Has anyone done valve adjustment along with their 105k service and actually noticed any improvement in engine idle/noise? A service rep at the dealership told me they no longer do valve adjustments as it doesn't really yield any improvements on the modern engines in the 4Gs.

 

I think you should find a new dealership...
...its about keeping valve clearance in spec and properly seal combustion chamber during intake, compression,ignition and exhaust cycles....
my 0.02

 

I have performed valve adjustments on 3 J-engines during the 105K timing belt change. Valves were still in spec but I adjusted them to the tight side. Did not do a thing for noise or performance. I shall not do it again because it seems to yield nothing. Reinstalling the valve covers are a PITA, especially in the rear. The spark plug tube seals are difficult to get aligned. Also, the upper manifold has to removed; or at least unbolted and moved aside. The engine needs to be rotated so it's best to have the spark plugs removed as well.

 

A valve adjustment will not affect performance unless clearances were totally out of whack and that is unlikely.
Valve clearance tends to increase with time as the parts in the valve train wear. The clattering noise tends to increase and at idle with the hood up you can hear the normal level of noise if you peek into the engine compartment. You don't normally hear much from inside the vehicle. If the noise gets very loud and obvious you should probably adjust the valves. The dealer comment above is pretty much nonsense. But the mistake some folks make is to try to eliminate the noise. In order to make the valves very quiet you have to adjust them too tight. This sounds better for a while but what it does is prevent the valves from properly bottoming out in their seats. This inhibits cooling of the valves through the needed heat transfer to the head. Result is burned valves which is a very bad thing leading to serious issues like no compression, poor power, hard starting, and complete head overhaul or replacement. There is another thread on here with a picture of a burned valve that people attributed to bad gas. It's not bad gas that burns valves. It's improper adjustment. So do not let anyone adjust your valves but a professional trained tech who has done it before.

 

I've adjusted valves before on my previous cars but they were all 4 cylinder dohc. Removal of the valve cover was a walk in the park and the adjustment took less than an hour. It seems that getting to the valves on these j-series will be a PITA so if there is no noticeable benefit I'm just going to skip this procedure altogether.

 

I like to do a fuel trim comparison before and after a valve job. Via the OBDII values for short and long term fuel trim you can tell if you did a good job with the clearances or you messed up. The closer to 0 the trims get, compare to before, the more accurate the job is. If you start getting any crazy > +/- 5 numbers then you better check again.

Ps. I also like to book a lower back massage after... Oh yes! Lol