One of the common issues with the L28E engines that could contribute to idling issues is the BCDD (boost controlled deceleration device) mounted to the bottom of the throttle body. This leak is a great reason to not only clean and rebuild the throttle body, but also check out the BCDD as well.
Removal was quite simple since I already had the air flow meter removed. Four bolts and the assembly came right off. In this picture I had already removed the dashpot and bracket.
Removal of the throttle valve itself involved removing the throttle return spring and linkage from the side of the body as well as the throttle position sensor.
Then the two screws holding the throttle valve plate to the shaft could be removed and the shaft slid out the side.
At this point I used throttle body cleaner to thoroughly clean the body, valve and shaft. Upon inspection of the shaft, I found no apparent wear and the shaft fit fairly snugly into the body. Not wanting to reassemble without addressing the leak around the shaft, I addressed the leak from two directions.
First, I used silicone grease to lubricate the shaft bushings. This likely would have addressed the leak for some time, but I wanted to take it a step further. I then decided that a perfectly spaced o-ring that barely touched the throttle body when it was closed would prevent a leak at idle (where the leak would make the biggest difference). To do this I mounted an o-ring on the throttle valve shaft and used a thin piece of aluminum (from the bottom of a Coke can) to space the o-ring.
The spacing is critical because no lateral pressure can be applied to the shaft otherwise the throttle valve will bind; it was meant to have some free play from side-to-side and the closing of the valve would center the shaft. In my case the o-ring ever-so-gently came to rest against the body when the valve was closed. Some silicone grease on the o-ring will hopefully prevent it from shrinking for some time.
I then moved-on to the BCDD. I once attempted to adjust the BCDD to see what impact it would have on my idle issues, but the set screw against the adjuster was frozen. With the throttle body off the car I should be able to loosen this set screw easily. Well...
So much for that idea; the set screw sheared. The good thing is that with the unit removed, I can drill out the screw easily. the other good thing is that the BCDD has two set screw locations, so I don't need to tap the hole that I drill and thus I won't have to disassemble the unit.
I reassembled the throttle body and reattached the throttle position switch. I adjusted the switch such that the idle position was triggered at the very end of the travel. I also verified that the "full throttle" position was triggered at about 30 degrees rotation per the FSM.
The throttle was then returned to it's home on the intake manifold using new stainless bolt and a fresh gasket.
Since my original throttle duct had a crack in it (that I fixed with Shoe Goo) I purchased a new unit from Z Car Depot. It turns out that Nissan still makes these! Based on the casting flaws matching on my original and new duct, the same mold is still used to make these.
With everything reassembled I ran another smoke test. The o-ring seal and greased shaft was holding pressure, along with the other fixes documented in previous posts also being effective.
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