Well I got bit with that bug too. Sorry if I missed something, although most common culprit has been long identified - VSV, most of ways to check its operation are rather tedious. Here is my easy way.
Disconnect VSV connector from main engine harness. Check for continuity between two terminals with yellow wires on opposite side. You must have something like 30-35 oms. No continuity = bad VSV.
Otherwise rig up something to connect those terminals to battery.
Then disconnect a vac hose from EGR valve. If you can blow air through or suck it out of it when terminals are connected to battery = bad VSV. (when disconnected - air should be able to flow).
Ones VSV is removed you might want to double check findings but I really fail to see why Toyota in shop manual recommends to remove intake plenum to just check VSV.
If all checks out, look elsewhere.
On the side note. I've seen a advice to clean passage through intake manifold. Well, clogged up passage could only conceivably cause code P0402 not P0401 since it would induce too much vacuum above membrane of EGR valve thus opening it too much. Role of VSV is to prevent vacuum built up unless ECU decides that now is a good time to close it.
Just my $0.02
Disconnect VSV connector from main engine harness. Check for continuity between two terminals with yellow wires on opposite side. You must have something like 30-35 oms. No continuity = bad VSV.
Otherwise rig up something to connect those terminals to battery.
Then disconnect a vac hose from EGR valve. If you can blow air through or suck it out of it when terminals are connected to battery = bad VSV. (when disconnected - air should be able to flow).
Ones VSV is removed you might want to double check findings but I really fail to see why Toyota in shop manual recommends to remove intake plenum to just check VSV.
If all checks out, look elsewhere.
On the side note. I've seen a advice to clean passage through intake manifold. Well, clogged up passage could only conceivably cause code P0402 not P0401 since it would induce too much vacuum above membrane of EGR valve thus opening it too much. Role of VSV is to prevent vacuum built up unless ECU decides that now is a good time to close it.
Just my $0.02
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