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Troubleshooting / Excessive Smoke

Introduction:

A small amount of smoke from the exhaust pipe of the car is normal. Excessive smoke is created by a malfunction. The smoke is cause by excessive fuel or engine oil. If this condition is allowed to continue it can affect the exhaust system. Failures can range from catalytic converter becoming plugged to engine overheating and poor fuel economy. Below we have listed the most common causes for this condition.

Troubleshooting Procedure

A. Testing for a Blown Fuel Pressure Regulator Diaphragm. A fuel pressure pressure by using a diaphragm/spring combination in conjunction with intake manifold vacuum. When the fuel pressure regulator diaphragm ruptures it allows raw fuel into the vacuum feed line and then into the intake system of the engine. This condition creates a rich black smoke from the tail pipe on start up and while running. To check for this condition remove the vacuum line from the regulator, if fuel is present in the line the regulator has failed. Some GM (CPI Injection) are designed with an internal regulator that you must remove the upper intake manifold to inspect. Most fuel pressure regulators are on the fuel rail on top of the engine in plain sight. If you cannot locate your fuel pressure regulator consult a repair manual.

Fuel Pressure Regulator
Removing the Vacuum Feed Line



B. Testing Fuel Pressure Regulator. The fuel pressure regulator adjusts fuel system pressure depending on the load of the engine. Vacuum is applied to the fuel pressure regulator to control system pressure. When the fuel pressure regulator is not functioning properly it causes the fuel pressure to be above normal operating pressure. This condition causes excess fuel to be injected into the engine causing a rich mixture that produces black smoke. To test for this condition (engine off) remove the protective cap and attach the fuel pressure gauge.

Attach Fuel Pressure Gauge
Attach Fuel Pressure Gauge



Support the fuel pressure gauge for observation, and then turn the key to the on position. The fuel pressure gauge should jump up to your system pressure. If you do not know your system fuel pressure consult a repair manual. Actual fuel pressure will vary slightly but should be fairly close to specification.

Fuel Pressure Gauge
Fuel Pressure Gauge

Then, start the engine, the fuel system pressure should stay the same. While watching the gauge, snap the throttle, the fuel system pressure should jump up about 5 psi. If the system fuel pressure does not respond the fuel pressure regulator has failed. If the fuel pressure gauge falls the fuel filter is plugged or the fuel pump is failing. When testing is complete, remove the fuel pressure gauge feed line and replace the protective dust cap.


C. Check for Fuel Injector Malfunction.  When a fuel injector malfunctions it can allow excess fuel into the engine. The fuel injector is manufactured with a control valve that allows fuel to flow at prescribed amounts. If this valve sticks open it will flood the cylinder with fuel. This excess fuel causes black smoke from the exhaust system. To test for this condition the injector fuel rail must be removed and injectors tested. It is difficult to fully test an injector, but an basic operation test would include pressurized air and a 12 volt power source. While an engine is running you should be able to hear the injectors clicking as the electronic valve opens and closes. To aid in this inspection use a stethoscope and touch it against each injector. If no audible sound is heard test fuel injector pulse and supply voltage output (this test is used for most cars).  This test will tell you if the computer system has operating voltage and injector trigger signal. Remove an electrical connector from a fuel injector (all injectors need to be tested) probe both sides of the connector with a grounded test light or multimeter (voltmeter) (black lead) switched to DC voltage (there are only two terminals). Have a helper turn the key to the "on" position without cranking the engine and observe the test light. The test light should illuminate one side of the connector only and the multimeter should be at about 12 volts. Next, switch the test light or multimeter (voltmeter) lead (black) or to the positive side of the battery to test the system ground injector trigger, probe the side of the connector that did not light up, have a helper crank the engine over and observe the test light, it should blink on and off or the multimeter (voltmeter) should bounce from 0 to 12  volts. (Note: if no injector pulse is present try disconnecting the remainder of injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse. If injector pulse returns plug the injectors electrical connectors in one at a time until the pulse fails and then replace that injector). If this test revealed that there was no pulse but system has power, the ECM is not generating a fuel injector trigger. If there is no trigger to the fuel injector it will not allow fuel to enter into the engine. Some of the most common reasons that can cause this condition include a shorted fuel injector, injector wiring damage or shorted ECM/PCM. While the injector trigger wire is off, test the fuel injector windings by setting the multimeter (voltmeter) to ohms and attach the leads to either side of the injector. Your reading should be between 11 and 18 ohms. Test all of your injectors; if one injector reading is considerably different from the others replace that injector. Check the fuel pressure regulator, remove the vacuum line feeding the fuel pressure regulator, check for the presence of fuel, if fuel is present the diaphragm inside the regulator has ruptured and is feeding raw fuel into the engine causing it to run rough.

Fuel Injector
Fuel Injector


D. Check Air Intake Boot.  An air intake boot is designed to transfer air from the mass air flow meter to the throttle bore. The ECM monitors this air flow and corrects fuel input. When an air intake boot fails it causes the ECM to respond by inputting more fuel than is necessary. This condition causes black smoke from the exhaust system. To check for this problem remove air intake boot to inspect for cracks or tears and replace if needed.

For exact specific repair procedures for any of the steps above visit our online repair manual page.

Troubleshooting / Excessive Smoke

Additional "How to" Car Repair or Services

1. How to Replace Air Filters?
2. How to Change a Cabin Filter?
3. How to Change Oil and Filter?
4. How to Tune Up with Throttle Bore Service?
5. How to Replace a Radiator?
6. How to Replace Fuel Filters?
7. How to Read Trouble Codes?
8. How Filters Work?
9. How to Replace Front Brake Pads?
10. How to Replace Rear Brake Pads?

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