• PRESSURE TRANSDUCERS by Bernie Thompson - ATS

HIDDEN PRESSURE HUNT by James Garrido - Have Scanner Will Travel

This month we will use our in cylinder pressure transducer to take a look at a defect that many shops have trouble nailing down even by other more time consuming methods. This defect is a clogged catalyst on only ONE bank of a 2 bank engine equipped with a dual catalyst system.

When only one catalyst on one bank has become restricted the typical complaint is a lack of overall engine power. And if the restriction is severe enough miss firing on only one bank and not necessarily on the bank with the defect! This bank isolated miss fire is due to the effects on airflow through the engine with one side of the engine restricted. When one bank is flow restricted the result is less than 50% of the total air entering the engine ends up being passed through the restricted bank. Yet 50% of the fuel mass is still delivered via the injectors to that restricted bank. As a result one bank ends up very lean and the other bank over rich. The first telling sign of an unequal airflow due to restriction on a V-type engine are the fuel trims. If the fuel trims are moving in increasingly opposite directions bank to bank as engine speed and load increase you need to check for a restriction.

Figure 1 is a P0300 Failure Record captured on code set from a 2003 Chevrolet pick up with a 4.8L cam in block V8. Notice the fuel trims in the capture. Bank one is the side with the clogged catalyst and as such has a negative total fuel trim (STFT+LTFT) correction of -21% while bank 2 is showing a positive total fuel trim correction of +25%.

How would you diagnose this condition to be a clogged exhaust before purchasing that expensive catalyst? Drop the exhaust and test drive with an open system? And risk needless broken bolts, not a great idea in the rust belt states. A vacuum gauge reading at idle and 2500 RPMs is only useful on severely restricted catalyst systems and will not show a partial restriction on only one bank. You could install a back pressure gauge in place of the front oxygen sensor and then power brake the engine under load at a specific RPM. Then move the back pressure gauge to the other side and compare at the same load. My experience with this is unless the catalyst is severely restricted the gauge bounces around so quickly it is difficult to see if the average needle movement is higher side to side. Pulling hard to get at O2 sensors on a hot engine is no picnic either.

I prefer removing one spark plug from each bank in turn and installing a pressure transducer. Then look at the saved pressure waveforms so that you can get an easily acquired and highly accurate comparison of in cylinder conditions bank to bank.

To do this test start at idle speed so that you have a relatively slow crankshaft speed and a plenty of engine vacuum. Then snap the throttle open. The engine vacuum drops as outside air rushes in to fill the low pressure area in the intake. Initially the crankshaft has not yet begun to spin faster so the extra air has lots of time to fill the cylinders before it attempts to exit the exhaust ports.

In figure 2 I have captured bank 2, cylinder 6, the good side of this engine. Compare these pressures waves seen from the good side to the clogged bank 1, cylinder 7 shown in figure 3. In the zoomed out capture shown in the lower right corner of the images you can see that the cylinder from the restricted bank built up more pressure on average than the cylinder from the unrestricted bank.

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If we zoom in on the highest in cylinder pressure peak section of the wave we can actually see the cycle by cycle difference between the cylinders on the different banks. On the bad cylinder you can see that the end of the exhaust stroke pressure wave plateau peaks at a high of 0.78V even after the pressure drops slightly as the intake valve opens during the overlap period. On the good cylinder you can see the exhaust stroke pressure wave plateau peaks at only 0.53V and then permanently drops off as the intake valve opens during the overlap period. The scaling on this sensor is 1.0V = 75 PSI so the variation between the restricted and unrestricted cylinders is 0.25V or 18.75 PSI! It is impossible to get this kind of accuracy and detail using older methods for testing for back pressure. Even a slightly clogged catalyst will show up by using in cylinder pressure measurements.

THE PRESSURE IS ON by James Garrido - Have Scanner Will Travel

While engine mechanical diagnostics dates back to, well, ever since the first engines were made, the way we diagnose those defects continues to evolve. The toughest aspect of engine mechanical diagnostics may not be as sophisticated as electronic control systems diagnostics but due to disassembly requirements inherent in mechanical inspection it can be just as time consuming. However there are some relatively new techniques we can use to pinpoint engine mechanical defects in both a timely and accurate manner without disassembly by using pressure transducers. A transducer is anything that senses one type of physical quantity and outputs another physical quantity in proportion to the first.

In this case I am speaking of an “In Cylinder Pressure Transducer” which reacts to cylinder pressure by outputting a corresponding electrical signal to an oscilloscope. This pressure transducer is installed in the cylinder in the place of a removed spark plug. Then the engine is cranked, started and run while the transducer is used to graph pressure changes in the cylinder as the pistons is moving and the valves are opening and closing the way they would during normal combustion. Only there is no combustion due to the spark plug energy being shorted to ground to facilitate use of the tester. In this way we can watch each of the mechanical aspects of the engine and determine if any part of the equation is defective with out disassembling the engine!

