Air injection pump mulfunction?
 

My air pump turns on at cold start-up then shuts off after 90 sec or so, just as normal. After a few miles of driving, idling at traffic stop, the pump often comes right back on for several seconds then shuts off again... What could cause this?

I thought the pump is triggered by engine temp/ cold start, so wodering if the temp sensor is gong out, or maybe any other component is suspect? My water temp is reading the way it is supposed to, on dash gauge and through 'hidden' climate control monitor.

I appreciate thoughts from those experienced.

PS. I'm planning t go through the secbdary air injection system to check vacuum leakage, etc.

Are you sure it's the SAI and not the fan?

Yes it is SAI pump. After a few miles of driving my engine is nowhere near hot to trigger the compartment fan...

Also when the air pump turns on for the second time within just a few minutes of cold start, it shuts off either after several seconds or as soon as I blip the throttle.

I believe this is normal. Mine does the same thing. I can attempt to find if need be. I think it is explained in the owners or Bently manual. If I remember correctly SAI comes on after rich start up to kick off the cats then it does a system check at the next idle period.

Correct.

The second time you hear the SAI, its running a pressure test to check the integrity of the system. If that test fails, it will run the test a few more times until eventually a CEL will be illuminated to let you know of a fault in the system.

Helpful Hint: When you do go through the SAI checking for leaks, make sure to check the vacuum reservoir. I chased a couple of codes - IIRC P0410 and P0411 - for over a year before I found it was the source of the problem.

Thanks for tips. I was not expected that the brief second run was as designed but it makes sense.

Actually it was during this short, second pump run when CEL was occasionally triggered (P0410 and P1411 if I remember...) and I was planning to go through SAI system, hopefully to fix a simple vacuum leakage. I was wondering if malfunction of some components are making the pump run erratically, but gladly it seems not to be the case.