I recently purchased a 95 Montero SR and have been slowly working my way through all the issues to make sure I don't end up stranded off of the grid, and a few weeks ago I discovered it still has the original plenum. After removing the plenum and taking it apart I realized that its basically a ticking time bomb, however I couldn't bring myself to buy the updated plenum for about $500. After TONS of research, I discovered that the early 2000s Kia Sorrentos had a similar issue that they issued a recall for, and it also happend to be on a 3.5L V6. After watching a little bit of this video https://www.youtube.com/watch?v=PJ7kkdiSrq8
, alot of the parts look very similar. Since they had a recall, they offer the middle plenum for much cheaper than normal and the new part number is 29222-39800QQK. I called the local kia dealership and they had one in stock and were asking $70 for it, so I stopped by on my way home to take a look at it. The sorrento plenum looked pretty much identical to the mitsu with the exception of the bolt pattern for the upper plenum so I bought it for $70. I took it apart when I got home to make verify that it will work, and sure enough the shaft, bushings, butterfly valve flaps, etc are all the correct size. The cam on the end of the shaft that connects to the vacuum actuator is even the same, so all you need to do is remove the butterfly flaps and shafts from both plenums, and the shaft with bushings and the cam from the kia can be installed directly into the mitsu plenum without any modifications. Since the butterfly flaps are the same size, you can install the shiny new flaps from the kia as well. The process is pretty straightforward; all you have to do is remove the flaps, then remove the two screws on the end that hold the little metal gasket/spacer in place that holds the shaft in the plenum and the shaft will slide out. I did learn a few things in the process, so here are a few pointers:
-Use channel locks to break the screws free that attach the butterfly flaps to the shaft, then use a phillips head to remove them the rest of the way. I learned the hard way with the mitsu plenum that those little screws are made of very soft metal and stripped just about every one of them during removal. The screws on the kia plenum seem to be stronger/harder, but they had some blue loctite on them so I used this process to avoid stripping them and it worked.
-Be careful when sliding the shaft out of the plenum, the small rubber gaskets that are on the shaft between the chambers have some sort of adhesive holding them in place on the shaft. I'm not sure how strong the adhesive is, but I'm sure if someone tried to force the shaft out they could rip them off.
I haven't completed the reassembly as I am going to clean up the old plenum first, however I did install the shaft to make sure everything fit correctly and it fit perfectly. The main differences between the old and new appear to be the material the two thin plastic sleeves around a couple of the the rubber bushings are made out of, and the bushing on the end of the shaft. The bushing on the new one is one piece, it looks and feels much stronger than the old one and it is fixed in place with a pin rather than having a small spring/clip holding it in place. I know many of you have probably already fixed this issue, but I seeing as how the Kia plenum can be had online for about $60 I thought I would share.