In figure 1 we will take a brief look at what a good in cylinder pressure waveform should look like relative to possible cam timing issues. For a much more detailed explanation of the information contained these types of waveforms visit: http://www.automotivetestsolutions.com/pressuretransducersarticle.htm#compressionanatomy

On a normal engine as the piston begins to rise from the 180 degrees of rotation the BDC marker should dissect the exhaust stroke pressure rise ramp at approximately 50% of the total height of the waveform. If the 180 degree marker falls as much as 10° below or 15° above the 50% point, exhaust cam timing is within a normal range.

When viewing the intake stroke pressure drop ramp, a cursor placed 20 degrees after the 360 degree TDC marker (or 380 degrees). This cursor should intersect the downward slope of the waveform at the 50% point, give or take 10°, to be within a normal range.
Normal cam timing design tends to fall in these ranges due to the influences of ambient pressures and the requirements of fuel mileage and emission control priorities. Non-normally aspirated engines, vario-cam and high performance engines may vary further from these general specifications but not drastically.

Now let’s look at a 1995 Toyota Tacoma with a complaint of poor idle and low power on acceleration. This truck has a 3.4L DOHC V6 engine. There were no DTCs stored and all the fuel trim values were a negative 5-8%. I put a vacuum gauge on the engine and found that at idle we had only 8”Hg of vacuum. At 3000 RPMs the vacuum just barely improved but did not drop so I did not believe the problem was an exhaust restriction. I asked the shop if they checked the camshaft timing. They stated that they had pulled to timing covers and checked the cam and crank gear timing marks and that they were exactly where they should be. I decided to check the CMP and CKP sensor waveforms to see if at least the bank #2 camshaft was lined up properly. The bank one cams had no CMP sensor to look at for comparison. The waveform shown in figure 2 matched perfectly with 2 known good waveforms I found on iATN. Still believing the cams may be out of time I installed a pressure transducer in the bank #1 cylinder #5 spark plug hole and ran the engine. As you can see in figure 3 the pressure rise is occurring late (retarded) completely after the BDC mark. Installing the pressure transducer in the bank #2 cylinder #2 spark plug produced exactly the same retarded pressure waveform figure 4.

With both sets of cams being out of time on both cylinder heads the exact same amount and a CMP/CKP waveform that was in alignment, there was only one possible explanation. The crank shaft had advanced independently of the crankshaft timing gear and the entire rest of the valve train. As you can see in the photo in figure 5 the crankshaft key way gouged into the crankshaft gear allowing the crankshaft to advance without the rest of the valve train.

When the pressure is on to come up with an accurate diagnoses in a short amount of time nothing beats pressure transducer waveforms! We’ll explore different engine mechanical defects using in cylinder pressure waveforms in future columns.

ANATOMY OF THE COMPRESSION WAVEFORM by Bernie Thompson - ATS

The compression waveform produced from the internal combustion engine holds the key to determine if the mechanical condition of the cylinder is in good working order or if there is a deficiency within the mechanical condition of the cylinder.

It is necessary to break the waveform down into several divisions in order to make a determination of the cylinder condition. In figure 1 (taken with a 300psi transducer) and in figure 2 (taken with a -30hg transducer to increase the resolution of the exhaust plateau) a compression waveform is shown from a good spark ignition engine at idle and has key points of the waveform marked. Note that the compression waveforms, figure 1 and figure 2, were taken from the same spark ignition engine. At the top of the waveform point A marks the peak pressure that occurred. This point will correspond to the point at which the piston position came within the closest distance to the cylinder head. This pressure is the sum of compression from point K to point A. The amount of pressure built will depend on the volume between the cylinder head and the piston, when the distance from the piston to the head is at its closest point. This peak pressure point will represent the top dead center (TDC 0º) position of the piston’s movement. This is the point at which the piston has come to rest and is no longer in movement. This occurs when the crankshaft has reached the end of its stroke. This pressure will change due to the operating condition the engine is running under.

When the engine is in a cranking condition the compression on a spark ignition engine should be about 130lbs/square inch (psi). When this cranking pressure drops below 90psi it is an indication that the pressure within the cylinder is no longer adequate to support combustion of the hydrocarbon chains. When the engine is in a running condition the compression at idle should be about 70psi. When this running pressure drops below 40psi a misfire will occur. This is an indication that the pressure within the cylinder is no longer adequate to support the combustion of the hydrocarbon chains.

During a snap throttle compression test the idle compression pressure should increase by about 3 times. As the crankshaft rotates past the top dead center position the piston starts to move away from the cylinder head. This allows the volume between the cylinder head and the piston to increase. Under this condition the peak pressure that has been produced will start to decrease.

If the compression tower is measured from its lowest point, D, to its highest point, A, and this pressure is divided in half; then this point should occur at 30º after top dead center. This point is indicated by point B, halfway down the compression tower. The piston will then continue to move away from the cylinder head increasing the volume between the head and piston. The piston velocity will continue to increase until the crankshaft has reached the 90º position. The piston was at rest at top dead center and, as the crankshaft rotation continues, the piston speed increased until it obtained its maximum velocity at the 90º point. From this 90º position to the bottom dead center (BDC) point the piston will slow its velocity down until it reaches BDC and stops. The piston movement has now reached its half way position at 90º of crankshaft rotation.

