G2: DIY: Diagnosis: Leakdown Test

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A "leakdown" test measures the pressure loss in a cylinder. It's similar to a compression test, but not the same. In a compression test, a pressure gauge is connected to a cylinder in place of a spark plug. The engine is then rotated a few times by the starter, and the gauge is monitored to see how quickly the pressure builds up and the maximum pressure that it reaches. In a leakdown test, air is forced into a cylinder by a separate air compressor, and the gauges show the amount of pressure that is leaking out of that cylinder. This is by far the most reliable way to tell if your engine has a leaking or blown headgasket. Before doing this test, make sure your cooling system is full of coolant and properly bled (see G2: DIY: Maintenance: Service the cooling system).


Air compressor

G2 Leakdown Compressor.jpg

This test will be run at 100psi. Your compressor should be able to reach at least 100psi, preferably higher so that the compressor motor won't need to run constantly during the test.

Leakdown test gauge

G2 Leakdown HarborFreightTool.jpg

A leakdown gauge consists of two air pressure gauges seperated by a 1mm flow-control orifice. The first gauge measures air pressure entering the tool, the second gauge (the differential gauge) measures the pressure in a cylinder. The difference between the two gauges tells you how much your cylinder is leaking.

These tools can be purchased from many tool manufacturers, like SnapOn, but it is not necessary to have an expensive one. Harbor Freight sells one for around $40, although this gauge will work best if you make some slight modifications to it. The Harbor Freight tool is intended to be used at 15psi, and no higher than that. If you go past 15psi, the differential gauge on the tool becomes useless. The tool uses standard NPT threaded gauges however, and you can easily replace both gauges with ones that go to ~150psi, purchased from a local hardware store. You'll want to use gauges that go to around 150psi because a gauge that only goes to 100psi will be far less accurate at its max reading.

Making your own

You can also make your own leakdown test gauge from scratch. I would still recommend using the Harbor Freight tool as a starting point, because it is cheap and includes all the necessary adapters and fittings you will need to connect the tool to your air compressor and then to your engine. All the other parts you need are available at McMaster-Carr, or possibly at your local hardware store.

  • Two ~150psi air pressure gauges
  • Air pressure regulator
  • 1mm (0.04") flow-control orifice
  • Quick-disconnect fittings
  • Air tool fittings to connect the above together into one tool

More on making your own tool will be added in the future

Additional Special Tools

  • 19mm deep-drive socket
  • 5/8" Spark plug socket

1. Stut Bar

To remove the coil packs, you will have to remove the strut bar (if you have an aftermarket bar, that will need to be removed also). The OEM strut bar is held on by five 12mm bolt (M8x1.25) and one 12mm nut (M8x1.25). Two bolts on each side (the "strut towers"), and one bolt and one nut along the firewall. Remove all the nuts and bolts, and remove the strut bar. If your car has a factory r134a AC system (some 93, all 94-95), you will also have an AC hose bracket connected to the strut bar. This bracket will come off when you remove the nut from the strut bar, and the AC hose can be moved aside and out of the way (but do not disconnect it).
G2 Leakdown StrutBolt1.jpg G2 Leakdown StrutBolt2.jpg G2 Leakdown StrutBolt3.jpg G2 Leakdown StrutNutBracket.jpg

2. Coil Packs


G2 Leakdown CoilConnector.jpg

Disconnect all 6 coil packs from the wiring harness.


G2 Leakdown CoilBolt.jpg

Each coil pack is held in by two 8mm bolts, remove them all (12 total).


Remove the coil packs from the engine. When re-installing the coil packs, you will want to put them back in the same locations. You can do this by labeling their cylinder number with masking tape before removing them, or simply setting them aside in the order they go in the engine.

3. Spark Plugs

Remove all 6 spark plugs. After removal I like to store them in their upside-down coil packs to keep them all in order (it's always suggested to replace the spark plugs and coil packs back to the same cylinders they were removed from).

4. Caps

Remove the oil cap, dipstick, radiator cap, and optionally the air intake tube from the throttle body. The most important one here is the radiator cap, but you'll want to remove the rest so you can still hear where the air is leaking into.

5. Tool preparation

Close the regulator on the leakdown tool. Set your air compressor to somewhere above 100psi, and connect the leakdown tool to your compressor. Set the regulator on the tool to 100psi. Both gauges on the tool should now read 100psi, and you should hear no leaks. If the tool or your compressor fitting is leaking, fix it before continuing. Also, if needed, recalibrate the gauges so they both read the same pressure. Set the regulator on the tool back to 0psi.

