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Re: EGR Valve
Guess I was confused by the term "manually".....
The Three EGR's GM used were the vac type, the 3 solenoid type, and then the linear type....
The vacuum type, you could manually check just by reaching under the top and moving the diaphram to open the EGR and see if the car stumbled, or by applying vac to the port.....
The second had three solenoids with different sized orifice ports, which you could electrically activate each circuit to check out.....
The last though, is linear.....that solenoid is "pulse-width" modulated, to vary the opening, along with a pintle position sensor.....only a scan tool can read what the PCM is seeing as the actual position of the pintle vs. what the commanded position is.....The test you are doing is full open, or 100 %.....and as you say, with the EGR wide open, this will cause the engine to stumble and vac pressure to drop......
That only tells you if you have MAJOR blockage or if the EGR is not working at all.....when the PCM runs this test, it does this at a very low percentage, and looks for a corresponding drop in Map pressure.......the flashing in the EGR passageway of the upper plenum could crud up just a enough, so when the test is done, it's enough of a restriction to block low pressure from the exhaust, but not enough to block high pressure from the exhaust(when pintle is commanded wide open)....that's why I say the only way to truly test a linear EGR is with a bidirectional scan tool.....
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