In this first 90º of crankshaft rotation the cylinder has totally decompressed and now enters into a negative pressure or vacuum state. The piston continues its downward travel building more negative pressure within the cylinder. At the point the exhaust valve opens, point D, the piston travel is still moving in a downward direction but, the cylinder pressure starts to rise. This is due to the pressure in the exhaust being higher than the pressure in the cylinder. The cylinder pressure will continue to rise until it is equal with the exhaust pressure, point F. This exhaust pressure change should occur at the point the piston has decelerated to a stop or has obtained BDC 180º. The pressure change from point D to point F is referred to as the exhaust ramp. The target point is for the center of the exhaust ramp to be equal with the BDC 180º point, figure 3. At this point the exhaust camshaft timing is correctly timed to the crankshaft. If this exhaust ramp crosses the BDC 180º position within -10º to +15º of this target the camshaft is in proper time of the piston position.

On some performance based engines it is normal for the exhaust cam timing to be advanced and can still be in proper time with a +20º of this target. The piston being at bottom dead center is not in movement. The crankshaft continues to rotate which in turn moves the piston. The piston now starts to accelerate in an upward direction on the exhaust stroke. This forces the contents of the cylinder out of the cylinder into the exhaust system. The piston crosses the half way point, 90º position, reaching its maximum velocity and then starts to slow down and stop as it reaches the TDC 360º position. Approximately 15º to 30º before TDC 360º the intake valve will open. This pressure change can be seen at point G however, in different engines this pressure change may not be apparent.

When the piston is coming to a stop and the intake valve opens the piston has very low velocity. The exhaust valve is still open at this point and will equalize the cylinder pressure to the higher pressure that is within the exhaust system. When the piston reaches TDC 360º and then starts to move away from the TDC 360º position in a downward movement, the negative pressure will overcome the exhaust pressure within the cylinder and the cylinder pressure will decrease. The pressure decreases until it equalizes with the intake manifold pressure. The intake manifold is in a negative state of pressure or a vacuum. This intake pressure change creates the intake ramp, point G to point I. The exhaust valve will now close at approximately point I. This intake ramp should start to drop at the TDC 360º position and equalize with the intake pressure by the 60º mark after TDC 360º, point I. If the pressure from point G to point I is divided in half this target point should occur at 20º after TDC 360º, figure 4. This indicates that the intake camshaft is in time with the crankshaft. If the intake ramp crosses the TDC 360º plus 20º position within -10º to +10º of this target the intake camshaft is in proper time with the piston position.

On variable valve timed (VVT) engines the target for the center of the intake ramp is TDC 360º +30º within +/-10º. The intake pressure at point J should be approximately equal to the exhaust pressure at point D. This is due to the intake manifold pressure, point J, being compressed to the peak pressure and then decompressed to this starting pressure, which should be equal to point D.

The exhaust plateau, point D to point I, is created by the pressure differential within the intake manifold or the vacuum that is contained in the intake manifold. As this intake vacuum changes so will this exhaust plateau. For example, figure 5, when the engine is in a cranking condition the engine can only produce 1 inch hg to 3 inches hg of intake manifold cranking vacuum. With this reduced intake manifold vacuum the exhaust plateau will also be reduced or will decrease in its definition. With this decrease in the exhaust plateau’s definition the exhaust plateau will change in the way that it appears and is used. Since the height of the plateau is based on only 1 to 3 inches hg, this plateau will no longer cross the bottom dead center 180º mark or the TDC 360º +20 mark. The intake manifold vacuum will need to be much greater in order for the exhaust plateau to have enough height or pressure change for these exhaust and intake ramps to cross their targets. Since the exhaust and intake ramps cannot be used to check cam timing during a cranking condition, the valve openings must be checked instead. The exhaust valve opening should occur 30º to 50º before BDC 180º. The intake valve opening should occur just after TDC 360º. The intake valve closing should occur 30º to 60º after BDC 540º. If these targets are met the camshafts are timed closely enough for the engine to start however, the camshaft timing could still be up to 1 tooth out of time. In order for the cam timing to be known the engine must be at a steady idle state. The piston then continues to increase its velocity in a downward direction until it reaches the 90º position. At this point the piston has reached its maximum velocity. The piston then continues to move downward, slowing until it reaches the stopping point or BDC 540º. The crankshaft continues to rotate and the piston starts to move in an upward direction but, the piston velocity is low. At this point the intake valve is still open so the pressure is equalized by the pressure within the intake manifold. The intake valve closes at point K and the cylinder pressure begins to rise. This intake valve closing should occur at approximately 50º after BDC 540º. The piston continues to travel in an upward direction, gaining velocity until it reaches its maximum velocity at 90º. The compression ramp at this point is clearly increasing in pressure. The piston continues to travel upward and is now slowing its velocity as it approaches 30º before TDC 720º point. At this point the compression should be halfway between the minimum pressure and the maximum pressure, point M. The compression then continues to build until the piston slows down and reaches a stopping point at TDC 720º. It is important to note that most of the compression pressure is produced in the last 30º of crankshaft rotation.