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Troubleshooting / Excessive Smoke

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

Troubleshooting Procedure

A. Testing for a Blown Fuel Pressure Regulator Diaphragm. A fuel pressure regulator is used in the fuel injection system. This component regulates fuel system pressure by using a diaphragm and spring combination. 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 vehicles are designed with an internal intake regulator you must remove the upper intake manifold to be inspected. Most fuel pressure regulators are on the fuel rail on top of the engine in plain site. If you can not 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 that is attached 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, 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 very slightly but should pretty 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 will allow excessive 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. Confirm that all injectors are still connected to the fail rail. Without cranking the engine over turn the ignition to the "on" position. Without the engine running the fuel injectors must not have any fuel exiting the injector nozzle. If fuel is present at any of the fuel injectors they have failed and need to be replaced.

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 that 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.

Troubleshooting / Excessive Smoke

Additional "How to" Car Repair or Services

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