How to Do a Leakdown Test on a 3S-GTE: Step-by-Step Procedure

ByKenji Alvarez

Why Bother With a Leakdown Test at All

A compression test tells you a cylinder is weak. A leakdown test tells you why. You pressurize a stationary cylinder with shop air and measure what percentage escapes — then you listen to where it goes. Rings, intake valve, exhaust valve, or head gasket: each leaks air to a different place in the engine, and a leakdown test distinguishes between them without tearing anything apart. On a 3S-GTE, where head gasket failures are a documented weak point on Gen 2 and Gen 3 engines and ring seal matters a lot under boost, that distinction is worth the extra half hour.

What You Need

  • A leakdown tester kit with an M14×1.25 adapter — the 3S-GTE uses M14 spark plugs
  • An air compressor capable of sustained output at the tester’s rated input pressure (typically 100 psi)
  • A 19mm socket and breaker bar for the crank pulley bolt
  • A long rod, dowel, or screwdriver to sense piston position through the spark plug hole
  • A helper, or a way to hold the crank from turning once air is applied

Warm the Engine First

Run the engine to normal operating temperature before you start. Hot rings conform better to the cylinder walls, so a cold test will show higher leakage than actually exists. More importantly, coolant near boiling temperature is far more likely to show bubbling if a head gasket is leaking — a cold test can miss a marginal gasket entirely. Shut the engine off, let it sit for ten minutes so it stops ticking, then begin.

Setting TDC on the 3S-GTE — the Step That Trips People Up

Every cylinder reaches top dead center twice per engine cycle: once on the compression stroke and once on the exhaust stroke. Both land at the same crankshaft position. The crank pulley mark cannot tell you which one you’re on. If you test on the exhaust stroke, pressurized air blows straight out through the open exhaust valve and the reading is meaningless.

The 3S-GTE fires in the order 1-3-4-2. Cylinder 1 is at the timing belt end of the engine.

Finding Compression TDC on Cylinder 1

  1. Remove all four spark plugs. This eliminates compression resistance and makes the engine easy to turn by hand.
  2. Drop a long rod gently into the cylinder 1 spark plug hole so it rests against the piston crown. Watch the rod, not the crank.
  3. Using a 19mm socket on the crank pulley bolt, rotate the engine clockwise as viewed from the front (timing belt end). Watch the rod rise. Keep rotating until it stops rising.
  4. At that point, align the 0° mark on the crank pulley with the pointer on the timing belt cover. You are at TDC — but you do not yet know which stroke.
  5. Look at the camshaft lobes for cylinder 1. Both the intake and exhaust cam lobes should be pointing away from the valve buckets, meaning both valves are fully closed. If either lobe is pressing a bucket down, you are on the exhaust stroke. Rotate the crank one full turn (360°) clockwise and recheck.

As a secondary confirmation, the camshaft sprocket timing marks should align with the reference marks on the No. 1 camshaft bearing caps. This applies across all 3S-GTE generations — the DOHC cam-cap reference system is consistent from Gen 1 through Gen 4 even though the intake manifold and turbo setup changed.

Moving to the Other Cylinders

Because cylinders 1 and 4 are 180° apart in crank rotation, they reach TDC together. When cylinder 1 is at compression TDC, cylinder 4 is at exhaust TDC — you still need to verify the stroke the same way. Move through the firing order (1-3-4-2), rotating the crank 180° between each cylinder. Check the cam lobes every time. Do not skip this step even after you get comfortable with the process.

Running the Test

  1. With the target cylinder at confirmed compression TDC, thread in the leakdown tester adapter by hand until snug. No crush washer needed.
  2. Before connecting air, note that once pressure is applied the piston can be pushed down, spinning the crank. Have a helper hold the crank pulley, or wedge a breaker bar against something solid.
  3. Connect the air supply. Set the input to the tester’s rated pressure (your tester’s instructions will specify — 100 psi is common). Let it stabilize for a few seconds.
  4. Read the outlet gauge. The percentage shown is your leakdown figure for that cylinder.
  5. While the air is flowing, walk around the engine and listen. Note the reading, then move to the next cylinder.

What the Numbers Mean

Under 10% is excellent. 10–20% is acceptable for a production engine with real mileage on it. Above 20% warrants investigation; above 30% means something needs to be addressed before you put the engine back under load.

Compare cylinders to each other. A uniform 14% across all four cylinders is a very different situation from readings of 9%, 10%, 9%, and 29%. Outliers matter more than the average.

On a boosted engine like the 3S-GTE, cylinder sealing is under more stress than on a naturally aspirated motor. A cylinder at 22–25% that might be tolerated in a commuter car is a real concern when you’re pushing boost through it.

Where the Air Goes — and What It Means

This is the whole point of the test. Each escape route points to a specific failure.

  • Hissing from the exhaust pipe or downpipe: the exhaust valve is not sealing. Burned valve, bent valve, or heavy carbon on the seat.
  • Hissing from the intake tract or throttle body: intake valve leak. Less common than exhaust valve failures but happens with worn seats or bent valves from a timing event.
  • Air blowing from the oil filler cap or dipstick tube: blow-by past the piston rings. The air is entering the crankcase. On the 3S-GTE it will also come out the PCV outlet.
  • Bubbles in the coolant overflow tank, or air escaping from an adjacent cylinder’s spark plug hole: head gasket. The 3S-GTE — especially Gen 2 and Gen 3 engines running stock headbolts and OEM gaskets — is known for this failure mode. If coolant starts bubbling with air in the cylinder, that is your answer.

A Practical Note on Engine Bay Access

In the MR2 SW20 and ST205 Celica GT-Four, the 3S-GTE sits in a tight space with the intake manifold running close to the firewall. Listening for intake valve leaks at the rear of the intake requires patience — a length of vacuum hose used as a listening tube helps. The exhaust downpipe also blocks easy access to the first section of exhaust. If the downpipe runs close to the engine, listen at the wastegate outlet as well, since pressure can bleed through there and make you chase a false reading at the pipe exit.

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