6. TDC

You will be testing each cylinder individually, and the cylinder you are testing needs to be set to top-dead-center (TDC) in the compression stroke. This ensures that all intake and exhaust valves are fully closed during the test. It's easiest to set TDC starting with the #1 (passenger side front-most) cylinder.

Setting TDC on cylinder #1

G2 Leakdown TDCSet1.jpg

Using the 19mm deep drive socket on the crankshaft pulley bolt, turn the crank clockwise until the timing marks on the pulley line up with the pointers on the lower timing belt cover. The crankshaft pulley has one mark for TDC, a few inches clockwise (to the right, if looking down from the front) are three more marks. The one mark by itself is TDC, the three others are for setting ignition timing (13-15-17 BTDC), you can ignore those. If you pass the TDC mark, don't reverse and spin the crankshaft counter-clockwise, keep going clockwise until you get back around to the marks. Once you have the mark lined up with the timing belt cover pointers, the #1 piston will be at the absolute highest point of its compression stroke travel and all 4 valves in that cylinder will be closed.

Setting TDC for remaining cylinders

Note: These methods can also be used for setting TDC on cylinder #1, but the above method is much easier and faster.

Visual method

The easiest way to set TDC for the remaining cylinders is visually, with a long object resting on the top of the piston. Some people use a screwdriver, DON'T do this. It's best to use something soft that won't risk scratching the piston or cylinder walls. I use a wooden dowel. Make sure it is long enough to not fall into the cylinder if the piston is at the bottom of its travel. Place the dowel into the spark plug hole so that it rests on the top of the piston. Slowly turn the crankshaft and you will see the dowel move up and down. Watch the top of the dowel as it travels, when it is at its highest position is when the piston is at TDC. Also note that when the piston is coming up on its compression stroke, you will hear air escaping from the spark plug tube. On the exhaust stroke, you will either not hear it or it will be much quieter, as the air will be existing through both the spark plug tube and through the two open exhaust valves.

Compression gauge method

Alternatively, you may use a standard gauge for testing compression (your leakdown tool will not work for this). Thread the gauge into the spark plug hole. Turn the crankshaft a few rotations and watch the pressure reading on the gauge. The highest reading you get on the gauge is when that piston is at TDC. Continue turning the crankshaft until the gauge shows that reading again, then stop. The piston should now be at TDC.

7. Adapter

Thread the adapter of the leakdown tool into the spark plug hole on the cylinder you are testing. It does not need to be tightened with a wrench, just tighten it by hand with the attached rubber hose until it's snug and the hose starts to twist and flex.

8. Connecting The Tool

Connect the leakdown tool (set to 0psi beforehand) to the end of the spark plug adapter. Raise the pressure in the tool to 100psi, going by the reading on the gauge closest to the air compressor. As you are raising the pressure, you will start to hear some air leaking out and possibly a whistling sound, this is normal. Listen carefully at the spark plugs fitting to make sure no air is leaking from there. Tighten the fitting if it is.

9. Readings

With the first gauge of the leakdown tool still set to 100psi, check the reading of the second gauge (the differential gauge). Subtract the reading on the differential gauge from the first gauge (again, this should be 100). This number is how much air is leaking out of the cylinder. Some leakage is normal, exactly how much will depend on the condition of the engine. If the leakage is not excessive, the important thing to check is that all the cylinders have around the same amount of leakage, so write down the number for each cylinder. Leave the gauge connected while you listen for where the air is coming from, use a mechanics stethoscope if necessary. Air coming from the oil cap or dipstick tube means that cylinder is leaking at the piston rings. Air coming from the throttle body means the intake valves are not sealing fully on that cylinder. Air coming from the exhaust pipe means that the exhaust valves are not sealing fully. It's very hard to hear a normal leakage from the exhaust pipe, if you hear it at all you may not be at TDC. Again, some leakage at all of these points is completely normal.

The area we are most interested in is the radiator neck. If there are air bubbles coming up out of the filler neck, then the headgasket is leaking. Also check for a leaking sound at the spark plug tubes that are next to the one you are testing, this is also a sign of a bad headgasket. The only other possibility here is a cracked cylinder wall, which is extremely unlikely on a Legend.

10. Repeat

Repeat the test procedure (steps 6 through 9) on all of the remaining cylinders.

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