FOUND THE MAIN PROBLEM!
Intake manifold leak!
I missed it on the earlier checks. After eliminating ALL the other alternatives, I started going back with more thorough tests. A manifold pressure reading was way low (idling down to 11 and 10 inches Hg, when should be above 19). To check for any manifold leaks, I wanted to eliminate vehicle noise when looking for a leak. So I removed the throttle body and made a special test fitting (piece of plywood with a 5/16" hose fitting on it), and bolted it in place of the throttle body. With a friend maintaining 10psi on the hose, I went right to the leak. The rubber boot just above the bypass valve has a 1" split in it.
This boot is called the No. 1 Intake Air hose, Toyota part number 90904-45003. It is more of a boot than a hose. The old one and a new one are shown in this picture. The split looks like classic dry rot (cracks) on the outside. The rubber has been softened by oil mist or other intake fumes and is much easier to stretch than the new one. Note the bulge of the old one.
The really embarrassing thing about this is that I had the same split in this boot on my '96 previa six or seven years ago but didn't think of it here. But in my defense, the leak on the '95 was on a different side of the boot and not as easy to detect. In addition, the '96 never showed lean codes like the '95 has.
Moral of story: Go old school. If poor idle, first put a manifold gage on it.
It may be worse in Texas heat, but judging by my two failures on two Previas, I suspect this is a common problem and may have caused more than one unnecessary engine removal or Previa sale.
I am doing this vehicle's second SAD shaft coupling replacement while working in the same area. First replacement only lasted 40k miles. I am using a genuine Toyota part this time instead of the ebay special. The bolts in the Toyota kit are definitely better quality than the ebay kit, and are exactly like the original equipment. And I suspect the rubber is a better mix as well. I will also pay closer attention to the shaft alignment as well. If anyone wants pictures of the new or the failed original equipment couplings (130k miles), as well as the new or the failied ebay couplings (40k miles), I have pictures of all of them and will post on a new thread.
Images (click to enlarge)
Saturday, February 11th, 2012 AT 7:57 AM