Some suggestions from an old thread, FWIW:
Throttle body: ============== Air blown into the hoses from the modulator's Ports P and R into the throttle body should flow freely into the throttle body.
To be tested: =============
Test 1 -- To rule out VSV as the problem. Test 2 -- Whether the EGR modulator is indeed the culprit.
Step 1 for VSV:
1A. Disconnect hoses from Port Q at the modulator and at the EGR valve. This leaves VSV with two long hoses to test. Mark them so you can put them back in the right places.1B. With the engine cold, stopped and ignition ON, Port Q hose leading to the VSV should hold vacuum, use about 15". And blowing through the EGR valve end of the hose you will hear air coming out of the VSV filter, which will help you locate it as well.
1C. With the engine warm and at 2500 rpm, Port Q end of the hose should loose its vacuum automatically as VSV is opened. This means the Port Q hose now opens all the way to the EGR valve end of the hose.This confirms VSV and its control circuit is working; otherwise the VSV or the vacuum hoses leading VSV to are leaking or VSV is not energized. Verify proper electrical connection to VSV. Clean or change the VSV and hoses.
Step 2 for VSV while also checking out modulator:
2A. Now your engine is warm in diagnostics mode from Method 1, tee Port R of the modulator to the MAP hose (getting full engine vacuum now).2B. At 2500 rpm, you should measure about 13" at Port Q of the modulator.
If the numbers are the same at Port Q of the modulator and at the EGR end of the hose, then VSV is working.
But if Port Q of the modulator shows a higher vacuum than at the EGR valve, then the VSV or the vacuum hoses leading to VSV are leaking. Clean or change the VSV and hoses. In this case of higher vacuum at Port Q, a direct hose from Port Q to the EGR valve should cause the engine to run rough in the diagnostics mode at 2500 rpm. If not, you may have problems at both VSV and modulator.