Buyer Beware - Engine Blues

There are many reasons why someone would be in the market for a used Pontiac engine--the desire for more cubic inches, a restoration project, or the building of a race motor. Not to mention that Pontiac has not produced a V8 of its own design for more than twenty years.

Due to these circumstances, a pre-owned engine is about your only choice. Generally, a used engine can be classified as the following: a running sample in a body of a Pontiac or on a test stand; out of the vehicle and not able to be test run; or in pieces laying in a field or in a garage. It goes without saying that the most desirable scenario would be test driving the engine, but if that is not the case, a little knowledge and some good detective work is all that's required.

A phrase like "show me the money" from the movie Jerry McGuire applies very well to this business. What's nice about the automotive hobby is that it only takes money. The fastest, prettiest, or best restored Pontiacs only have their title because huge sums of cubic dollars have been tossed at them. This simple fact makes the art of negotiation the key to successful used engine purchasing, since the price paid determines if you got taken or not. A complete 455 that was represented as being rebuilt but only has fresh paint is still a bargain if the proper amount of dead American presidents changes hands.

The more you know about engines, the easier it will be not to make a mistake. Regardless of whether the engine is in or out of the vehicle, a good visual inspection is the first place to start. Look for any signs of cracks on the block or cylinder heads, oil residue from leaks, or discoloration of any metal surface from overheating. In addition, remove the dipstick and examine the oil's condition and, if possible, look in a valve cover for sludge or a milky white substance that may indicate moisture in the combustion process, an inoperative PCV system, or lack of maintenance.

Query the seller about the history of the engine. Often a discussion will lead to facts that can determine internal wear. A low-mileage powerplant that was used frequently with many cold starts in its history may seem to be a bargain until you realize that this type of operation leads to excessive piston ring, cylinder bore and bearing wear. Think years and mileage, not just what the odometer says. A 1970 engine that is still in use with 200,000 miles was only driven 6,250 miles per year. This would usually indicate that it has seen many undesirable short trips and extended idle conditions. In contrast, a two-year-old powerplant with 120,000 miles will show less wear since it completed fewer warm-up cycles.

The following are test procedures that help to indicate the condition of a used engine. Obviously, some can only be accomplished on a running engine.

Vacuum Test
The vacuum produced in an engine is an indicator of its ability to seal and pump air, thus creating a pressure differential that can be quantified. To perform this procedure will require a vacuum gauge that is available from most auto parts stores, tool jobbers or a mass merchandiser such as Sears. The cost of the gauge is around $25.

Connect the vacuum gauge to an intake manifold vacuum source. There are two distinct vacuum signals in an engine: manifold, which can also be referred to as full-time, and ported, or part-time. A full-time vacuum source is one that is connected below the restriction of the throttle plates, while a timed signal is sourced from the carburetor and is referenced above the throttle plates. Manifold vacuum is available on most carburetors. The easiest method to determine the source of the vacuum is to start the engine and, at curb idle speed, remove a vacuum hose; if there is suction present then it is a manifold source. If a signal is only created when the throttle is opened, that is a ported source. With the engine idling, monitor the vacuum gauge reading. An engine in good health (with a stock cam) should have a rock-steady gauge needle with a reading of 17 to 20 inches of mercury. A rapidly shaking needle means a burnt valve, while very low vacuum can indicate retarded ignition or camshaft timing.

Leakdown Test
A leakdown test requires a leakdown meter and differs from a compression test in that the piston is stationary. Air is pumped into the bore and the amount of leakage recorded. Leakdown gauges can be purchased for approximately $100, but to perform the test a compressor is required. This makes it difficult if the seller is not willing to bring the Pontiac or the engine to a specific location.

The leakdown test is an excellent indicator of the piston ring and intake and exhaust valve sealing. In theory, an engine should have zero leakage, but due to the necessary ring gaps and piston-to-cylinder wall clearance, an acceptable amount of leakage is up to 15 percent, for a used engine. If the results are higher, that should not be interpreted that the engine does not have many more miles of use, but that it's getting tired. Any value over 30 percent would be of concern.

If any leakage is detected, the location from which the air is escaping will reveal the extent of the problem. Air escaping into the oil pan indicates worn piston rings; from the tailpipe, poor exhaust valve sealing; from the carburetor, faulty intake valve sealing. Any leakage past the valves is usually indicative of a valve job.

Compression Test
In contrast to a leakdown, a compression test is dynamic and measures the piston's ability to pump air. With the engine warmed up to operating temperature, all of the spark plugs need to be removed and the battery should be in a good shape, or an assist in the form of a battery charger attached. With the throttle wide open to limit pumping losses, crank the engine over at least four complete revolutions and record the reading. The first puff of the gauge should be approximately 40 percent of the final reading if the cylinder walls and rings are in good shape. Most Pontiac engines will have 175 to 180 psi, if stock, and in good mechanical health. What is just as important as the final reading is the variation between cylinders. Always compare the cylinder-to-cylinder results in percent by dividing the highest and the lowest bore. Accept a maximum differential of 10 percent. (Of course cranking compression will be lower in a 7.6:1 compression engine than in a 10.75:1 engine.)

Excessively high compression in one or more cylinders can indicate excessive carbon buildup, while a low reading is usually poor ring seal or valvetrain issues. When questioning a low-performance cylinder, shoot some oil in the bore and recheck. If the results improve, the piston rings are worn.

