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="" />
