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If you have the engine out of the car, it's a good idea to mike all the major wear components and check to make sure they're still in spec.
Really the biggest determiner of engine longevity is maintenance. If you've been up on yours, chances are you'll find very little wear and things should be fine. The 6G72 is a decent engine. Mitsubishi made some engines that weren't really spectacular (i.e. the 2.6 found in the early K's), but I wouldn't say they're bad engines. I prefer the 2.5 because... well it's an American pride thing, mostly, but that is a truly bulletproof design that belongs up there with the slant six for reliability. That being said, I would still buy a 3.0 car. Really the biggest problem with the 3.0 was the valve seals. So does it equal the 2.5 in reliability and ease of maintenance? In my book, no. Is it a ticking time bomb like the early Ford SHO V8's (very bad issues with the camshafts)? Far from it. I would say it's an above average engine.
 

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Remember that wear parts are wear parts, even if they're not a frequent replacement item. After about 100k (miles, which is right about what you're coming up to, I figure), things like shocks, struts, brake rotors, wheel bearings, etc are reaching the end of their service life. Say you put $400 in the car for a full suspension. A suspension is a job you only have to do every 10 years or so, so you're talking $40 a year, which is FAR less than the monthly cable bill at my house. Really, the only thing in my book that condemns a car is severe structural rust, with strong second going to engine and transmission failure. Even in case of powertrain failure, I would carefully consider the options. 300 klicks is about 150k miles, which is still relatively young for a car, even under severe service. You'll need to replace suspension parts more often and need to change the oil more often, but that's something that will be true for ANY car. All buying a new car does is "reset" the date when the job has to be done. A new shock is a new shock, regardless if it's under a 1990 or a 2013. If your driving is killing the shocks in 75k, say, it's going to kill any shock in 75k.
 

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A well-maintained engine should not need mains or cam bearings at 125k. Replacing the mains also entails removing and miking the crank, and while you have the engine open and the crank out, you'd be insane not to Plastigauge the big ends and mains, and since you're going THAT far you might as well pull the pistons and check the bores and rings, and deglaze the bores... you can see how this turned into an engine rebuild that it doesn't really seem your engine needs, and a rebuild is something you really need to do with the engine out and in a relatively clean place.
It seems to me that really the worst problems with the 6G72 were valve seals. I wonder sometimes if the 24V DOHC twin-turbo version of this engine had similar problems, or if it was just limited to Chrysler engines. Unfortunately, valve guide seals require removing both cylinder heads which will probably be very difficult in the vehicle (I have never worked on a 3.0, so I don't know for sure, but I have seen the engine compartments of a couple 3.0 AA's, and it does not look like a fun place to have to work). I do know that the head on my 2.5 is heavy enough to basically require two people to get it on the engine properly. Valve guide seals are more of an annoyance than anything else; they really only cause oil consumption.
I'm curious about your choice of 5W-50... why so heavy?
Having heard the horror stories about the use of non-ATF+4 in the A604, I wouldn't recommend straying from that spec. It's superior to D/MIII and is a fully synthetic fluid by definition.
I don't think your engine needs most of the stuff you're talking about. It's not really that old; you're talking about seals that usually last well over 200k. If you've done a head gasket in the car, you can do the valve guide seals without pulling the engine, but I'm looking at my FSM and I'm not so sure that I would want to replace some of these seals, mainly the crank seals, in the car. It's probably doable, but it looks like a headache. In fact, the manual seems to assume that the engine is out.
 

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The conception that a transmission works better when it's full of gunk, and that changing the transmission fluid will make things worse always baffles me. I've seen it several times on here. Automatic transmissions are the products of years and millions of dollars of research, engineering and development. If the unit worked better gunked up, the transmission would be "gunked up" from the factory by changing tolerances inside. I agree with you that the TCM is probably re-learning in your case, though. As for proven fluids, D/MIII has been proven to destroy the A604. I would advise using whatever oil weight the factory advises. There are high mileage oils with extra conditioners if leakage is a concern.
 

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Might have something to do with the driving cycle of the vehicle. From what I understand, the D/MIII was nastiest to the transmission's torque converter lockup clutch, since the controller essentially pulses the clutch on and off for partial engagement under some conditions. When the clutch is fully open or closed, the wear is much less. The wear on the clutch packs would also be accelerated, but from what I understand, the TCC got it worse than any other component. However, if the vehicle isn't shifting, there's little to no wear occuring. If the van was driven mostly highway at sustained speeds, the transmission wouldn't have had to shift very much compared to a vehicle accumulating the same miles in stop and go service. The clutch in a stick shift car really doesn't wear when it's engaged (or it shouldn't); this is a similar concept.
ATF+4 is of much less benefit in the A413. The TCC in this unit is on or off; it doesn't pulse. By the early 80's, when the A413 was developed, most major manufacturers had a lockup torque converter of similar operation. Because the A413 is so similar to other traditional automatics, it can tolerate fluids designed for them. ATF+4 is still optimal, but you can use nearly anything in a pinch. The A604 was so radically different when it was introduced that it required a radically different fluid to tolerate things like pulse width modulation to smooth engagement of the TCC, which would have been difficult, if not impossible and impractical, to implement in a traditional hydromechanical automatic. It certainly doesn't deserve the bad rap it's gotten: after all, the 41TE family became Chrysler's go-to transmisson for the next 10 or so years for most of the higher-powered (i.e. V6) FWD vehicles it built, and I would say it's more than proven itself.
 

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The A670 is an uprated A413. It has a TCC, but it works exactly like the A413 otherwise. I would still recommend ATF+4, but I imagine that with the 3-speeds, this spec is more to keep the business in-house rather than speccing another manufacturer's ATF.
OklahomaWolf dismantled his solenoid pack a while back and rebuilt it, and made an excellent (as always) article on it for the main site. You may want to look here if you think that's the issue. The A604 cannot be read using the key dance, it requires a (hideously expensive) DRB to pull its codes.
The general public doesn't tend to be as conscientions as us about maintenance. The transmissions may have run for a long time with worn fluid (in addition to clutch particles, the heat of use breaks down some of the molecules in the fluid. Some fluids are also hygroscopic, meaning they absorb water extremely readily, which can cause accelerated wear and corrosion), or have been abused in other ways. You really never know how the previous owner treated the car. In the past, at least here in the States, Chrysler advertised the Torqueflite as nearly maintenance-free, with some fluid change intervals being blank, meaning they didn't expect the fluid to need to be replaced during the life of the vehicle. While this probably isn't a terribly good idea in a hydromechanical unit, the A604 is a VERY different box. Unfortunately, people tend to assume a trans is a trans and none of them need service, EVER. Not really something that you can get away with in an A604...
 
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