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Engine Mechanical Integrity: What's the Best Test?Posted 2/17/2003By Brian Manley
Perhaps it's just the way my brain is built, but I never liked the following math statement: "To every rule there is an exception." And then there's the one writers know, that goes "I before E except after C, and ..." Good thing I'm not a writer. Just give me a basic engine - a block and a crank and some pistons and rings. No exceptions to the rules when performing basic engine tests, right? Well, sort of. An engine can have high cranking compression readings, but still have an engine mechanical issue that will require parts replacement. So, what is the first, the best, the most accurate test for analyzing an engine for mechanical integrity? Let's use a recent customer vehicle while discussing a few test methods. 1985 Honda Accord Analysis - 215,000 miles - Very Rough Running This vehicle rolled into my bay running very rough and spewing some grey smoke from the tailpipe. As the mileage shows, this Accord's engine may be breathing its final breaths, so I set out to find the root cause(s). My first plan of attack was to gather as much information as quickly as possible, so I grabbed my vacuum gauge. Hooked to a manifold vacuum source, the gauge jumped erratically from 12 inches to 16 inches. An Engine is Just an Air Pump This means that the amount of power that can be obtained from a given displacement engine is determined by the amount of air it is able to breathe in a certain period of time. Any flaw in the engine's ability to breathe, such as a worn camshaft lobe, will reduce the ability of that cylinder to contribute to the overall performance of the engine. Based on the above vacuum readings, I determined that one cylinder of my "air pump" wasn't creating much, if any, negative pressure, so I moved on to the next test that would help me isolate the weak cylinder. Relative Compression Test
To prepare for this test I pinched the fuel line to the carburetor and ran the engine until it died, then blocked the throttle open for even airflow. So, with all of the preparation involved, why didn't I just screw a compression tester into my four cylinders and take readings? Speed is often the reason, although it may have been equal time spent for either test, but isn't it nice to show your customer a nice, clean scope readout? Also, access is a big issue when choosing this test over the manual method. Can you think of any engines that have difficult spark plug access? No, I can't either. Figure 1 shows the results from my first test. Since the firing order is 1-3-4-2, cylinder 4 is currently only working 60 percent as hard as cylinder 1, and cylinders 2 and 3 are also a little lazy. I repeated this twice more with the same result. So, what conclusions can we draw from this? Cylinder 4 is not breathing or sealing as well as it should, so now it's time to pull the plugs. Leak-Down Test
OK, so we've confirmed a blown head gasket or worse, a cracked head or block, but are we finished with our diagnosis? No. The leak-down reading in Figure 4 (0 percent) is what I expect to see in a "good" cylinder, and that's what I saw in cylinder 1. But when I let the air fill cylinder 2, I got the reading you see in Figure 5 (65 percent leakage) with a corresponding hiss of air from the tail pipe - a burned exhaust valve. That leaves just cylinder 3, and when filled with air, it created a 30 percent leak and a rush into the crankcase - worn rings.
Cranking and Running Compression Testing Do we need to perform more tests on our high-mileage Honda? No, but before it goes off to Honda Heaven, it will breathe a few more times for us. Figures 6 through 9 show the cranking compression in cylinders 1 through 4 in order: 130 pounds per square inch (psi), 110 psi, 115 psi and 50 psi. These readings mirror the synchronized relative compression test that we've already done, and now a squirt of oil into cylinder 4 is in order. A retest showed no increase in compression, which illustrates another exception to the rule when compression testing: If a wet compression test isn't higher than the dry test, then the valves are probably the culprit. That doesn't hold water any better than the fourth cylinder of our Honda does, but remember, our Honda is an exception to the rule. At this point, our Honda is so full of coolant it won't run right, so we'll continue our discussion using another test subject that was a customer at Linder Technical Services. Running Compression Testing with Michele "the Sleuth" Winn Michele, a diagnostic technician at LTS, recently worked on a 1999 Mercury Cougar with a 2.5L V-6 engine and 54,000 miles. The customer complained of a slight misfire at idle that seemed to go away with an increase in rpm. There was also a P0304 code (cylinder 4 misfire) and a check engine light "on." OBD II is so helpful; it already identified the problem cylinder for her! After ruling out ignition, fuel, vacuum leaks or an EGR issue, Michele decided to perform cranking and running compression tests. Here are the results:
Michele said even without knowing the compression specifications for this engine, it is easy to tell there is a problem with cylinder 4, and just like doing a quick check with an ignition scope, you are looking for one or two cylinders that stand out as being different from the others. After the engine was torn down, a defective piston was found. But why did Michele perform a running compression test? Didn't the cranking test show her a low cylinder? Read on. Volumetric Efficiency Engine volumetric efficiency refers to how much of a cylinder's volume is filled during different running conditions. Flow of air is related to the opening it must flow through, so for a fixed pressure the flow through a fixed opening is time-related. If pressure is increased, flow will also increase, but only until the area of the hole stops the increase in flow. If an engine has a worn intake camshaft lobe it will have low compression on that cylinder because the volume will be decreased due to the shorter time the valve is open. Cranking Compression Pressure vs. Running Pressure During cranking, the cylinder fills completely with air because the valves are open for a long period of time, which allows the pistons to pull in a full load of air. This will result in a cranking pressure of around 150 psi or less if you live in Denver. When an engine is running, the throttle is closed, the valves are open a shorter period of time, and the pistons are moving four times faster than cranking speed, which reduces the total volume of air being pulled into the cylinder. The piston is simply moving faster than the air can move. This is why Michele performed both tests; she knows cranking compression will test for cylinder seal leaks, but running compression will test for volumetric efficiency, or "breathing" problems. Snap Acceleration Compression Pressure Idling compression pressure is usually about 50 percent to 70 percent of cranking compression, but when you "snap" open the throttle, the pressure should rise to at least 80 percent of the cranking value. This is because we briefly increase air volume while maintaining the same basic idle speed. If one or more cylinders are higher than the rest, then there could be an exhaust restriction or an exhaust valve lift issue. If the number is lower, then there could be an intake restriction or valve issue. Other Compression Issues Problem cylinders may have trouble pumping up and may increase by only 10 psi per stroke. You may be able to crank these cylinders enough times to come close to the other cylinder's total psi, and this is one reason to limit the number of compression strokes. Poor rings usually cause this condition. Be aware that a cylinder suffering from excessive oiling, even from bad rings, can yield high compression test results because the excess oil in the cylinder seals the rings. Other symptoms may give you a clue to the problem (a smoking engine). There are some variables that affect the readings obtained from compression testing. They are cranking speed, altitude, temperature, worn camshaft lobes and high-performance, long-duration profile camshafts. The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed. There are factors to compensate for the different altitudes and the corresponding temperature differences. These are as follows: 1,000 feet = .9711, 2,000 feet = .9428, 3,000 feet = .9151, 4,000 feet = .8881, 5,000 feet = .8617, 6,000 feet = .8359, 7,000 feet = .8106, 8,000 feet = .7860. The equivalent compression reading for a cylinder that should be 135 psi by the data at 5,000 feet would be 135 x .8617 = 116.33.
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