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An R107 that smells of fuel, blows black smoke, fouls plugs or gets poor fuel economy is running rich. On K and KE-Jetronic cars, the causes are usually a few specific parts.
Signs of running rich
- Black smoke from the exhaust, especially on acceleration
- Strong fuel smell, particularly at idle
- Sooty, black spark plugs
- Poor fuel economy
- Rough idle when warm
- Failed emissions test with high CO or HC, where testing applies
Warm-up regulator
On K-Jetronic cars, a warm-up regulator with low control pressure makes the mixture too rich. Test with a fuel pressure gauge. See fuel pressure test.
Cold start valve leaking
The cold start valve sprays extra fuel during cold starts. If it leaks, it floods the intake and makes the engine run rich, especially at idle after a hot shutdown. Test by removing the valve and checking that it does not drip with the fuel pump running.
Injector leaks
Leaking injectors drip fuel after shutdown, making hot starts difficult and the mixture rich. Poor spray patterns also affect combustion. See fuel injectors.
Mixture adjustment
The mixture screw on the fuel distributor adjusts the basic mixture. If it has been set too rich, often by a previous owner, the car runs rich. Adjust with a CO meter or duty-cycle meter. See lambda duty cycle.
Oxygen sensor
On cars with lambda control, a failed oxygen sensor may not correct the mixture. Check the sensor. See O2 sensor.
Other causes
- A faulty coolant temperature sensor on KE cars, telling the control unit the engine is cold.
- The EHA on KE cars. See EHA valve.
- A sticking airflow sensor plate. See airflow sensor plate.
Quick answers
- Is a fuel smell always running rich? Not always; check for fuel leaks. See gas smell.
- Can running rich damage the engine? It can wash oil from the cylinders and foul plugs; fix it.
- Will a new O2 sensor fix it? Only if the sensor was the problem.
- Should I adjust the mixture myself? With the right meter and knowledge, yes.
- Can a leaking fuel pressure regulator cause richness? On KE cars, pressure problems can affect mixture; test pressures.
- How do I check the cold start valve? Remove it with the fuel lines attached, place it in a container and check for drips with the pump running.
- Will running rich damage the catalytic converter? Yes, on cars with one; fix it promptly.
