regulated air from a compressor and shows how much escapes. A twin-gauge kit such as the 35884 cylinder leakage kit shows cylinder pressure and the percentage of leakage side by side.

Petrol or Diesel: Which Compression Tester Should I Buy?

Match the gauge to the engine, because a diesel builds far more pressure than a petrol engine. The petrol kits in our range read to about 20 bar (300psi): the CT955 petrol compression test kit, for example, has a 300psi gauge with 10, 12, 14 and 18mm plug adaptors.

Diesel kits read much higher, such as 70 bar on the 35878 diesel compression test kit, and connect through dummy glow plugs or injector adaptors that must suit your engine. For a tractor or truck, pick a kit that lists agricultural or commercial diesels, such as the VS2044.

What Leak-Down Percentage Is Acceptable?

There is no single pass mark, but as a rough guide 10% or less is healthy, 10 to 20% shows wear worth watching, and over 20% often means repair work.

Readings also vary between testers, because the size of each tester’s internal orifice changes the result. So compare cylinders on the same tester, at the same pressure and temperature: one cylinder well above the rest tells you more than any single number. The VSE2020 twin-gauge cylinder leakage tester suits petrol engines with 10, 12 or 14mm spark plugs.

Air Escaping on a Leak-Down Test: Is It the Rings, Valves or Head Gasket?

Listen and look while the cylinder holds pressure: where the air comes out shows what is leaking. A hiss from the oil filler, dipstick tube or breather points to the piston rings. Air from the intake or throttle body means the inlet valve is not sealing; from the exhaust pipe, the exhaust valve.

Bubbles in the coolant reservoir point to a leak into the cooling system, past the head gasket or through a crack in the head. Leakage kits such as the 35884 are built to help find these faults, from worn rings and valves to a cracked head or a blown head gasket.

What Should I Check Before I Buy?

• petrol, diesel or both, and the engine’s expected compression from the workshop manual, so the gauge range covers it
• the spark plug or glow plug thread, and whether the plugs sit deep in the head
• whether you also want a leak-down test, and have a compressor and air line to run it
• a hose with a one-way valve that holds the reading and a release valve to reset it
• any kit you already own: some leakage testers accept the adaptors from the same maker’s compression kits

Browse the Compression & Leakage Testers range, or contact the PartsForMachines team by WhatsApp, telephone or email with the engine make, model and fuel type.

Product options, delivery options and returns terms are shown on each product page. A 30-day return policy applies to eligible items.

Frequently Asked Questions

What does a wet compression test tell me?

Whether the rings are to blame for a low cylinder. Squirt a small amount of clean engine oil through the spark plug hole and test again. A big rise suggests worn rings; little or no change points elsewhere, often a valve.

Should the throttle be open for a petrol compression test?

Yes, as a closed throttle can pull the reading down. Hold it fully open, but on an electronic throttle follow the workshop manual instead. Disable the fuel and ignition so the engine cannot start, and crank each cylinder for the same number of strokes.

Should a leak-down test be done hot or cold?

Warm, ideally. Run the engine to normal working temperature and test soon after stopping it, because a change in engine temperature can shift the reading. Test every cylinder in the same state so you can compare them fairly.

Do I need a compressor for a leak-down test?

Yes. The tester runs on compressed air from a workshop compressor and regulates it to its test pressure. The 35884, for example, comes with a 1/4in BSP air coupler and a 600mm hose. Follow the tester’s instructions on inlet pressure before you connect.

Why does the engine turn when I put air into the cylinder?

The piston is not at top dead centre, so the air pushes it down. Return it to TDC on the compression stroke and hold the crank still, for example with a spanner on the crankshaft pulley bolt. Keep hands clear in case it moves.

Is a cheap compression tester good enough?

Often, yes, for occasional checks, if it has adaptors for your plug size, a hose and a one-way valve that holds the reading. The 35880, for example, has a Schrader one-way valve and a release valve, but suits 14 and 18mm plug threads only.

Can one kit test both petrol and diesel engines?

Yes, a master kit does both. The 35886 master compression test kit pairs a 0–600psi (0–40bar) gauge with M10, M12 and M18 petrol adaptors and 22 dummy glow plugs. Check that your diesel’s expected pressure sits within the gauge range.

Will a compression tester fit my motorcycle?

It will if the kit has an adaptor for your spark plug thread, so check that first. The MS100 motorcycle set takes 8, 10, 12 and 14mm plugs, while the CT955 covers 10, 12, 14 and 18mm.

My spark plugs sit deep in the head: will a tester reach?

Yes, with a long-reach adaptor or a tester made for deep plug ports. The 94269 set adds a 200mm long-reach adaptor for 14 x 1.25 and 18 x 1.5mm threads and a 65mm extension for 10 x 1.0 and 12 x 1.25mm.

Is a digital compression gauge better than a dial gauge?

It can be easier to read. The 35885 digital kit shows psi, bar or kPa on an LCD, and Sykes Pickavant says its digital tester shows small pressure differences more clearly than an analogue gauge. Either way, compare cylinders on the same gauge.