If you want to buy a car or an engine and want to know if the engine is any good BEFORE you buy it, there are some tests that can be run to be somewhat sure you are getting a decent engine.
The engine is basically divided into two sections, the top end and the bottom end. The top end is the cylinder head or heads and the bottom end is the cylinder block. It does not take a highly skilled mechanic to do the tests, but does take a few tools and a little work. How extensive the tests are will be determined by the particular situation. Meaning, how much effort do you want to spend to know how good the engine is? Most men know when they are going to buy a used car to check the oil level and how black it is. (not much of an indicator) They look for smoke out the tail pipe and listen for knocking noises when the engine is running. They may also listen how smooth it runs and how it pulls the car and to drive the car for a while to see that it does not overheat. This is about all you are going to be allowed to do on a used car lot.
What is the next level of testing to see if you are getting a good engine? This is where the tools come in. A plug wrench, a compression gauge, oil squirt gun and an oil pressure gauge (if a gauge is not on the dash). A compression gauge, an oil pressure gauge and an oil squirt gun can be purchased at most any auto parts store. These tests can determine if the top end and bottom end are basically sound where the visual checks above can miss.
To do an oil pressure test you need to locate the oil pressure switch or sending unit on the engine. Many of these are 1/8 pipe thread but you will have to check the fitting to be sure. Some oil pressure gauges come with an assortment of fittings. First check the oil level and how the oil on the stick looks. The condition of the oil will effect the oil pressure. If possible do a cold oil and hot oil test. The following are bits of information you are looking for.
1. Oil pressure during cranking after an engine has set overnight. Oil pressure should come up quick and be good.
2. Oil pressure at idle and light rev-up just after start. (cold engine) Should be good and climb quick with RPM.
3. Oil pressure at idle and light rev-up after engine temp is normal. 20 to 35 psi at idle and 40 to 80 psi on a light rev-up for most British cars but lower for some domestic engines. (look up the specs)
4. Oil pressure at idle and light rev-up after oil temp is up to normal. (this requires expressway driving) (Oil temp does not come up with engine temp) Still should read close to the above figures but may show a quicker needle movement.
5. Time that it takes Oil pressure to come up after oil temp is up and the engine has been shut down for 5 or 10 min. The quicker the oil pressure response the better.
Oil pressure is achieved by the restriction of the thin clearance between the engine bearings. Most oil pumps have the capacity to blow an oil filter off the engine and can reach pressures of over 300 + psi. The thing that prevents this is the Oil Pressure Control Valve or Relief Valve. This valve is located usually in the filter block or engine block on most import cars and in the oil pump on most domestic cars. It is easy to adjust or modify on an import car and more difficult on domestic cars not just because they are usually in the pan but because they are staked in not bolted in the pump. It is not uncommon on old domestic engines for the oil pump to ware out before the engine and thus show low oil pressure when the bearings are good. Another factor is the oil filter and or the oil filter bypass valve. This is built into the filter on most cars but a few imports have it built into the filter block. This valve allows unfiltered oil to access the bearings if there is too much resistance in the filter itself, like when the filter is dirty and clogged "OR" when the oil is very cold or too thick and can't get through the filter. Very cold oil in the winter will affect the oil pressure a lot. You might think that very cold oil would show very high pressure readings when you first start but an MG for example may show very low pressure on crank up when it is very cold. I found two factors here, one is on the MGs with a hydraulic pressure gauge have a very small thin tube that does not want to transmit the actual pressure along the length of that tube. Also, when the oil is cold and thick it is able to push the pressure control valve open easier. The cold thick oil will not want to go through the filter and can open the filter bypass valve and run unfiltered oil through the bearings. For this reason, you should not rev the engine much until the oil temp has time to come up a little.
Low oil pressure readings can mean bad bearings but it is not a sure thing. A leaking oil pressure control valve can leak internally or be stuck in an open position. Thin oil due to a change being necessary will show a low reading. Electrical oil pressure gauges are always suspect and if a low reading is noted, you should check it with a hydraulic gauge (called a mechanical gauge).
An extremely high reading will usually mean the pressure control valve is jammed or stuck closed. Reving this engine will usually result in the explosion of the oil filter. What is a normal oil pressure is different on many cars, so you should check a service manual for that car. Everything above will apply except the numbers will depend on what brand and model car.
The top end is checked with a compression gauge. This tests the general condition of the valves and rings. This will not detect a crack in a head nor a leak in a head gasket unless it is a badly blown head gasket. This test should be done with the engine at operating temperature. Remove all the spark plugs. Disable the ignition by removing the power wire from the ignition coil or the wire/s from the negative side of the coil. Do not just pull the high tension coil wire out of the coil or cap. A spark can jump to the primary post of the coil and destroy an electric ignition system this way. Be sure the wires that you remove from the coil do not contact anything during the tests. The first test is called a "Dry" test and is done by opening the throttle and contact the starter so as to rotate the engine at least 4 or 5 revolutions with the compression gauge installed. Here as with the oil pressure the readings should be looked up in your service manual. Generally, a reading of under 100 psi just about will not run. Most engines will produce from 125 to 180 psi on a compression test. Look up the specs to see what it should be for that engine. Also, there should not be a large difference between cylinders. The next test you run is called a "Wet" test. On this test you first put about a table spoon of engine oil into the spark plug hole just before doing another compression test. If you got the oil squirt gun, put about 4 or 5 squirts of engine oil in the plug hole then do the test on that cylinder. You will see a slightly higher reading this time but it should not be a lot higher. A large difference between the dry and the wet test indicates a ring problem. A low dry reading and a low wet reading indicates a valve problem is possible.
