Nissan AD (Y10) 1.3 (1993): Diagnosing Rough Idle and Stalling at Idle

Part numbers are from the manufacturer's original catalogue. Check them against your VIN before ordering.

• Set of 10-17 mm wrenches and sockets
• spark plug socket
• screwdrivers
• feeler gauge
• multimeter
• vacuum gauge or fuel pressure gauge
• timing light (or a high-voltage spark tester)
• carburetor or throttle body cleaner spray
• compression tester

1. Determine the fuel system type: a carburetor (round housing with a fuel hose and linkages) or a throttle body with injectors and a sensor wiring harness. The remaining steps differ only in steps 7 and 8. If the Check Engine light is on or has been on, read the self-diagnostic codes from the control unit.
2. Warm the engine up to operating temperature. Check whether it misfires evenly or intermittently, and whether the engine speed changes when you press the accelerator. If it runs smoothly under throttle but misfires at idle, look for the cause in the idle system, an air leak, or the carburetor.
3. Remove the spark plugs and inspect them. Black deposits mean a rich mixture, a whitish coating means a lean mixture, and oil means worn valve stem seals or piston rings. Check the gap with a feeler gauge, using the GA13 specification, usually about 0.8-1.1 mm. Replace suspect plugs as a full set. Spark plug tightening torque is 20-25 N·m (15-18 lb-ft); do not overtighten.
4. Inspect the spark plug wires and the distributor cap. Cracks, signs of arcing, white deposits on the contacts, and a worn carbon contact and rotor cause misfires. Check wire resistance with a multimeter; values are per specification. If in doubt, replace the wires, cap, and rotor. In the dark, start the engine and look for spark "jumping" to the body.
5. Check the ignition timing with a timing light at the mark on the crankshaft pulley with the engine warm; the value is on the plate in the engine bay. Incorrect timing or a stuck vacuum or centrifugal advance mechanism in the distributor causes an unstable idle.
6. Check for air leaks. Inspect the vacuum hoses and their tees, the carburetor or intake manifold gasket, and the brake booster hose. To find a leak, briefly spray carburetor cleaner on the joints with the engine running. A change in engine speed points to the leak location. Do not use an open flame for this.
7. For the carbureted version: check the air and fuel filters, the fuel level in the float bowl, the operation of the power valve and accelerator pump, and the cleanliness of the jets and idle passages. Adjust the idle speed and mixture screws per specification (usually idle 700-800 rpm; verify on the plate). If the carburetor has not been serviced for a long time, remove it, disassemble it, clean it with cleaner and blow out the passages with compressed air, and replace the gaskets. Check the operation of the automatic choke on a warm engine: it should be fully open.
8. For the fuel-injected version: clean the throttle body and the idle air passage with cleaner, and check the idle air control valve, the throttle position sensor, the coolant temperature sensor (a fault in it causes a rich mixture and stalling), the air flow meter or pressure sensor, as well as the connector contacts and ground points. Check the fuel pressure with a gauge per specification.
9. Check the fuel delivery: the condition of the fuel filter, the pump output (on the carbureted version this is a mechanical pump driven by the camshaft, and its diaphragm may be cracked), and the absence of air in the hoses. A clogged filter causes stalling under load and sputtering.
10. Measure the compression in each cylinder with the engine warm and the spark plugs removed. A variation between cylinders of more than 10-15% indicates a mechanical problem: valves, piston rings, or a blown head gasket. While you are at it, check the valve clearances, which on older engines often drift out of spec and cause an unstable idle.
11. After fixing the cause, take a test drive and make sure the idle is smooth and the engine does not stall when you turn on a load (lights, heater, fan).

• When disassembling the fuel system, work only on a cool engine and keep open flames away from gasoline
• Always disconnect the negative battery terminal before removing the carburetor or fuel lines
• If you cannot find the cause after checking the ignition, air leaks, and fuel delivery, take the car to a repair shop with diagnostic equipment: on older engines the problem is often a combination of several worn components at once