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PRESSURE TRANSDUCER TRAINING
The Pressure is On
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 see the following article: "Anatomy of a Compression Waveform"
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.
Automotive Test Solutions is a multi-national award winning company for innovative thinking that produces patented tool solutions for the automotive industries.
Established in 2001, ATS manufactures oscilloscopes, OBD II generic scan tools, misfire detectors, pressure transducers, leak detectors, gas analyzers, borescopes,
engine simulators, ignition analyzers, and a number of high tech training products to serve the automotive technician.
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