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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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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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