This is a problem with the DOHC motors and NOT the SOHC 3.5, right?

The reason that disconnecting the vacuum diaphragm does not solve the vacuum leak is that the butterfly valve shaft extends through the forward outer wall of the plenum. It connects to the diaphragm actuator here. The bushing where the shaft comes through the plenum wall is one of the ones that gets sloppy, increasing clearance between bushing and shaft. This is essentially a hole that allows unfiltered air into the plenum. Since this air is also not metered, it can cause problems for the ECM and fuel mixture. Its been awhile since I've had a faulty plenum in hand, but it seems like there was more than one bushing for the valve shaft that was exposed to external air, but for sure the front one.

It is the front bushing, right by the vacuum diaphragm, that can be heard leaking at idle if the bushings are really sloppy. Also, if one or more of the bushings have worked their way out of the aluminum plenum and slid down the shaft to one of the valve blades, you may be able to hear it rattling on the shaft. Disconnecting this thing is NOT a fix, like I said above. Replace the lower plenum or your very expensive four cam motor will be a not so good boat anchor (not heavy enough)!

I honestly don't know if its a problem on the SOHC 3.5L. Was 96 the first year for the updated motor?

Well you guys have done a good job of scaring the crap out me on this one and looking up past posts doesnt make it any better <img src="/forums/images/graemlins/shocked.gif" alt="" />.

MontyMCV sent me one of those manuals on cd but it doesnt work so I am out of any referencing tools for this as well until I can find a service manual so...

Can anyone post pics of where to start assessing this potential nightmare and/or some scans from the manual???

Also, to fix/replace this valve (0r do you have to replace the whole plenum?) what are we talking in parts $$$(if I can't get Mitsu to deal with it) and how much time if I do it myself???



P.S. Augie if you happen upon this one I'd love to hear if you had a look at it when you rebuilt the top end.

Is the updated piece easy to identify? Maybe the previous owner of my truck already did this job. How would i know? I got it with 86,000 miles and now it has 140k+.

Externally, the only thing that would distinguish the new piece from the old would be if the lower plenum looks shinier or newer than the upper plenum. Otherwise, they are identical. The rate of corrosion between the two would be a givaway. It the look equally corroded, then I would bet that it was not done.

I have just replaced the lower plenum on my 95 SR (92,000 miles) and have the following to contribute for the sake of completeness. All comments are my humble opinion, so please don't take offence <img src="/forums/images/graemlins/kewl.gif" alt="" />

1. My first comment would be that if you suspect this is a problem you really need to replace this part not just unplug the vacuum servo. The plenum I have just removed shows very considerable wear in all of the bushings (7 total: 1 plastic, 6 metal) supporting the shaft holding the butterfly valves. This wear will mean that this shaft will rattle, or vibrate, even if the vacuum servo is disconnected. With the servo disconnected, you may not hear this rattling at low revs but at high revs it may become a problem.

2. The replacement part has a different part number to the one I have just removed:

Old Part Number - MD313333 (VACV48-001, 5109)

New Part Number - MD335940 (VACV48-002, 9512)

The numbers in brackets were also printed near the part number. If you are lucky, it may be possible to see these numbers through the gaps in the upper plenum casting as they are printed on inside face of the lower plenum. The fact that these part numbers (& VACV codes??) are different may suggest that Mitsubishi has introduced an updated part, but I could not see any obvious differences between the 2 parts.

3. The warning signs for this problem are:

a. Rattling from front of engine. Check using the screwdriver stethoscope method at the front uppermost part of the plenum. If suspected, look for the lever where the vacuum servo attaches to the butterfly valve shaft and press a screwdriver against this lever. The rattling should stop.

b. Hissing noise from same area as above. Only the front plastic bushing is "unsealed" so when this wears unmetered/unfiltered air will be sucked in. The rearmost bushing is sealed with a metal cap pressed into the plenum casting.

c. Possible engine vibration at around 3000rpm. Diminishes at about 3200rpm.

4. I don't think this part is repairable unless you have access to a machinist who can do an in-line rebore of lower plenum casting and insert all new bushings. I doesn't seem an overly difficult job for a competent machinist but may be more expensive than the replacement part.

5. Replacing this part is a big, expensive task for the DIYer. Additionally, you need to be quite experienced and confident of your mechanical capabilities. This job has just taken me 10 hours of seriously hard work, and cost $550. I reckon I could have done it in about 6 hours, but I replaced the spark plug leads, tested the coil packs & did a compression test. I also spent alot of time making notes and taking photos to help me with reassembly and for an article I am going to submit to this site.

6. I am not sure about the 3.5 SOHC but this seems to be a persistent problem with the 3.5 DOHC (6G74) engine. I think that this engine (6G74) is used in several other Mitsubishi vehicles, so it would be interesting if these have the same problem.

If anyone is now thinking of doing this job and need any help, wait for my article to be published and then if you need anything else let me know.

Cheers <img src="/forums/images/graemlins/smile.gif" alt="" />

Just got back from the stealer. The part listed at $365 but I talked him down to $300. If anyone else wants one of these I may be able to get it for you at that price. If you're serious about it I would need the cash before next Wed (1-14-04)

Rafbloke,

Looking forward to seeing your writeup especially since I have yet to find a service manual. In the meantime, any shots of the new part vs wear on the old part and where this thing fits in the engine would be appreciated.

Thanks
Phil

I would keep working on your dealer. As I recall, I was able to get my dealer down to around $265. He said I was basically getting it at cost as he had personally seen several vehicles come thru their service dept. with the same problem. Ask your parts guy to see what his cost is!!

I ended up getting it for US$292.62 (with a military discount) - US$300 is pretty good.

I am still trying to finish the writeup, but it's taking longer than I expected. I have finished the text, which runs to about 10 pages, and I just need to do the pictures (I have got about 14 to format and annotate). I can post just the text straightaway if folks need it, but it will be better with the pics. I am happy to send CD's with it all on, including the original pics which are about 1.3Mb each. I actually took 28 pics, which include several pics of the new lower plenum. I have still got the old one if folks wants pictures of that as well.

Bottom line here is - if you want it and I've got it, I'll get it to you. <img src="/forums/images/graemlins/kewl.gif" alt="" /> Phil, if you want some quick & dirty JPEG's, let me have your e-mail address and I will beam them through. If anyone else wants them, PM me with your e-mail and I will send them all out together.

Cheers <img src="/forums/images/graemlins/smile.gif" alt="" />