Flow Control on intra-vm connections?

All

We are using a Swiftlet that creates messages based on database content and places it on a queue.
This Swiftlet shall be delayed if the queue reaches the flow control start size and therefore we are using

The particular code is:

tx = sender.createTransaction();
message.setJMSDestination(new QueueImpl(queueName));
if (this.ttl > 0)
    message.setJMSExpiration(System.currentTimeMillis() + this.ttl);
tx.putMessage(message);
tx.commit();
Thread.sleep(tx.getFlowControlDelay());


So far so good, but the Swiftlet never throttles down. We can see it, because the queue usage information for FC Delay is always 0, even if the queue depth is multiples times of the FC Start value

Any hints?

Dirk