The Good, Bad, and Ugly: Testing a Used Engine

Building a trick engine for a classic truck project can be an expensive proposition, something the average person's cash flow doesn't always accommodate. Most of us would like to have the latest, greatest powerplant under the hood of our hauler, but compromise often becomes the name of the game.

For those of us building on a budget there are a variety of methods to cut down costs, one is a used engine. Granted, it may not be the blown big-block of your dreams, but a used engine can get your truck on the road rather than being parked in the corner of the garage. And who knows, you may be pleasantly surprised by an "experienced" engine.

Recently we did some horse-trading and came up with a used 350 Chevy from a pickup for next to nothing. The small-block had accumulated approximately 40,000 miles since being rebuilt and was reported to be a good runner that didn't burn oil. We're certainly not anxious to put a tired, oil-burning piece of junk engine in our truck, rather the point to be made that with some testing a good used engine can be found (of course the same procedures can be used on the engine in a truck you may buy).

Visual
While this would seem obvious, start by looking for oil leaks at all the usual places, but pay particular attention to the area around the crankshaft snout and the back of the pan. While gaskets are easy to change, crankshaft seals aren't.
If you're dealing with a running engine, once it has reached operating temperature remove the PCV valve hose, remove the breathers (or oil filler cap), and look for blow-by vapor coming out of the openings. A small amount of blow-by from a used engine is to be expected, but where there's lots of smoke there is a worn-out engine.

Dipstick
Check the oil level; clean oil is a hopeful indication of adequate maintenance. Look for white foamy goo or rust on the dipstick, which can indicate coolant in the pan (the same residue on the inside of the breathers is also a sign of excessive moisture). A drastically over-full pan can also indicate coolant in the oil. If there is any doubt, remove the drain plug—water will be at the bottom of the pan and will come out first.

Sounds
If the engine can be run, listen for any strange sounds—in general the heavier the sound, the worse it is. A hard metallic rap from the bottom end is not what you want to hear because it's probably a bearing problem. A lighter ticking sound from the rocker covers usually indicates cam/lifter issues. However, if that sound goes away right after the engine starts a sticky hydraulic lifter, or one that has bled down after sitting, is the likely cause, which is not a big deal.

Oil Pressure
At operating temperature most stock engines will show around 20 psi at idle and go up to 40 or more as rpm increases. If low oil pressure (in the 10-psi range at idle and 20 at higher speeds), a new oil pump may help—but don't count on it. The usual culprit is excessive internal caused by excessive bearing clearances—which indicates a rebuild is in its future.

Compression Test
With all the plugs removed, the throttle blocked wide open, and the ignition disabled, crank the engine at least five times (you'll see the gauge jump and hear the sound of the starter change when the piston comes up on compression).
Cranking pressure should be over 100 psi, 140-160 is normal in most cases, but check a manual for factory specs. If pressures are low, under 100 psi, squirt a little oil in the cylinders and retest. If the cranking pressure comes up the oil has temporarily sealed the rings and they are the problem; if the pressure stays the same the valves are at fault.
When doing a compression check, look for consistency between the cylinders; they shouldn't vary more than 10 percent. Close readings mean the cylinders are in uniform condition; one cylinder that is lower by 20 percent or more indicates a mechanical problem, such as a burned exhaust valve, broken rings, or other damage.

Vacuum Testing
A vacuum gauge is a good indicator of an engine's condition. At sea level with a stock cam normal readings should be between 17 and 22 inches of mercury. Low readings, fluctuating readings, or the needle on the gauge vibrating are all indications of problems.

Leak Down
A great way to evaluate an engine is with a leak down tester. Air is introduced into each cylinder—the pressure is controlled by a regulator and indicated on a gauge. A second gauge indicates the amount of pressure the cylinder will hold, which is another way of saying how much leaks out. The advantage to this process is that escaping air can be heard—if it's coming out the carburetor the intake valve is leaking, out the tailpipe means the exhaust valve isn't sealing, from the breather means compression is slipping past the rings, and finally if there are bubbles in the coolant there is a leaky head gasket, a cracked head, or block.
In terms of numbers, a fresh engine will generally show only 5-10 percent leakage; a good engine in reasonable condition may be as high as 20 percent and than 30 percent or more indicates problems.

