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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.

It's also a good idea to have a buddy follow you on the testdrive to see if there is blue smoke on acceleration or deceleration. On acceleration, blue smoke is usually an indication of poor piston ring seal. If the engine puffs on deceleration, this is usually caused by worn intake valveguides and/or seals. If the engine smokes almost all the time, it is likely past its prime. Or if you see engine blow-by oil dripping from the bottom of the hood, you might be wise to pass on that car. Blue smoke can also be the result of a poorly installed intake manifold on a small-block Ford or Chevy V-8. On these engines, the intake manifold must seal to the heads at the bottom of the intake ports. If not, the engine will pull oil from the lifter valley. We've had this happen to us more than once.

Case of the Bad Intake Manifold Seal
Several years ago, a good friend of ours (we'll call him Mickey) pulled off the deal of the century that is a great lesson in engine diagnosis. At the time, he worked at the parts counter for a Chevy dealership when a wannabe car crafter came in to buy an intake manifold gasket. Several weeks later, the customer returned and was less than thrilled with his engine. He had reached his limit and wanted to buy a new GM crate engine. When Mickey asked about the old engine, the customer offered to sell it on the spot for a righteous price. The first thing Mickey did after he took possession of the engine was remove the intake manifold. That's when he discovered two intake manifold gaskets laid on top of each other. Mickey removed them and properly installed a fresh set. The engine instantly ran like a champ for years in his '64 Chevelle wagon. Knowledge is power, baby.

The Engine Doctor Is In
These next steps demand a more involved testing that will require removing parts such as spark plugs. Obviously, the seller will have to be agreeable to this. If he balks, it might be a sign he has something to hide and perhaps you should look elsewhere. Or you could use this as a bargaining chip to see if he'll come down on the price. Either way, you have more information from which to deal.

The first parts to remove are the plugs. We do this even on engines we know intimately. The first bad sign is if you see oil dripping off a plug or a noticeable buildup of chalky residue inside the plug around the center insulator. This fouling often indicates oil control problems. Another possible explanation could be a dead plug or perhaps one that has too cold a heat range. The only way to know that is to find out what the proper heat range selection is for the engine. A thin, sooty black covering over the entire working end of the plug means the engine is running way rich. This is not necessarily a bad thing, although a rich mixture can lead to increased cylinder bore wear because the extra fuel has washed the cylinder walls of oil. If the working ends of the plugs look exceptionally shiny, then the plugs are glazed, usually the result of a too-hot heat range selection.

On our Olds, we noticed a plug that looked a little suspicious, which led us next to perform a quick compression test. The best way to do this is to remove all the plugs, disable the ignition (so stray voltage doesn't turn you in a ground path), and block the throttle wide open. If you are going to compare cranking pressure with a factory rating, the throttle should be propped open.

Typical production engine cranking pressure ratings can be found in a Motor's Manual (remember them?), but a typical number is between 170 and 190 psi. The higher the number, the more cylinder pressure the engine makes and the greater potential torque and low-speed power it will make. As you can see from our Cranking Pressure chart, our Olds is a little on the weak side. We actually tested our Olds with throttle closed and open and found a 20-psi cranking pressure difference coming in at a lame 135 psi .

Cranking Pressure And Leakdown Chart
Cylinder PSI Leakage
Cylinder PSI Leakage
1 155 22 2 135 70
3 150 24 4 152 24
5 150 25 6 148 29
7 155 20 8 155 23

It's clear from these numbers that our Olds motor is a little soft. Going forward, we wanted to know just how badly cylinder No. 2 leaks. Leakdown testers are designed to pump air into a sealed cylinder and measure the amount of air required to maintain a given pressure in the cylinder. Both valves must obviously be closed to use this tool. The most common version leakdown tester is a two-gauge unit that features a line pressure gauge and a leakdown gauge that reads in percentages. We quickly discovered why the cranking pressure was weak when the leakdown tester indicated 70 percent leakage. Yikes. To make sure our gauge was not misleading us, we tested the rest of the cylinders with the results indicated in the cylinder pressure chart.

The last thing you might consider doing is yanking a valve cover. Just be aware that if the engine has not been apart in a long time, the valve cover gasket will likely tear and the owner probably won't be thrilled with that prospect, so tread carefully here. Valve covers are great places for sludge to hide, so it's a good idea to peek under the covers. Sludge is a good indicator that the engine may have been neglected.

Junkyard Hunting
The other major way car crafters locate used engines is at the boneyard. Apparently, even pouring liquid glass down the throats of all those cash for clunkers (CFC) engines wasn't enough for the simpletons that write the rules, so we can't even buy one of these engines to rebuild. Even without these CFC engines, there are plenty of powerplants to choose from, but it takes a natural skepticism to pick out the darlings from the junkyard runts. The beauty of the boneyard is you can tear that motor all the way down if you like in search of clues. The best place to start is obviously at vehicles with low miles. That's rare in the boneyard, but it is possible. The engines receiving the most attention right now are late-model GM LS motors; Ford Modular 2Vs, 3Vs and rare 4Vs engines; and, of course, Chrysler Hemi engines. We're seeing more and more of these engines being laid to rest. Finding an engine with around 100,000 miles on it may not be a bad thing. Combining today's EFI-maintained air/fuel ratio with better metallurgy and far superior lubricants means the 100K number is not necessarily the de facto rebuild point any longer. On the negative side, if the previous owner's idea of engine maintenance was adding oil only when oil pressure dropped to zero, it's still possible to latch on to a loser.

In the junkyard, it's always a good idea to yank the plugs and valve covers to get a better picture of the engine's past life, and reading the plugs is still wise. Since compression testing probably won't be convenient, you might be able to rig up a way to test leakdown with an air tank and leakage tester. This is even a good idea with a used engine you find at the swap meet, since this type of testing is minimally invasive. The problem with pulling the heads is it will only indicate if the engine has a serious flaw like a scored piston or burned valve. Other more subtle issues will likely not be noticeable to the naked eye.

Tool List
Description PN Source Price
Matco computer tester CT66K Matco Tools $69.60
Matco leakage tester CLT2APB Matco Tools $124.95
Summit diagnostic 900009 Summit Racing $94.95
Summit Racing
Matco Tools
4403 Allen Rd
OH  44224
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