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Tech Tips
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Fuel System
L-Jetronic: Troubleshooting Symptoms
L-Jetronic is an electronically controlled injection system: the quantity of air drawn in by the engine is determined in accordance with the air-flow sensor flap principle with electromagnetic control of the fuel injection into the intake manifold. All changes pertaining to the engine are detected by numerous sensors and processed by the ECU. Consistently good exhaust gas emission levels and low fuel consumption are the results.
SYMPTOM: ENGINE CRANKS BUT DOES NOT START
Cause |
Remedy |
| Defect in ignition system |
Check battery, distributor plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Blockage in fuel system |
Check fuel tank, filter and lines for free flow |
| Relay defective; wire to injector open |
Test relay; check wiring harness |
| Fuel pump not operating |
Check pump fuse, pump relay and pump |
| Fuel system pressure incorrect |
Check pressure regulator |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Cold start valve leaking |
Check valve for leakage |
| Thermo-time switch defective |
Test for resistance readings vs. temperature |
| Auxiliary air valve not operating correctly |
Must be open with cold engine: closed with warm |
| Air flow meter defective |
Check pump contacts; test flap for movement |
| Loose connection in wiring harness or syst. ground |
Check and clean all connections |
| Control unit defective |
Use known good unit to confirm defect |
SYMPTOM: ENGINE STARTS BUT THEN DIESEL
Cause |
Remedy |
| Leaks in air intake system (false air) |
Check air system for leaks |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Blockage in fuel system |
Check fuel tank, filter and lines for free flow |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Thermo-time switch defective |
Test for resistance readings vs. temperature |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Idle speed incorrectly adjusted |
Adjust idle speed with bypass screw |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
| Loose connection in wiring harness or syst.ground |
Check and clean all connections |
| Control unit defective |
Use known good unit to confirm defect |
SYMPTOM: ROUGH OR UNSTABLE IDLE
Cause |
Remedy |
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Blockage in fuel system |
Check fuel tank, filter and lines for free flow |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Thermo-time switch defective |
Test for resistance readings vs. temperature |
| Auxiliary air valve not operating correctly |
Must be open with cold engine; closed with warm |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Throttle butterfly does not completely close or open |
Readjust throttle stops |
| Throttle valve switch defective |
Check with ohmmeter and adjust |
| Idle speed incorrectly adjusted |
Adjust idle speed with bypass screw |
| Defective injection valve |
Check valves individually for spray |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
| Loose connection in wiring harness or syst.ground |
Check and clean all connections |
| Control unit defective |
Use known good unit to confirm defect |
SYMPTOM: IDLE SPEED INCORRECT
| Cause |
Remedy |
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Thermo-time switch defective |
Test for resistance readings vs. temperature |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Throttle butterfly does not completely close or open |
Readjust throttle stops |
| Idle speed incorrectly adjusted |
Adjust idle speed with bypass screw |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
SYMPTOM: CO VALUE INCORRECT
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Idle speed incorrectly adjusted |
Adjust idle speed with bypass screw |
| Defective injection valve |
Check valves individually for spray |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
SYMPTOM: ERRATIC RUNNING
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Defective injection valve |
Check valves individually for spray |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
| Loose connection in wiring harness or syst.ground |
Check and clean all connections |
SYMPTOM: ENGINE MISSED WHEN DRIVING
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Defective injection valve |
Check valves individually for spray |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
| Loose connection in wiring harness or syst.ground |
Check and clean all connections |
| Control unit defective |
Use known good unit to confirm defect |
SYMPTOM: FUEL CONSUMPTION TOO HIGH
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Blockage in fuel system |
Check fuel tank, filter and lines for free flow |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
SYMPTOM: NO MAXIMUM POWER
| Defect in ignition system |
Check battery, distributor, plugs, coil and timing |
| Mechanical defect in engine |
Check compression, valve adj. and oil pressure |
| Leaks in air intake system (false air) |
Check all hoses and connections; eliminate leaks |
| Cold start valve not operating |
Test for spray,check wiring and thermo-time switch |
| Air flow meter defective |
Check pump contacts; test flap for free movement |
| Throttle butterfly does not completely close or open |
Readjust throttle stops |
| Throttle valve switch defective |
Check with ohmmeter and adjust |
| Defective injection valve |
Check valves individually for spray |
| CO concentration incorrectly set |
Readjust CO with screw on air flow meter |
| Control unit defective |
Use known good unit to confirm defect |
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