General Powertrain Diagnosis

Despite the sophistication of today’s powertrain control systems, even the most advanced software program cannot disguise a worn camshaft lobe, a burned valve or a loose timing chain. Unfortunately, even though these types of mechanical failures create recognizable symptoms of poor performance, many technicians get caught up analyzing pulse trains without ever considering valvetrains! The fact is, illuminated MILs (Malfunction Indicator Lights) and/ or stored DTCs (Diagnostic Trouble Codes) are often the result of faulty mechanical components. That’s why it is so important to follow a logical diagnostic routine. In this section, we’ll look at basic troubleshooting procedures that can effectively isolate the root cause of many powertrain­ related failures.


While it may be lowtech, a vacuum gauge is a powerful diagnostic tool that can help you quickly determine overall engine performance. A ‘vacuum’ is simply a condition in which the pressure of a gas, in this case air, is less than normal atmospheric pressure. At sea level, atmospheric pressure is approximately 14.75 psi, which corresponds to a barometric pressure of 29.5 in. Hg (inches of mercury). This means that if you placed a long tube into a bowl of liquid mercury and removed the air from the opposite end, the surrounding air pressure would force the mercury almost 30 inches up the tube. This works out to 2 inches of mercury for every 1 psi of air pressure. Understanding the relationship between pressure and vacuum is important to avoid misinterpreting certain scan data readings. For example, in the generic mode, an OBD II (On Board Diagnostics Genera­tion 2) scan tool displays the MAP (Manifold Absolute Pressure) value in inches of mercury (in. Hg). Under KOEO (Key On, Engine Off) conditions, the MAP reading will be about 29.5 in. Hg, which is the equivalent of atmospheric pressure at sea level (29.5 + 2 = 14.75). Once the engine is idling, the MAP reading will drop to about 11.5 in. Hg. At first glance, you may think that this reading is indicative of a serious mechanical problem. However, while this may appear to be a low vacuum reading, it is actually a normal pressure reading. Remember, the MAP sensor measures pressure not vacuum. Consequently, the 11.5 in. Hg value reflects the pressure inside the intake manifold. To determine actual engine vacuum, simply subtract the running MAP value from the KOEO value. This means that if the running value is 11.5 in. Hg, actual engine vacuum is 18 inches (29.5-11.5=18). A healthy engine will typically produce between 18 and 20 inches of vacuum. A vacuum gauge will indicate a steady reading of 18-20 in. Hg on an engine in good condition. Vacuum at a no-load idle. With that in mind, let’s examine some of the more common causes of abnormal vacuum readings.

Misrouted Vacuum Lines:

When the MAP value on the scan tool indicates an incorrect reading at closed throttle, misrouted vacuum lines could be the cause. This is especially true if the engine was recently replaced or major engine work was just completed. In these cases, the sensor may be mistakenly connected to a source of ported vacuum. As a result, the MAP sensor will output a high voltage signal, since ported vacuum is practically zero at closed throttle. In order for the MAP signal to reflect actual engine vacuum, the sensor must be connected to a source of manifold vacuum.

Leaking EGR Valve:

A leaking EGR (Exhaust Gas Recirculation) valve will cause a low but steady vacuum reading at closed throttle. This is because EGR raises manifold pressure (lowers vacuum). While the engine can tolerate metered amounts of EGR as the throttle is opened, volumetric efficiency is too low at idle for the engine to accept exhaust gas dilution. In cases where the EGR valve is stuck wide open, the engine will fail to start. Under this condition, cranking vacuum will be close to zero, rather than the normal reading of 3 to 5 in. Hg.

Restricted Exhaust System:.

In order for an engine to operate efficiently, it must discharge combustion gases at the same rate it draws in fresh air. When an exhaust system becomes restricted, this ability is severely diminished. Instead of combustion gases exiting through the open exhaust valve, the gases back up into the cylinder in search of another way out. The path of least resistance then becomes the open intake valve, which allows the high-pressure gases to enter the intake manifold. Under this condition, manifold vacuum drops significantly. To check for a restricted exhaust system, connect a vacuum gauge to a source of manifold vacuum. Record the vacuum reading with the engine at normal operating temperature and running at 1000 rpm . Next, slowly increase engine speed to 2500 rpm and note the vacuum reading. If the reading gradually drops more than 3 in·. Hg from the 1000 rpm reading, the exhaust system is restricted. The most likely cause of a restricted exhaust is a clogged converter, however, don’t overlook the possibility of a collapsed pipe or blocked muffler.

Sticking Or Tight Valves:

An engine can’t develop sufficient vacuum unless the valves seal properly. When the valves hang open because of carbon deposits or over tightened adjusters, air leaks are created in the cylinder. This condition causes the cylinder to partially misfire, resulting in reduced piston speed and a reduction in vacuum. When the valves have been over tightened, the engine will idle rough and the vacuum gauge will show a low but steady reading. Where sticking valves are concerned, the gauge needle will drop abruptly each time the offending valve or valves fails to close.

Broken Valve Spring:

A valve spring maintains tension on the valvetrain and ensures that the valve closes when it should. A broken valve spring allows the valve to hang open, resulting in a significant reduction in engine vacuum . This can be seen on a vacuum gauge as a sudden drop in the reading (as much as 10 in. Hg) each time the valve tries to close.

