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Scope & Scan: The Pressure is On

Friday, August 1, 2008 - 00:00

While engine mechanical diagnostics dates back to, well, ever since the first engines were made, the way we diagnose defects continues to evolve. The toughest aspect of engine mechanical diagnostics may not be as sophisticated as electronic control systems diagnostics, but because of disassembly requirements inherent in mechanical inspection, it can be just as time consuming.

However there are some relatively new tools we can use to pinpoint engine mechanical defects in both a timely and accurate manner without disassembly: pressure transducers. A transducer is anything that senses a physical force and changes it to an electrical signal.

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 place of a removed sparkplug. The engine is cranked, started and run while the transducer is installed. The oscilloscope will then graph pressure changes in the cylinder as the pistons are moving and the valves are opening and closing the way they would during normal combustion (the sparkplug is shorted to ground to facilitate use of the tester). In this way we can watch the mechanical operation of the engine and determine if any part is defective without disassembling the engine.

In Figure 1, we see 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 in these types of waveforms, visit www.automotivetestsolutions.com/pressuretransducersarticle.htm#compressionanatomy.

Keep in mind that one normal engine cycle is 720 degrees, so pistons will reach Top Dead Center (TDC) and Bottom Dead Center (BDC) twice in each cycle. On a normal engine, as the piston begins to rise from the first marker, the marker should dissect the exhaust stroke pressure rise waveform at approximately 50 percent of the total height of the waveform. If the that marker falls as much as 10 degrees below or 15 degrees above the 50 percent point, exhaust cam timing is within a normal range.

When viewing the intake stroke pressure drop waveform, a cursor is 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 percent point, give or take 10 percent, 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. Turbo-or supercharged engines, variable cam timing engines and other high performance engines may vary further from these general specifications, but not drastically.

A Closer Look

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 negative 5 percent to 8 percent. I put a vacuum gauge on the engine and found that at idle we had only 8"Hg vacuum. At 3,000 rpm, the vacuum 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 they had pulled timing covers and checked the cam and crank gear timing marks, and they were exactly where they should be. I decided to check the crankshaft position (CMP) and camshaft position (CKP) sensor waveforms to see if at least the bank No. 2 camshaft was lined up properly. The bank No. 1 cams had no CMP sensor to look at for comparison. The waveform shown in Figure 2 matched perfectly with two known good waveforms I found on iATN.

Believing the cams may be out of time, I installed a pressure transducer in the bank No. 1 cylinder No. 5 sparkplug 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 No. 2 cylinder No. 2 spark plug produced the same retarded pressure waveform.

With both sets of cams being out of time on both cylinder heads the same amount, and with a CMP/CKP waveform that was in alignment, there was only one possible explanation. The crankshaft had advanced independently of the crankshaft timing gear and the rest of the valve train. As you can see in the photo in Figure 3, the crankshaft key is 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 diagnosis in short time, nothing beats pressure transducer waveforms. We'll explore different engine mechanical defects using incylinder pressure waveforms in future columns.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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