There are other things to check first.
Cam timing...older vehicles needs checking.
Fuel pressure control unit....again, pressures need checked.
Fuel accumalator...would be checked for operation when checking all fuel pressures....rest/run/pump stall/volume...etc.
Injector/s....dribble and spray pattern.
Air flow ratio....with c/o setting screw, after all tests indicate everything working within factory spec's.
One more thing...Run the timing with out any vacuum applied set to 10 to 12 degrees BTDC at 700 rpm.
Watch for smooth advance from the mechanical advance weights and also smooth decay back to idle speed....weights wear and get irratic...they have a tendence to hang up, so settings get thrown off. Weight springs get weak and do not pull advance back to basic after engine speed returns to idle.
This help ?
PS...If memory serves...that vac' switch turns on either egr, charcoal canister or vac' advance and switches off at 50 degrees C coolant temperature. It is a vacume switch valve. If it's plumbed into the distributor advance via a three way connector then it's for emmision control.(Purge canister.).. the vac' lines for the other switch go to the control pressure regulator and the intake manifold prior to the throttle plate, it's also a 50 degrees C switch. This vac' signal causes a little extra fuel when throttle opened .
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