Mechanical know how in our case has come largely from two sources. There have been those kind enough to show the way, and there has been the hard way. A long and proud lineup of 500-dollar cars has steered us mostly down the latter route. The way we learned about how and why to run a compression check on an engine came only after bolting on every conceivable replacement part to a 318 V-8 in a 500-dollar '67 Plymouth Barracuda in an effort to make the thing run better. A compression check revealed that the engine was closer to a V-5 and-a-half than a V-8.
An engine makes power by taking in a mixture of air and fuel, pushing the mixture into a confined space, and lighting it aflame. The energy from this explosion pushes the cylinder back down into the hole. Internal combustion! The valves and seals, pistons and rings, and cylinder wall surface all work together to create a tight seal. If any or all of these parts that confine the explosion inside the cylinder get beat up or wear out, engine performance suffers. The reason the old Plymouth was going nowhere despite a carburetor rebuild, tune-up, and so on, was that number two, number seven, and half of number five cylinders had almost no compression. The rest of the cylinders were not in much better shape. Confinement had been lost.
In a perfect world, the parts inside an engine slowly wear out in unison. In every other world, this rarely happens. If an engine burns voluminous amounts of oil, is down on power, or is just plain running like crap, a compression test is a good way to check what's going on inside the engine without taking it apart. The thing to hope for from a compression test is even numbers. Good news if the cylinders check out within 10 or so PSI of each other, and those numbers sync up with the factory pressure specs. Bad news if one or more of the cylinders show a difference of 15 or more PSI. The service manual will supply guidance on drawing conclusions from test results, and will also list compression service limits. A leakdown tester, which fills the cylinder with compressed air, with can peer deeper into engine problems if the compression test reveals problems.
If one cylinder shows a low reading, remove the compression tester and squirt some engine oil inside the spark plug hole. Test again. If the second test reveals a higher reading, then worn piston rings or cylinder walls may be the culprits. If the reading stays the same then suspect worn valves or other top end issues. If the gauge shows a very low or zero on any one or more of the cylinders then you too may be the proud owner of a V-5 and-a-half. The 4K-C in this Starlet is a four-stroke gasoline
peanut grinder engine. Rotary, two-stroke, diesel, and perpetual motion engines call for a different riff. In any case an engine with low sealing compression in one or more cylinders will never run right - no matter how many new parts are connected to it. The compression test is a good baseline diagnostic and long-term financing tool. Where to spend money will soon be obvious!
Stuff You'll Need:
· About an Hour
· Service Manual
· A Healthy Battery
· Spark Plug Socket and Extension
· Hand Tools
· Compression Gauge Set
· Helper [Optional]
Prep for the compression test by making sure the battery is up to snuff, and warming up the engine. Warm parts make for a better seal and more accurate reading. Look in the service manual and see how to disable the ignition, and fuel system if required. In this case we just yanked the coil wire and the ingnitor connector from the distributor.
Remove all the spark plugs. Removing the spark plugs will let the engine turn over with ease. Don't mix up the wires. Like a slant six the Toyota 4K-C has spark plug tubes that like to come out with the plugs. Determine which tester adapter will work for your engine.
Some kits come with threaded adapters. If you're going solo then use the threaded bits. If you have a helper, then use the rubber-tipped extension deals. Either way the idea is to create the same seal as a spark plug while the engine is turning over. Don't over torque the adapters.
Mount the gauge to the adapter. Crank engine over a few times until the needle on the gauge stops climbing. Holding or propping the throttle plate open can speed up air intake. Record final reading on a scrap of paper or with a Sharpie on hand.
Hit the button on the gauge to release the pressure. Repeat compression test for each cylinder. Try not to knock your head on the hood when Beavis lays on the horn when you tell him to turn over the engine on number three. Record and compare final readings to factory specs.