Turn the Engine Over by Hand
Removing the spark plugs and cranking the engine over with a breaker bar or ratchet attached to the harmonic damper bolt is a partial indicator of the its ability to pump air and also to determine friction. Each rotation of the crankshaft should require the same amount of force, turning nice and smooth. The test results are inconclusive unless there is a major internal problem with the engine, and should only be done if a leakdown or compression test cannot be performed.

Smoke
The exhaust is the window into the combustion process. When oil burns, it produces a blue-tinted smoke, while coolant creates a white steam-like smoke. When antifreeze burns, it also emits a sweet smell.

Oil smoke after startup from an engine that has been shut off for a while is usually the result of worn valve guides or valve stem seals. When the engine is shut off, the hot oil leaks past the seal or guide and drips down on the piston top. When the engine is started, the oil burns off and creates the smoke for the few seconds of operation.

If oil smoke comes from the tailpipe during idle or high vacuum conditions such as a coast-down at highway speeds when the throttle is rapidly shut, it is a sign of poor oil control at the piston rings.

Also, perform a visual inspection of the tailpipe or the exhaust manifold or cylinder head exhaust port for oil residue. Stick your finger in the passage and see if it attracts carbon or a wet oily substance. Any oil burning issue other than valve stem seals will require either partial or complete engine disassembly to repair.

Engines that are attached to a Turbo-Hydramatic 350 or 400 can smoke if the vacuum modulator diaphragm is faulty. To diagnose this, remove the vacuum line from the modulator and see if it is wet with transmission fluid and if the smoking stops.

With the engine running remove the oil fill cap or gain access to the valve cover through a breather hole, etc. With the engine hot, look for a plume of rising smoke. This is a poor man's leakdown test. The PCV will do a good job of hiding this blow-by, but any excessive smoke indicates poor piston ring seal.

Listen...
A good set of ears can go a long way in determining an engine's health. First, listen to the engine as it starts. Does it crank smoothly? Sound dry and knock as it fires? Once running, listen to the valvetrain for lifter noise and wear. Do a snap throttle test to approximately 3000 rpm. The engine should respond quickly and smoothly with no foreign noises. Gently raise the engine speed just off idle and listen for any knocks. If a knock occurs at any engine speed that's very rhythmic then it usually means a rod bearing or wrist pin. Try to isolate the sound by grounding each spark plug wire, one at a time, until the noise disappears. When you get to the bad cylinder, the noise will either stop or diminish greatly. This is due to the drop in cylinder pressure without the firing of the spark plug.

Use either a mechanic's stethoscope or a piece of metal pipe to listen to different parts of the engine. Place the listening device on the valve cover, oil pan, timing cover, intake manifold. Pay attention to any unusual sounds that are not similar to the other cylinders.

Take the Pontiac for a test drive and listen for detonation. Abnormal combustion, known as ping, can be the result of an engine with excessive compression ratio, poor combustion from oil in the cylinder, or excessive carbon deposits. When oil is burned during combustion, it produces carbon deposits at an extremely high rate. While driving, check for adequate power for the vehicle and any unusual sounds. In addition, have someone follow you and look for any smoke from the exhaust.

Oil Pressure
The ideal situation would allow the installation of a mechanical oil pressure gauge. During cold engine operation, the oil pressure should be near 60 psi and, with the coolant and oil hot, should drop to a minimum of 30 psi at idle. (For years the automobile industry has stated that 10 psi for every 1000 rpm is acceptable, meaning 7 psi for an engine that idles at 700 rpm. It is the author's opinion that though considered commercially acceptable, oil pressure so low is a sign of excessive bearing clearances and a rebuild is in order.)

Then let the engine sit for 15 to 30 minutes and watch how the oil pressure builds on startup. The oil delivery system should be able to build at least one half of the idle pressure during crank.

Timing Chain Wear
A worn timing chain certainly is not a cause to not purchase an engine, but is a good point of negotiation for price. To check for timing chain wear, remove all of the spark plugs and the distributor cap. With a wrench on the harmonic balancer bolt, turn the engine at least two complete revolutions backward to remove any slack from the timing chain. Still turning the crankshaft against its natural rotation, bring the timing mark to TDC on the timing tab. With a helper watching the distributor rotor, gently turn the crankshaft in the proper direction of rotation. As soon as the rotor tip starts to move, stop. Read the amount of stretch on the timing tab. For example, if the rotor moves at 10* ATDC, as measured at the timing mark, the chain has 10* of stretch. Any reading more than 4* or 5*, bargain the price down for a new timing chain.

What Will It Cost to Fix?
It would be impossible to predict every possible repair cost but the following are average prices for quality machine shop services:

Complete Engine Rebuild: Disassembly, bore and hone block, align-hone main caps, deck block, resize the connecting rods, polish crankshaft, balance rotating assembly, deck cylinder heads, valve job, complete engine assembly. $1,800 to $2,000, plus parts for a nearly stock rebuild.

Valve Job
Disassemble cylinder heads, check guides, mill deck, install hardened seats, machine valve angles, reassemble and set spring pressures. $400, plus parts.

Conclusion
There is a ton of money to be saved in purchasing a used engine. Smart shoppers will reap the rewards of their investigative efforts, while the rest will incur the cost of the engine plus a complete rebuild, putting them bucks down in the long run. Use of the information in this article will place you in the former category and not the latter.