Model: Micra LX
Engine Code: CG10DE
Initial Fault Description by owner:
Frequent and intermittent hesitation and extremely poor drivability
Initial Fault Description by us:
From the symptoms we thought the problem probably stemmed from a fuel starvation problem causing a misfire.
Steps of Diagnosis:
First of all we tried to recreate the problem in the workshop but of cause the problem was not present then. So a test drive was needed, this did show the problem again, but it did now seem to be unlikely to be a fuel starvation problem because the misfire could be driven through, meaning that fuel was not the problem.
Reassess the fault:
The problem now needed to be recreated on our rolling road. Using the gas analyser we could see that there was 0% CO and a high lambda value (air/fuel ratio), 3% Oxygen and little or no HC. After this we thought it was likely that one or more cylinders was running lean or with no fuel.
The lambda sensor showed the expected value for this problem and there was no visible or audible sign that the exhaust system or inlet system was faulty, so we had no reason to doubt the lambda sensor.
Due to a 3% oxygen value and a very low HC value we can tell that it is not an emission fault as low HC tells us that there is no excess fuel in the system and so a fuel starvation problem is assumed.
The next step was to remove the injectors and bench flow test them, they where fine. So then we started to look at the output from sensors on the vehicle. The first of which was the throttle potentiometer, this showed all the expected values and was ruled out as being the problem. We then moved on to the coolant temperature sensor that also showed all the correct values. Finally we moved on to the airflow sensor that showed the expected value of 1.9-2v on the rolling road under load and 0.7v at idle, but when the symptoms showed again, it’s value dropped to between 0-0.5v, less than idle. The ECU’s reaction to low air flow is to weaken the mixture and that was what was causing the initial symptoms.
At this point we checked the cost of the air flow sensor and in this case as it is only possible to buy it as part of the throttle body assemble from Nissan and it was nearly £500 pounds, so we contacted the customer before we went ahead with the repair.