2500 Beijing Jeep Cherokee sport utility vehicle, equipped C498QA1 type inline 4-cylinder 2.5 L engine, mileage of 140,000 km, running for some time after the vehicle due to the total loss of antifreeze, and some burning oil, so the overhaul of the engine. But the overhaul is completed, the engine can not be started.
maintenance personnel check the battery voltage normal start, the starter motor is running correctly, that there is no sign of the car. The car’s engine is equipped with an older engine on starting, ignition and fuel control is relatively simple, if the starter motor is operating normally and should not be with nothing more than the following vehicles, the point of failure: lack of fuel or no fuel pressure pressure, ignition system failure or insufficient ignition, cylinder pressure is insufficient, the engine mechanical failure, the system failure and the start timing related failure and an engine control sensor unit failure.
In the ideas from simple to complex, the author is first connected to the fuel system when the fuel pressure gauge pressure detecting start the car fuel pumps will work for a few seconds when the ignition switch is opened, the hydraulic pressure of 350 kPa, start-time of about 380 kPa, fuel pump is working properly, the fuel pressure is normal. Then check the ignition system, because the replacement overhaul, new spark plugs, spark plug problems the chance is very small, it can be ruled out. Is also new for high-voltage, remove the ignition plug connection situation when the high voltage test is started, the ignition intensity well, so the possibility of failure of the ignition system can be excluded.
From the traditional point of view of maintenance, oil fire is vain car, is it a problem when the assembly? Detecting the pressure of each cylinder, each cylinder pressure were normal. When installing high-voltage lines, I suddenly thought of the maintenance staff is not 1, 4-cylinder high-voltage line connected to the wrong cause the car engine is not it? Check the distributor cap alignment mark is not wrong, after consulting maintenance staff, maintenance staff, said of the ignition timing is labeled in accordance with 1 cylinder piston to top dead center, but did not observe whether 1 cylinder intake valve actuation. Dead center because the piston cylinder of the engine 1 is a 4-cylinder top dead center of the piston, once the cause of the engine will not start trans. After checking, high-voltage plug on the distributor really wrong. Re-start of the engine proper position high voltage, smooth start of the engine. Troubleshooting the thought, but after a simple test, turn off and then restart the engine, the engine still does not start. To start the motor continues to run 6 ~8 s can start. At this time the engine has reached normal operating temperature, the high idle speed when the engine is running. Turn off the heat again to start the engine, the engine is still difficult to start.
connected to the vehicle fault diagnostic apparatus for detecting an engine control system, the system is not stored fault codes. Can be determined, the failure of the car is a typical hot car starting difficulties. So what would cause not easy to start the engine hot car? I Remove the spark plug, it was discovered the spark plug has been soaked in fuel, engine fuel injection can be determined excessive. After washing and drying the spark plug refitted, the throttle opening is adjusted to the maximum position, and then start the engine, the engine start smooth. After starting the engine control system of the data observed, it was found significantly higher idle speed, coolant temperature has at 100 ℃. Will not be a problem or the coolant temperature sensor circuit, causing the engine control unit from vehicle start state according to the thermal state of the cold supply, the engine starting mixture is too thick results in difficulties? See fault diagnostic apparatus using the engine data stream unremarkable. When starting the engine, the temperature suddenly found diagnostic coolant temperature display and the instrument displays a large gap, the coolant temperature is displayed on the diagnostic apparatus 39 deg.] C (FIG. 1), the meter has been indicated nearly 100 deg.] C (FIG. 2 ), it seems that the problem is in terms of the coolant temperature sensor. However, according to the user reflects the car due to engine failure warning light has replaced a coolant temperature sensor in the repair shop.
FIG 1
FIG. 2
uses two car engine coolant temperature a sensor, mounted on a rear side of the cylinder head, the engine operating temperature to provide to the instrument; a cover mounted on the thermostat, providing the coolant temperature to the engine control unit. The engine control unit provides a reference voltage to the sensor 5 V, the engine control unit detects a signal flowing through the sensor to the sensor ground. Since the coolant temperature sensor provides a signal to the engine control unit is new, then causing the control unit to identify the reasons for low coolant temperature sensor will be in the line or the coolant temperature of the engine control unit. After careful observation, can be determined once the vehicle due to coolant temperature results in the head-side melt through the harness. The coolant temperatureSensor wires to the engine control unit is wherein the line so it can present a problem. Refer to the circuit diagram (FIG. 3), the engine control unit is disconnected connector, the plug is disconnected coolant temperature sensor, a digital multimeter for measurement of engine control unit 15 feet between the coolant temperature sensor to the No. 1 pin off line measurement results normal; continue measuring this grounding line is grounded, normal measurements; measuring an engine control unit 46 to pin 2, pin between the coolant temperature sensor is off line, the normal measurements. Whereby the coolant temperature sensor can be determined to the line between the engine control unit no problem. In view of the problems are less likely to engine control unit, we suspect replaceable sensor can be problematic decision to replace the coolant temperature sensor again provides a signal to the engine control unit.
Figure 3
After replacement of the coolant temperature sensor, the test comparison of water and temperature water has reached a normal temperature, can be determined thermostat It has been opened, the mechanical problems and the engine coolant pipe line is not present. But the water temperature diagnostic display temperature and display of the dashboard there is still a lot of deviations, the engine control unit is it a problem? For insurance purposes, I got a car with an engine control unit Substitute. After replacing the engine control unit, after the starting operation of the engine coolant temperature 20 min See, instrument display 96 deg.] C water temperature, coolant temperature fault diagnostic display is 40 ℃. The engine control unit has been replaced, the coolant temperature sensor associated line may be determined without problems, problems do replacement coolant temperature sensor? But it had been replaced two of the coolant temperature sensor, supposed to be no problem. But in order to determine whether it was faulty coolant temperature sensor, it decided to remove my station rescue vehicle to replace the coolant temperature sensor test. After replacing the coolant temperature sensor, re-use of fault diagnostic data reading engine control system, found in a case where the engine does not start, the coolant temperature has reached 90 ℃, and almost the same temperature display instrument.
Figure 4
After careful observation, I found that the coolant and the coolant temperature sensor used in the car rescue vehicles withThe temperature sensor is somewhat different color, slightly lighter color. Upon inquiry fitting portion that the sensor is a non-original accessories. Since it difficult to buy original parts, had to temporarily rescue vehicle sensors to malfunction cars.
To determine the fault has been ruled out, repeatedly starting the engine, the engine hot car started smoothly, the engine idling speed has reached the normal range.
Summary: The coolant temperature sensor is a negative temperature coefficient thermistor of a 2-wire sensor, to sense the temperature of the engine coolant. The engine control unit recognizes coolant temperature sensor cold and hot car, is cold, for the fuel system to provide a richer mixture, higher engine speed; hot car, the engine control unit based on the sensor signal, the correction injection pulse width. Since this car coolant temperature sensor signal is supplied to the engine control unit is incorrect, resulting in that the engine control unit is always in the engine cold state, causing partial mixed gas has been concentrated. Engine failure inconspicuous cold state, when the car engine is hot, giving rise to a difficulty starting the engine idle speed and the high symptoms. Failure by the maintenance of the car, reminding us, as long as proper maintenance of ideas, we have faith in his own judgment. In addition, as maintenance personnel, the replacement parts be sure to use the original or better quality accessories to ensure quality of maintenance. Furthermore, if we can identify what parts quality attention to strengthening the ability of exercise, it will give us some help in the daily maintenance.
(Chenghuai Yu)