If however, the engine was out of the car and you had attached a starter & battery just for this test and the engine had not been run in a time, then you may find a large difference between the two tests. In this case you should let it set for several hours with a little oil in the cylinders and do the dry test again. An engine that has sat for several months in or out of a car and not been run will show a bad dry test. You may even reverse the two tests by first put oil in all cylinders then with the plugs still out just run the starter for a good length of time, checking the starter with your hand to be sure it does not get overheated. A starter can run a good length of time without damage without the plugs in. Then do a dry test and a wet test to see the results. Keep in mind that an engine that has sat for a long time can have a temporary valve problem. Some valve is sure to be in the open position when an engine is stopped so that valve can get rust on the valve face and seat and on the valve stem and thus stay partially open when doing a compression test. If you have a cylinder that shows little to no compression and spinning the engine with oil in the cylinders or starting and running did not correct it, remove the valve cover and check that cylinder's valve clearance. If the clearance is about right, then that cylinder either has a burnt valve, badly blown head gasket or worse yet a hole in the piston. If the clearance is extremely high, then put that piston up on the compression stroke and then turn the crank about 90 degrees in either direction to get the piston away from the top with the other valve closed. Now, depress the valve with a tool. I often use a hammer handle as the spring is strong and requires a lot of pressure to do this. Many standard engines use 40 to 80 lbs of spring pressure on the valve in the closed position. A penetrating oil will help if you can get it sprayed in a intake or exhaust port and in the valve spring area. If you work the valve up and down many times and there is no improvement in the clearance then you probably have a bent valve. If the clearance gets back to normal and you get compression back, then it was just rust and some light running will probably ware that away. Another possible cause of a failed test is if the engine has hydraulic lifters. After setting for a long time a lifter can bleed down and take a while to pump up. This is a reason to do a oil pressure test first. Usually a lifter will pump back up by itself with oil pressure applied. A lifter that has bleed down will show an excess of valve clearance and make it difficult to diagnose. Another possible problem is that a gummed up hydraulic lifter can pump up too high and hold a valve open. This can be noted with the valve cover off and the suspected cylinder on TDC of the compression stroke, note if you can rotate the pushrods and look at the level of the valve springs in relation to the others.
How about that mild head gasket leak or a cracked head? Can you check that from the outside? The answer is yes, somewhat. This is a simple test for a leaking head gasket or a crack in the cylinder head. This is not a 100% test but is an indicator of a possible problem. This should be done if you have the following symptoms, No or light over heating at idle and low speeds and light to severe over heating at speed or long drives. A simple test is to wait until you can remove the radiator cap an fill the radiator or reserve tank with water. Then with the cap off and filled to the brim have someone start the engine and put the RPM on a fast idle right away. A leaking head gasket will blow water out very quickly as soon as the engine fires up. A slow raise in water level and overflow is normal. There are some more extensive tests that can be done in this area. If you have CO equipment available, you can sniff the air in the radiator or expansion tank to look for exhaust gases.
There is another test that is very good but requires some work. This requires air pressure (about 150 psi) and a fitting for the spark plug hole to connect the air hose to. (This is available from any automotive tool company). This is very difficult to do to a car with an automatic transmission but easy to do on a standard trans car.
Remove the plugs and put the cylinder to be tested on TDC of the compression stroke with the hose adapter in place and put the car in high gear with either the hand brake on or a person with the foot brake applied. With the radiator or expansion tank full, apply the air hose to the fitting using about 150 psi air pressure. Be sure the brake is applied hard as the engine my lurch as soon as the air is applied. If you have even the slightest leak in the head gasket or a crack in the head, you will see a raise in the coolant level. A blow out of coolant or bubbles or raise in level indicates a leak. How quick the raise of level indicates how savvier the leak is. I used this test to find an over heating condition on several E-Type Jags and a Healey 3000 that overheated at 100 MPH + but no signs of overheating during normal driving. A friend once said he used a pull handle on the front pulley bolt on a car with an automatic to hold the engine in place while doing this test but had the wrench slip once and destroyed a fan and radiator in the process. I did have a Healey 3000 give me a surprise once. In a dealership in FL once I set a Healey up for this test and pulled up the hand brake and it felt good. (tightened up about 1/3 to 1/2 it's lever travel). But when I applied the air, it barked the rear tires on the concrete floor and it was in 4th gear. So be careful, 150 psi of air in a combustion chamber has a lot of power if the piston is not exactly at TDC and it never is.
With air pressure in the combustion chamber you can also check for any leakage in the valves by opening the throttle and listen in the intake then listen at the exhaust pipe. Even the slightest leak can be heard. You can also remove the oil filer cap and listen to the hissing in the engine which is bypassing the rings. This is more difficult to diagnose as a problem as even a new engine will hiss. We were able to compare good vs bad as in a dealership we had many good and bad engines to listen to of the same brand to get a picture of what was a good hiss vs a bad hiss since both did hiss.
There is another tool that is very good that can test the % of leakage from a combustion chamber and is available from any automotive tool company and can test the % of leak down from a applied air pressure. This tool is a little more expensive but is a good tool if you are more serious about testing engines.
If you have any questions, e-mail me and I will try to answer them as best I can.
Past tech pages
Tech 1 - Lucas Ele. & polarize a gen.
Tech 2 - Alt. & Gen. tests
Tech 3 - Brake pad installation problems
Tech 4 - Prolong gearbox life
Tech 5 - Engine Oil
Tech 6 - Ignition System
Tech 7 - Cooling System
Tech 8 - Instruments reading wrong