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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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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 .

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