Checking secondary ignition, (spark),  with the Fluke 97 Lab Scope. 
 
 
 
 

 
The connector  inside the cabin at the right side kick panel, used to check for on board computer fault codes. 
 
 
 
 
 

 
Back probing the Manifold Absolute Pressure, (MAP), sensor signal wire with the Fluke 97 Lab Scope. 
 
 
 
 
 
 
 

 
Back probing the Oxygen, (O2), sensor with the Fluke 97. 
 
 
 
 
 
 
 

 
Back probing a connected fuel injector with the Fluke 97 so we can see the computers command signal at the injector and the injector's waveform. 
 
 
 
 
 
 
 
 

 
View of the waveform of a fuel injector's short pulse with, (on time), while applying the gas pedal during the symptom window. This was caused by the O2 sensor incorrectly reporting a rich signal to the car's computer. As a result, the computer purposely did not spray a lot of fuel into the engine, thus causing the hesitation. 
 
 
 
 
 
 
 
 

 
This is the waveform of the MAP sensor showing normal operation during a load. 
 
 
 
 
 
 

This is the waveform of the O2 sensor when it failed and was stuck reporting to the computer that the exhaust was rich. This caused the computer to hardly open the fuel injectors, thus causing the hesitation. 

  Those darn intermittents. 
  The symptom was simple enough, the customer reported that when driving, the car would momentarily loose power, whether the accelerator was depressed or held steady made no difference. The problem was that it was very intermittent. 
 
  Oh yes, those darn intermittents, not exactly every technicians favorite type of diagnostic problem. They bring back memories of intermittent problems that cost endless hours of diagnostic time only to find a loose fuse connection as the cause of it all. Any customer complaint we receive about any intermittent problem is usually followed by a long explanation to the customer of how complicated some systems can be. A typical powertrain control module monitors at least a dozen sensors, through more wires than a small home, to control fuel and spark delivery as well as other sub-systems. The point we stress the most to customers, is that we most likely will need to be able to duplicate the problem in order to diagnose its cause. 
 
  System specifics 
 The Honda in question is typical of the modern offerings from the orient, but a few not so typical features are worth mentioning. The PGM-FI, (Programmed Fuel Injection) control unit does not measure intake air with any kind of sensor, instead injector duration is based on a preprogrammed duration that is derived from RPM signal and  MAP sensor signal, and then modified by other sensors. The Engine Control Module will protect the engine from over revving by cutting off fuel at any RPM reading above 6300. The Engine Control Module also controls charging system voltage in accordance with charging system load so fuel economy can be increased during times of light electrical load. 

  Were do we begin 
  Two or three road tests later we had some useful information to start with. The symptom, although very intermittent, could always be duplicated the same way. When the car was driven, allowed to hot soak for a short time, and then driven for a few minutes, the Honda would always suffer a loss of power that would only last a few seconds, and the customer was correct in reporting that accelerating made no improvement. 

  We started by checking for factory service bulletins, it is the best first diagnostic effort if you have access to the information, but in this case none were listed that matched our Honda's symptoms. Next we checked for trouble codes stored in memory. Jumpering the two wire connector located under the passengers side of the dash, is all it takes to make the dash check engine warning lamp flash out codes. No codes were present, but that is what we expected because the problem was so intermittent and never had illuminated the check engine lamp. 

  With in shop diagnostics used up, no we do not have a dyno, we decided it was time to hit the road. First we tried to duplicate the condition with our lab scope monitoring a fuel injector waveform. After a few more tries we finally got it to act up and found the pulse width, or injector on time , was being decreased as the Honda lost power. This explains the power loss, but now we realized we would need to find the input signal that led to this decreased fuel supply moment. With one power loss event sometimes taking hours to duplicate we searched for an easier solution. 
  We talked to the local Honda dealer for advise, and they reported the distributor recall on the 1991 Hondas had been changed to now include 1992 models. This cars symptoms do not match any of the failed Honda distributors we had seen in the past, but with the condition being related to a hot soak, and being so hard to duplicate, it seemed worth a try. Well one day later our Honda was back with its brand new distributor, compliments of Honda, but as the drivers brought the keys back to the service desk, they reported the intermittent power loss was still present. 
  Back on the road with the lab scope, we decided to monitor the MAP, or manifold absolute pressure sensors output next. Any false indication of reduced load could cause the lessened injector pulse width our Honda was plagued with. Hondas MAP sensor is a silicon diaphragm strain gauge style that directly varies voltage output in relation to manifold pressure. When the symptom occurred MAP voltage gave no indication of a load change, meaning the MAP sensor signal was not our problem, and that we had just wasted a few more hours of diagnostic time driving the Honda. 

  In another effort to save time we put in a call to one of our favorite diagnostic hot lines and asked for their advise. Trying to decide when to call a hotline for help can be a problem itself. If you call to early in the diagnosis they will tell you to check the basic and easy to verify things first, but you probably already knew to do that. If you call them to late you can find out you have been wasting time because the problem is common enough that they have seen it before. We were calling just a little late. They suggested we rode test and monitor the oxygen sensor, that on this vehicle , the oxygen sensor was the primary fuel control when ever the vehicle is above idle speed. Maybe we should of have known that already, but we work on more domestic car problems and they typically do not rely so strongly on oxygen sensor input. 

  Wait a minute, lets place the blame were it belongs. Should car companies supply us with more information when it comes to operation logic? This is one of our biggest pet peeves, and about once a week at our shop a job will come in, and we will notice that if more information were available the diagnosis would have taken less time. I realize car companies want to keep operation strategies secret but we are not looking for a lot of numbers or specifics. Just give us some simple guidelines like which sensor has what priority and when. Like what sensor signal amount would be drastic enough to over ride another sensor. Many technician taking the ASE advanced level L1 test report how difficult the test is, yet more information is given about the generic vehicle used in the test than some major car companies actually supply for there own cars. Hopefully the L1 test will give some auto manufactures a clue as to how much information a good technician will need to repair their vehicles. 
 
  On the road again 
  Driving the Honda again, this time monitoring the oxygen sensor circuit, we found the sensor signal looked good while the car was not acting up. The sensor moved rapidly between a low voltage reading, lean indication, to a high voltage reading, rich indication, just as it should. But when the symptom appeared again the oxygen sensor stopped swinging lean to rich, and just held a steady high voltage signal for a good three seconds. This false rich indication tricked the Electronic Control Unit into reducing fuel injection delivery and resulted in our power loss symptom. Intermittent oxygen sensors problems are not very common, but this Honda proved anything is possible. This particular oxygen sensor also has a heater element that could explain how the sensor failed with a fixed higher than normal voltage reading. Somehow the hot soak and short test drive would make the sensor fail allowing  voltage from the heater circuit to bleed into the sensor side of the circuit. 

  With the new oxygen sensor installed we repeated our road test routine and kept monitoring the oxygen sensor circuit. The symptom and hung up oxygen sensor reading never resurfaced after five road tests verifying the repair was complete. Three days after the car first rolled in the shop, the problem was finally solved with a simple oxygen sensor replacement. Those darn intermittents. 

Glenn Giammalvo MAT, L1