Valve cover gasket
Most Previa specific issues result from its unusual “slant midship” layout.
The forces of gravity and age mean that many Previas leak a small amount of oil from the top (effectively the side) of the engine. It’s not a big enough amount of leakage to damage the engine but the driver picks up a noticeable smell of burning oil since the engine leaks onto hot exhaust and it’s all happening directly under him.
In the worst case scenario, it could catch fire, though it hadn’t been a problem as of yet.
Because Nick CZ said to, I went strictly OE for all the replacement parts I needed. The Toyota gasket was actually pretty inexpensive on eBay, and given the complexity of the job, I don’t know why people cheap out on it.
Nick provided a tube of Genuine Toyota FIPG sealant as well, which as with the gasket, works much better than aftermarket alternatives. The forums indicate that it’s possible to do this job without the sealant, but not in a lasting way.
Since I was going into the engine anyways, I took Chris’s advice and splurged on some other parts I could use to really make the van purr. OE spark plugs, OE spark plug wires, OE distributor cap, OE distributor O-ring. I couldn’t find the OE ignition rotor anywhere, so I went with the Bosch. When it showed up, it was made in Japan, presumably by Denso, so it may well have been OE, more or less. I also went with a cheap air filter because it’s just an air filter.
People sometimes complain that the Previa is hard to work on because of the engine placement, but with the large access panel and ample ground clearance, it wasn’t actually a problem in practice. Name another car where you can reach the head from underneath.
To prep, I took out the passenger seat, the flat fixing kit, and the carpet on the right side. Then I started preemptively PB-Blasting the bolts holding on the engine cover. Only one of 10 sheared off, that’s improvement!
I sprayed the bolts on the valve cover as well and waited for Chris to arrive.
Chris has a lot of experience working on Toyotas, and I am really happy he was here to help out with the especially complicated stuff.
I managed to get the valve cover bolts off without breaking any, which is just great. The gasket and distributor O-ring broke apart into crumbs, having ceased to serve their purpose years ago.
I got to work cleaning all the crusty old oil-dirt-sludge around the engine with a rag, a big flathead, and a bunch of Clean Streak. After it was all neat and tidy, we started putting it all back together. Chris put the new distributor cap on (easiest from under the car) and I got sidetracked and put a new air filter and plugs in my daily driver, since it was a mechanical day and all the tools were already there.
When it all went back together, the engine was noticeably smoother and quicker to start and though I haven’t checked back yet, hopefully it’s now leak free as well.