diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index 4299802..c682277 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -226,10 +226,6 @@
     def refresh(self, request):
         self._update_token(request)
 
-    @property
-    def expired(self):
-        return _helpers.utcnow() >= self.expiry
-
     def _update_token(self, request):
         """Updates credentials with a new access_token representing
         the impersonated account.
@@ -239,8 +235,9 @@
                 to use for refreshing credentials.
         """
 
-        # Refresh our source credentials.
-        self._source_credentials.refresh(request)
+        # Refresh our source credentials if it is not valid.
+        if not self._source_credentials.valid:
+            self._source_credentials.refresh(request)
 
         body = {
             "delegates": self._delegates,
@@ -347,7 +344,9 @@
 
         headers = {"Content-Type": "application/json"}
 
-        authed_session = AuthorizedSession(self._target_credentials._source_credentials)
+        authed_session = AuthorizedSession(
+            self._target_credentials._source_credentials, auth_request=request
+        )
 
         response = authed_session.post(
             url=iam_sign_endpoint,
diff --git a/tests/test_impersonated_credentials.py b/tests/test_impersonated_credentials.py
index 19e2f34..e0b5b11 100644
--- a/tests/test_impersonated_credentials.py
+++ b/tests/test_impersonated_credentials.py
@@ -172,6 +172,44 @@
         assert credentials.valid
         assert not credentials.expired
 
+    @pytest.mark.parametrize("time_skew", [100, -100])
+    def test_refresh_source_credentials(self, time_skew):
+        credentials = self.make_credentials(lifetime=None)
+
+        # Source credentials is refreshed only if it is expired within
+        # _helpers.CLOCK_SKEW from now. We add a time_skew to the expiry, so
+        # source credentials is refreshed only if time_skew <= 0.
+        credentials._source_credentials.expiry = (
+            _helpers.utcnow()
+            + _helpers.CLOCK_SKEW
+            + datetime.timedelta(seconds=time_skew)
+        )
+        credentials._source_credentials.token = "Token"
+
+        with mock.patch(
+            "google.oauth2.service_account.Credentials.refresh", autospec=True
+        ) as source_cred_refresh:
+            expire_time = (
+                _helpers.utcnow().replace(microsecond=0)
+                + datetime.timedelta(seconds=500)
+            ).isoformat("T") + "Z"
+            response_body = {"accessToken": "token", "expireTime": expire_time}
+            request = self.make_request(
+                data=json.dumps(response_body), status=http_client.OK
+            )
+
+            credentials.refresh(request)
+
+            assert credentials.valid
+            assert not credentials.expired
+
+            # Source credentials is refreshed only if it is expired within
+            # _helpers.CLOCK_SKEW
+            if time_skew > 0:
+                source_cred_refresh.assert_not_called()
+            else:
+                source_cred_refresh.assert_called_once()
+
     def test_refresh_failure_malformed_expire_time(self, mock_donor_credentials):
         credentials = self.make_credentials(lifetime=None)
         token = "token"