Timing Chain
Worn timing chains are common, however, they're easy to check without disassembling the engine. Remove the distributor cap and then turn the crankshaft clockwise until the timing mark on the damper is almost past the timing tab. Now turn the crankshaft counterclockwise until the rotor starts to move and count the number of degrees of travel—useable wear limits are 3-5 degrees of slack.

Cylinder Balance Test
This is a quick and easy test to see if all cylinders are performing equally. With the engine warm, running around 1,000 rpm, pull one plug wire at a time and ground it (to prevent damage to the ignition system or spark jumping around trying to find a ground). The rpm drop should be the same for all cylinders—if they vary, or when it makes no difference in engine speed, further investigation is necessary.

Radiator Pressure Check
Remove the radiator cap (before the engine gets hot and the cooling system is under pressure) and look for bubbles that may indicate a blown headgasket, cracked head, or some other problem that allows combustion pressure to get into the cooling system. A cooling system pressure tester will provide the same information.

Summit's Engine Test Stand
We've always wanted a test stand and recently Summit Racing Equipment introduced an affordable version in kit form. With a rated capacity of 1,500 pounds, it will hold anything we'll be working on. It's great for checking out used engines, such as ours, breaking in rebuilds, as well as fine-tuning and making adjustments.
After testing we found our used engine doesn't make any strange noises, there are no puddles of oil on the floor, the compression and leak down tests are within specifications. The only weakness is the timing chain, but that is within tolerances. Our small-block may be used, but it's certainly not used up.

The engine attaches to the stand by way of telescoping mounts at the front and bolting the bellhousing to the stand in the back.Assembling the stand is quick and easy. After attaching the casters, the gauge panel and bellhousing mount are slipped in place and are secured with pinch bolts.Summit can supply all the components necessary to complete the stand, including a radiator. Ours was robbed from an abandoned project.

To keep engine temperature under control we mounted a shroud and electric fan. The ignition, fan, fuel pump, and ignition switches came from our collection of electrical components.We used a canteen for a radiator overflow tank. Relays were used on the fan and fuel pump to lessen the load on the switches and the mechanical temperature gauge probe was installed in the upper portion of the radiator tank.Our engine lacked a few items, such as motor mounts. These polyurethane motor mounts came from Trans Dapt—we'll use them in the truck the 350 will eventually call home.We picked up a sleek new Contour starter from PerTronix. They are designed so that the body can be rotated for improved clearance around headers and other parts in tight engine bays.To provide fuel to the engines being tested we mounted a Summit electric fuel pump on the stand. We added a pressure gauge and a shut-off valve.Although our engine was a quart low, the oil was reasonably clean and showed no signs of coolant.With the engine at operating temperature and at 1,500 rpm, the AutoMeter gauges showed good oil pressure at 55 psi and 23 inches of vacuum.For a cylinder balance test we pulled plug wires one at a time and let them go to ground. A PerTronix Flame Thrower coil supplies the spark.The Sun tach we mounted to the stand showed each cylinder dropped approximately 120 rpm, indicating they were all producing equal power.The spark plugs were well worn but showed no indication of oil getting into the cylinders—that's a good sign.

Cranking compression pressure ranged from a low of 145 psi to a high of 155, well within used engine tolerances.The cylinder leakage test showed 15 percent with the engine cold. At operating temperature it dropped to slightly under 13 percent.To check the timing chain we rotated the engine clockwise and marked the vibration damper and timing tab.The cap was removed and the position of the rotor noted.The crankshaft was then turned counterclockwise until the rotor moved—in this case the crank moved 5 degrees.Pressurizing the cooling system indicated there were no leaks in the system.