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Used Motor Buyer's Guide - Buy A Used Engine

Avoid Those Cracked Piston And Burned Valve Blues With A Few Time-Honored Tricks To Help You Buy A Used Engine

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If you're a young car crafter in search of his first car, it's one of the most exciting times of your automotive life. Purchasing a performance car is usually approached with loads of zeal, enthusiasm, and more than a little bit of romance. And if this is your first car, one of the last things to consider is if the engine is in good shape. "It spins the tires, right? What more do I need?" All that passion is fine as long as it's tempered with a healthy dose of mechanical reality. After all, your new ride should be in good enough shape to at least get you home. This story will focus on several time-honored yet simple engine diagnostic techniques that will help you make the right decision before you lay down the cash. The key to any approach is to pay attention to the details and the engine will tell you how it's doing. All you have to do is know how to read the signs.

Before You Break Out the Tools
Let's assume you've found the car of your dreams. This will be your daily driver and also your first shot at car crafting, so it's important you buy a machine that's not going to fall apart before you can begin to work your magic. But before even opening that toolbox, any engine will exhibit many telltale cues as to its condition. Leaks are an obvious hint, but they may not necessarily be a bad thing. Frankly, all '60s and '70s engines leak. It's just a fact of mechanical life. Next, pull the dipstick and look at the condition of the oil. If it's fresh and clean, it could be an indication that the owner was conscientious, or it could be that the fresh oil is an attempt to mask other problems. For example, if there is water in the oil, it will tend to turn the oil a milky gray color. Ask him how often he changes the oil and what kind of oil he uses. If he mentions 20W-50, that could be a sign that more viscous oil is used to mask an oil control problem.

Jump in the car and hit the starter motor. Pay attention to how the engine cranks. If it spins evenly, that's a good sign. If the engine speed seems to rise and fall, that could be an indication of a weak cylinder. Also listen to see if the starter motor grinds or whines. If so, the starter gear or flexplate/flywheel teeth could be damaged. Have a friend stand by the tailpipe to see if the engine smokes on start-up. Blue smoke is oil, while black smoke is just unburned fuel. Blue smoke at engine start indicates residual oil in the cylinders that leaks past worn valveguide seals and/or worn valveguides. If the car is equipped with an automatic transmission and you see white smoke from the tailpipe, this is most often an indication that the transmission's vacuum modulator valve has failed and is pulling automatic transmission fluid into the engine through the vacuum line connected to the intake manifold. Disconnect the vacuum line, plug the manifold, and the smoke should disappear-or feign ignorance and tell the unsuspecting owner the engine is blown up and buy the car for a song.

With the engine up to temperature, listen to how it idles. If this is a fairly stock car, you're looking for a smooth idle. If you hook a vacuum gauge to the manifold vacuum, the gauge needle should hold steady. This is in reference to an engine with a stock or mild cam. If the vacuum gauge displays erratic needle movements, this could be an indication of a bad intake or exhaust valve. But before you blame the engine, it could also be a sign of a poorly adjusted carburetor or minor vacuum leak. Engines equipped with longer-duration camshafts with more overlap will also produce more erratic idle vacuum gauge readings. You will need to learn how to listen to an engine to tell the difference between an engine with mechanical issues and one with a big cam. This is where input from the owner is helpful as to whether the engine "has a cam in it."

You should also make the short trip back to the exhaust tips to listen for popping noises. One old-school trick is to hold your palm about 1/8 inch away from the end of the tailpipe. If occasionally the exhaust pressure pulls your hand up against the tailpipe, it is often a sign of a burned or poorly sealed exhaust valve. Your palm is pulled toward the exhaust tip because the piston's downward stroke takes exhaust gas from the pipe on the intake stroke due to the unsealed exhaust valve.

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