Exhaust Gas Recirculation (EGR) System Diagnosis. GMC Truck GMC C-Series Medium Duty Truck B7 1999

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Exhaust Gas Recirculation (EGR) System Diagnosis

Engine Controls Components
Cell 20: Engine Controls, Exhaust Gas Recirculation Valve
Handling ESD Sensitive Parts Notice

Circuit Description

The Exhaust Gas Recirculation (EGR) system lowers the Oxides of Nitrogen (NOx) emission levels created by the high combustion temperatures by suppling cooler exhaust gasses to the combustion chamber.

The EGR valve pintle is opened by the PCM when the conditions of operation have been met. The exhaust gas flows from the exhaust manifold through the EGR pipe, intake manifold passages, and the EGR valve orifice into the intake manifold. The exhaust gasses mix with the intake air and enters the combustion chamber.

Diagnostic Aids

Too little EGR flow or no EGR flow allows the combustion temperature to increase. This causes the following symptoms:

•	Spark knock (detonation)

•	Engine overheating

•	Emission test failure

•	Poor fuel economy

Restrictions or blockage in the EGR passages may be caused by the following conditions:

•	Contaminated fuel

•	Poor fuel quality

•	High mileage or excessively worn internal engine components

Too much EGR flow causes any of the following conditions to occur:

•	The engine stalls after a cold start

•	The engine stalls during closed throttle condition

•	A rough idle

Test Description

The numbers below refer to the step numbers on the diagnostic table.

DTC P0404, P0405, P1404 a problem in the EGR valve electrical circuits.

This test will determine if any EGR passages are restricted.

EGR System Diagnosis

Step

Action

Value(s)

Yes

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

Are there any EGR DTC's set?

Go to the applicable DTC

Go to Step 3

Install a scan tool.
Idle the engine.
Command the EGR to the specified value.

Does the engine run rough or stall?

50%

Go to Step 6

Go to Step 4

Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .
Visually and physically inspect the EGR valve passages for a restriction.

Did you find and correct the condition?

System OK

Go to Step 5

Remove the EGR pipe from the exhaust manifold and the intake manifold. Refer to Exhaust Gas Recirculation Pipe Replacement .
Inspect the manifold EGR ports and the EGR pipe for a blockage caused by excessive deposits or other damage.
Correct the condition as necessary.

Did you find and correct the condition?

System OK

Visually and physically inspect the following items for a leak:

•	The EGR gasket

•	The EGR pipe

Did you find a problem?

Go to Step 7

System OK

Repair or replace the components as necessary.

Is the action complete?

System OK