Tech to Tech
Learn To Begin At The End
The time has come for me to give away my best driveability secrets. In past Tech to Tech columns, I've discussed O2 diagnostics (February 1995) and fuel injector diagnostics (March and April of 95). Now is the time to tie all of these together.Where do we begin? At the end! What? Let me explain. The signals that will provide the most information are the end products. Why? They are influenced by all the inputs. A defective input will result in defective outputs. So the starting points at my shop are: the tail pipe (four-gas), the O2 waveform and the injector waveform or mixture control solenoid waveform.
You may argue that the O2 signal is an input and not an end product. However, it is influenced by everything that happens upstream. This makes it an invaluable diagnostic tool. Sure, you can use a voltmeter to look at the O2 sensor, but to see the whole picture, a scope is by far the best tool.
The injector pulse width is determined by the computer after polling all the sensors and calculating the best fuel delivery. While the pulse width can be read from a scanner or DMM for some cars, others will require a scope for accurate measurement. Please refer back to the injector articles.
Referring to a four-gas "tail-pipe" test may be a misnomer. Sometimes it becomes necessary to test the gases before the catalytic converter to get true gas values. A good converter can mask problems by efficient conversion of gases. I have never understood four-gas supporters saying that O2 and CO2 are not affected by the converter, considering the fact that the converter's function is to use O2 to catalyze CO and HC into CO2 and H2O. Don't get me wrong, I couldn't live without my four-gas, I just think the testing before the converter is necessary in some circumstances. I have seen new cars with a 100 percent misfire on one cylinder with great tailpipe gases from a good converter.
OK, so I have my gas values and my pulse width and my O2 signal so now what? Now it's time to put on the old thinking cap and go to work. A few examples should illustrate the power of this information.
In the first scenario, we have an engine that is up to operating temperature and the O2 is also at operating temperature. The four-gas shows high CO at 8 percent; the HC is high at 500 ppm; the O2 is low at .1 percent; and CO2 is low at 10 percent. This is definitely a rich mixture. The O2 reads .8 volts and the pulse width is 5.0ms, which is a wide or rich command. Now stop and think. What might cause this? We know that the car is rich from the gases, the O2 sensor is telling the computer the engine is rich, but the computer is commanding a wide (rich) pulse width. Obviously the O2 sensor is not in command. So what inputs have authority over the O2 sensor? Well, the MAP sensor, the TPS sensor, the coolant temperature sensor and the air charge temperature sensor. It stands to reason that one of these sensors is out of specification, but not so far out to set a code.
Let's look at one of the most common modes of failure. The tail pipe readings are the same as above (rich). The O2 sensor reads 0.0 volts a lean signal (commanding enrichment). The injector pulse is wide, as in the first example. The computer is responding correctly to the lean signal by driving the injector rich. So the O2 is in command, but why is the tailpipe rich? In this example, the O2 sensor signal is false! Why? The most common failure is a dead sensor that doesn't respond correctly. Another possibility is false O2 on the sensor from secondary air injection, an exhaust leak or even an ignition misfire. (O2 sensor fakeouts were discussed at length in the February 1995 Tech to Tech.)
For another example, let's consider a rich tailpipe, a rich O2 signal and a lean injector pulse (1.0ms) command. Well, the O2 is in command because it knows it's rich and the injector is being commanded lean, but the tailpipe is rich. Why? The engine is getting fuel from somewhere else. Where? Possibly a ruptured fuel pressure regulator diaphragm. How about high fuel pressure? How about fuel in the oil being sucked through the PCV system? Perhaps a canister problem is supplying fuel. What about leaking injectors? A quick process of elimination should resolve this problem.
All of the examples above ended with a rich tailpipe. How about a lean tailpipe reading, a lean O2 signal and a wide pulse width? Low fuel pressure? Could be. Plugged injectors? Possible. A bad MAP sensor commanding a lean signal? No way then the pulse width would be short (lean command).
OK one more lean tailpipe, lean O2 signal and short pulse width (lean command). Hmm, looks like the O2 isn't in command. So once again we look at which sensors have authority over the O2 and look for a defective specification there.
If you refer back to the January 1995 article, "Baffling Bronco," you will see another example using this procedure. For those of you who missed that one, I'll review. This Bronco with a V6 and FBC (feedback controlled carburetor) had a rich tail pipe, a rich signal at the O2 sensor and a full enleanment signal output from the computer to the FBC solenoid. This system runs a rich carburetor calibration all the time and the FBC solenoid acts as a controlled vacuum leak to adjust the mixture. So was this a case of fuel from somewhere else? Actually, further testing revealed that someone had hammered a cork in the vacuum tube from the FBC solenoid, causing the FBC system to be inoperative.
Let's review; first, does the O2 signal match what the tailpipe gases are showing? Next, is the O2 in command of the fuel mixture? This is demonstrated by the injector pulse width correlating with the O2 signal. This is where techs that are new to dual trace digital scopes should start their learning experience.
We have covered many possible problems and their responses to the three parameters we check first. I have a list covering problems and their responses, which I will publish later. What I want now is your help in adding to this list. (See excerpt below for information on how to get your examples to me.)
Next month I'll cover four-gas analysis to help clarify this procedure. I look forward to seeing some faxes.
Fax In Examples Of Outputs Analysis
In this month's Tech to Tech, Mark Warren discusses several possible problems as analyzed by O2 sensor signals, injector pulse widths and four gas readings. Warren will publish a more complete list in a later issue of AUTOINC., and wants the help of readers in adding to the list.Please list possible failures in a vertical column on the left and then the responses to O2, pulse width and the four gasses across in a row. Fax your list to Warren at 1-520-790-9173. Please list your name, address, phone number and fax number in case clarification is necessary. Warren's fax machine is always on, and it's computer-based, so running out of paper isn't an issue. However, if it's busy, he is receiving a fax, and you can try again later. Please don't fax Mark requests for information, since he may not have the time to properly respond.
Examples may also be emailed to Mark at mwarren@azstarnet.com.
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AutoInc. Magazine ®, Vol. XLIV No. 3, March 1996