Worn Piston Rings:

Vacuum in an engine is created by the downward movement of the pistons on the intake stroke. If the piston rings are sealing properly, manifold vacuum will rise considerably during deceleration. This is because closed throttle piston speed is higher than normal whenever the wheels are driving the engine. As a result, engine vacuum will spike as much as 5 in. Hg during this time. A snap throttle test will have the same effect on the vacuum reading as long as the rings are doing their job. In addition, vacuum will be higher at a steady 2000 rpm than it is at idle.


Performing a power balance test is the most effective way to determine if each cylinder is contributing equally to the engine’s overall power output. On an engine in good condition, shorting out each cylinder should result in a significant drop in rpm. This is the ‘power’ part of the test. Cylinders that produce little or no rpm drop need to be checked for fuel or ignition problems, or a more serious condition, such as a burned valve. ‘Balance’ is evaluated based on the variation in drop between cylinders. Under normal conditions, the variation between cylinders will be less than 50 rpm. Variations greater than this indicate problems such as air leaks, clogged injectors, and/or carbon deposits. Before performing a power balance test, engine speed should be stabilized at 1000 rpm, and the fuel control system should be in open loop. A power balance test can also be conducted using an exhaust gas analyzer. This is accomplished by measuring the amount of HC (Hydro­ carbon) increase each time a cylinder is cancelled. A substantial increase in hydrocarbons tells you that the injector is delivering enough fuel and that the valvetrain is allowing the fuel to enter and exit the cylinder. If HC does not increase or rises only slightly when a cylinder is shorted out, then either the injector is malfunctioning or there is a problem in the valvetrain. Keep in mind that a powerbal­ance test does not provide a specific diagnosis, but instead narrows down which cylinder or cylinders require additional diagnostic attention. The cause of a dead cylinder can range from something as simple as a fouled spark plug to problems as significant as worn piston rings. The power balance test will direct you to the area of the engine that needs attention.


Measuring compression and leakage can isolate the mechanical problems that cause cylinders to misfire. Although these test procedures are relatively basic, neglecting several preconditioning steps can lead to misleading test results. To begin a compression test, remove all of the spark plugs. This allows the cylinder under test to reach its maximum pressure, since it will be unaffected by the pres­ sure drop in adjacent cylinders. Next, connect a battery charger so that cranking speeds will remain consistent throughout the test. Finally, be sure to prop the throttle plate wide open so that the engine can take in the maximum amount of air. If the compression reading for a particular cylinder is below normal, perform a ‘wet compression’ test by squirting about a tablespoon worth of oil into the offending cylinder. If the new reading is normal, or has increased significantly over the initial reading, then the piston rings are worn . If a wet compression test produces the same results as the initial ‘dry’ test, then a cylinder leakage test should be performed . Before doing a leakage test, make sure that the piston is at TDC (Top Dead Center) on the compression stroke. This is a critical, but often overlooked, first step. In many cases, a technician will claim that a cylinder either has a burned valve, or even bent valves, simply because piston position was not checked beforehand. Once the piston is in the right spot, remove the oil fill cap so that excessive ring leakage can be identified. After filling the cylinder with air, look/listen for air escaping out of the tailpipe if the exhaust valve is leaking, or the throttle body if the intake valve is at fault. Excessive leakage from the valve cover indicates that the piston rings are not sealing properly. However, be aware that the rings actually seal better with the engine running. As a result, a small amount of ring leakage is considered normal. Finally, remove the radiator cap to check for air bubbles in the coolant, which would indicate that the head gasket is defective. If compression and leakage tests are both acceptable on the offending cylinder, then the low compression reading is due to problems outside the combustion chamber. Assuming the fuel and ignition systems are OK, look for worn or broken valvetrain components (e.g. worn cam, collapsed lifter, broken rocker, etc.)


Determining whether a performance problem is the result of a mechanical or electronic fault requires two important skills. First, you must understand the relationship between mechanical and electronic components. For example, a worn camshaft can easily trigger a DTC since it causes a reduction in engine vacuum (MAP signal). Likewise, a faulty coolant sensor can cause a carbon coated tailpipe, even if the fuel system components (i.e. pump, filter, regulator, injectors) are in good working order. Secondly, you need to employ a logical diagnostic approach when attempting to find the root cause of the problem. That means to start with a road test (if necessary) to confirm the symptoms, followed by a visual inspection to uncover any obvious faults. Next, check for DTCs, and then refer to the appropriate diagnostic chat in the shop manual. The ‘symptom charts’ are designed to help you isolate no-code problems or history DTCs, while ‘code charts’ are used exclusively for diagnosing current DTCs.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Don Sindledecker says:

    2008 Tribeca won’t go forward or reverse when accelerate a month ago it the same thing I accelerated in reverse and it made a loud crack sound and broke loose any suggestion on this problem

    • Martin Hand Martin Hand says:

      Something probably broke in the transmission differential. Check the axles and drive line and if they’re okay then I would remove the transmission for a tear down and inspect.

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