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How to : Solve EGR Problems

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Old 04-11-2010, 03:43 AM
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Default How to : Solve EGR Problems

EGR PROBLEMS

With earlier-style vacuum-operated EGR valves, failure of the vacuum diaphragm or carbon buildup on the pintle valve are the main causes of trouble. A leaky diaphragm will prevent the EGR valve from opening, causing not only an increase in NOX emissions but often detonation (spark knock) when the engine is under load. Carbon can also build up inside
the manifold passageway and block the flow of exhaust. Carbon deposits that prevent the valve pintle from closing will produce driveability
symptoms like a vacuum leak (hard starting, rough idle, lean misfire, hesitation and possibly stalling after cold starting).

With digital EGR valves, the driveability symptoms are the same: elevated NOX emissions and detonation with loss of EGR, or idle problems if the valve fails to close.

HOW TO FIND OUT WHATS WRONG

On earlier vehicles, visually inspecting the EGR valve to see that it moves when the engine is revved is one way to check its operation. Another is to apply vacuum directly to the EGR valve and listen for a drop in idle speed when the valve opens (but this only works if the valve is not a back-pressure-sensing type).

Back-pressure-sensing EGR valves only open when back-pressure exceeds a certain level, so be sure to raise the engine speed to about 2000 rpm before applying vacuum.

On vehicles that have a vacuum-type EGR valve with a solenoid in the vacuum supply line, vacuum should pass to the EGR valve when the
engine is warm and above idle.

No vacuum? Check for voltage at the solenoid. No voltage would indicate a wiring problem.

The computer should energize (ground) the solenoid when EGR conditions exist (engine warm, running above idle). If EGR is not occurring, the problem may be a bad solenoid, wiring, computer or sensor problem. The solenoid itself can be checked by jumping it to battery voltage. No click will tell you the solenoid needs to be replaced. The next most likely causes would be a loose wiring connector, faulty coolant sensor, or failed driver circuit in the computer.

With digital EGR valves, a “Check Engine” light accompanied by any of the following fault codes may indicate a problem in the EGR system:

On OBD I Vehicles

Ford: 31, 32, 33, 34, 83, 84
Chrysler: 31
General Motors: 75, 76 and 77

All OBD II vehicles (1996 & up): Diagnostic Trouble Code (DTC) P0401 insufficient flow, P1406 EGR sensor

On some vehicle applications, a scan tool can also be used to access and display EGR valve position and status.

BENCH TESTS FOR A DIGITAL EGR VALVE

To check a three-solenoid GM digital EGR valve, measure the resistance of each solenoid:

Terminal A to B: 20 ohms
Terminal A to C: 10 to 17 ohms
Terminal A to D: 20 to 30 ohms

If any resistance reading is outside the range of specifications, the unit is defective and needs to be replaced.

Using a 9-volt battery or sensor tester, run battery positive (+) current to the “A” terminal, then ground each of the other terminals (B, C & D) one at a time .

Each solenoid should click and open its respective pintle valve. No click or movement would indicate a faulty solenoid.

With the battery and jumpers disconnected, spray a small amount of carburetor cleaner into each orifice, being careful not to get the cleaner in the center hole. If the cleaner seeps out of any orifice hole into the center, the pintle is not seating properly and the EGR valve needs to be replaced.

If a digital EGR valve passes the above tests but there are fault codes present, the problem may exist in one or more of the following:

the powertrain control module (PCM)
manifold air pressure (MAP) sensor,
the oxygen (O2) sensor,
throttle position sensor (TPS),
a plugged intake manifold passageway,
a clogged catalytic converter,
a blown fuse or a wiring problem.

TESTING A DIGITAL EGR VALVE ON THE VEHICLE

The same resistance checks can be made between the terminals (A to B, A to C and A to D). If resistance is within specs, check the operation
of the EGR valve by back-probing and grounding each solenoid terminal (B, C & D). This must be done with the engine at normal operating temperature, idling in closed loop. When each solenoid is grounded (energized), there should be a momentary drop in idle speed if the EGR valve is opening and the intake passageways are unobstructed. The computer will immediately compensate for the drop in idle speed by opening up the idle air-control valve to increase rpm, so you may have to use a tachometer to detect the rpm drop. No change in idle speed would indicate a faulty EGR valve or blocked passageways.

Note: This test can also be done with a bi-directional scan tool by selecting EGR output test and then energizing each solenoid to check for the rpm drop. If the EGR valve passes these tests but is not functioning, the problem is in the wiring, PCM or one of its sensor circuits (coolant,
MAP, TPS or O2). These items should all be checked to isolate the fault.
 
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