EGR operation questions

[SchoolBoy]

Ok so I understand that a egr system that is in good working will allow a percentage of exhaust gasses under load to reduce nox and chamber temps. Now my question is with our cars does the egr open under vacuum or boost? I assume it is only open under vacuum other wise you would have a massive boost leak straight into the exhaust correct?

[MikeFleming]

It uses a manifold vacuum source, with a check valve to keep the valve from ever seeing positive pressure, and an in-series solenoid to control the valve opening and timing.

So, you are correct. It should never see positive pressure in normal operation.

[SchoolBoy]

How would you test the egr to see if its operating correctly under load and boost, hook a vacuum gauge in line to it?

[MikeFleming]

Start with a KOER (engine running) self-test. Then stick your little finger into one of the diaphragm openings on the back side and blip the throttle. You should feel it open briefly. Be careful to not get burned (it can be hot). You cna also manually list (open) the valve this way, You should see a 1=8-15% drop in engine RPM. Very difficult to detect at idle as the EEC will also dry to keep the RPM constant. Remember the goal is to eat only the apple.

Lastly check/verify that the one-way check valve is still working properly.