Failure phenomenon: a 2007 annual Nissan Qashqai 2.0 CVT SUV, with mileage of 65,000 km. When users complained that the car is about 36,000 km from poor acceleration began to appear a failure, this failure has two kinds of performance: one is to accelerate sometimes feel powerless, especially the emergence of a higher frequency of this phenomenon when climbing, and intermittent a few seconds for an acceleration of the engine can not fault, the fault occurs to return to normal; the other is at high speed, duration accelerator pedal is depressed, the vehicle speed can not exceed 140 km / h. Symptom of two or more at an early stage is not obvious, when the vehicle traveling 60,000 km, failure began to appear difficult cold start, the engine needs more than 3 s ignition switch is operated to start.
Check Analysis: I start from the above symptoms, after which the car is first analyze the causes hot car can not exceed the maximum speed 140 km / h to see if the engine or be caused by CVT transmission. Found by test, when the vehicle speed reaches 100 km / h, the engine speed is 3 500 r / min or so, normal; when the vehicle speed reaches 130 km / h, the engine speed is 5 000 r / min, this speed is too high, this It shows that when the vehicle speed is constant, CVT transmission is a certain degree of skidding. I use the vehicle fault diagnostic detection, fault code is not found, then check the CVT transmission oil quality, we found darker oil, light brown (new light green transparent oil), indicating that CVT transmission oil is likely to have there have been high temperature overheating, the internal transmission had some wear and tear. In addition, when the vehicle speed drops from 50 km / h 20 km / h, sometimes appear speed hysteresis. It is not due to the presence of a fault inside the transmission so that the transmission stopper, locking the gear ratio upshift, downshift necessary to make the conversion time becomes longer, so that the fault occurs in a short time can not speed up. In order to verify the suspicion own, when the fault occurs in the vehicle, I N first transmission gear engaged, and then linked into the D block, the vehicle speed can immediately. Thus, the author determines the initial transmission fault is an internal fault actions, according to the manufacturer then sends the transmission outside the repair advice.
The maintenance of the transmission means re-testing the car at this time, the speed of more than 140 km / h, the vehicle traveling on a flat road is normal, but climbingWhen, sometimes to accelerate bad situation will arise when the deceleration by the deceleration zone, and then speed up when driving, and sometimes accelerated lag phenomenon. So I of the stall for the transmission experiments, the measured speed gains and losses speed of 1 700 r / min, the normal range, but sometimes at 1 200 r / min will pause, and then to reach 1 700 r / min. From analysis of the phenomenon, I believe that the transmission shift speed is too slow, can not shift in time in accordance with the load. Taking into account the failure of the transmission control unit TCM will cause this failure, so I replace an available TCM determined, and learning the B-mode (internal computer and the data transmission matching the CVT TCM) test, the fault persists.
FIG 1
After careful test, I found that in the case of slow acceleration, the TCM fault diagnostic data monitoring display speed gear ratio of about 1.9 (typically as a change in the speed ratio between 0.47 and 2.34), slip between 0 and 127, data is normal (FIG. 1). I also found in the test, the engine speed can not exceed 1 700 r / min, the speed should not rapidly indicating insufficient engine power at that time and related, because the speed 20 km / h or more, no matter in what gear CVT, the fierce when the accelerator pedal is stepped on rapid acceleration, the engine speed should exceed the stall speed. From this point it can be described, when the rapid acceleration speed lag before decelerating, actions by the insufficient power of the engine at low speed rapid acceleration. The reasons for this may include: a timing system, fuel system, ignition system and the variable intake cylinder pressure abnormality exists. So the author of these possible points of failure to check one by one, and finally found the fault, the fuel pressure is only 300 kPa (standard 400 kPa), and when the engine stalled, oil pressure will quickly drop, oil pressure also in the sudden acceleration cases Some will fall again, reaching 230 kPa. After examination, the cause of the malfunction to the bottom, because of the failure of the pressure regulating valve on the pump, the fuel pressure regulator is not well, and the low oil pressure can not be maintained, so that each time the engine is started to be more than a few seconds to start successfully, and resulting in lack of engine power.
Troubleshooting: replacement fuel pump test, troubleshooting.
Review Summary: car fault is integrated by a transmission fault fault and engine fuel system failure intertwined formed from the failure phenomenon seen more easily judged transmission failure, but ignored the failure of the fuel system. When carrying out vehicle maintenance, often encounter the illusion, the maintenance staff thinking is locked in somewhere, but ignores some inconspicuous place, making slow progress of maintenance work or can not find the start of the method, which reduces troubleshooting s efficiency. Therefore, maintenance personnel should open mind on the job, out of the fixed mode of thinking, to avoid being blinded by the illusion.
(Wu Hua)