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Why ATF+4? I've been having a harder and harder time finding Dexron 2D (which is called for in the manual), but am afraid to switch to any of the ATF's in my 3 speed as I lost a transmission shortly after switching to ATF +3 in the early 00's.
The switch to +3 certainly wouldn't have been the cause or your transmission failure. It was obviously close to failing when you went to the +3. Most common causes if failure in the legacy transmissions was incorrect in kickdown adjustment, followed by incorrect fluid level.
 

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Definitely go through the fuel system. Clean or replace the tank and sending unit. Blow out the supply and return lines, test the supply line for leaks. One little trick I used to do was take a length of fuel hose, slice it down the middle, and slide it over the fuel supply line as insulation. These cars like to vaporlock.

Change the secondary filter near the pump, use the correct 3-nipple filter, orientated properly. If the pump works, great. If not, replace it with a Carter pump. Buy a spare. I would normally say go through the carb, but if it looks rough, you'd be better off finding a good 6280 out of an '85 or newer M body to rebuild. It will bolt right in and work the same as the BBD.

Champion plugs, Echlin cap/rotor/wires (or replace the whole dizzy with a new (not used) unit. Test the SCC and set the timing per FSM, but be aware of timing chain slop or worn harmonic balancer.

Last thought - these cars, 2bbl and 4bbl, were all flat hydraulic lifters through 1984. The very best lube today, in the 2bbl car up through '84, would be a Rotella 10W30. I wouldn't use anything heavier, nor would I bother with a synthetic. Just run a regular oil like the Rotella or the Pennzoil high mileage is fine, too. Definitely also clean or replace the PCV and breather, as well.
 

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Fond memories of the 1984 Diplomat SE my wife and I ordered back in late winter 1984 and received late April. We kept that for 13 years. I do remember test driving one on a lot, and was NOT impressed with the handling, and it sounded/felt like a front tire was rubbing somewhere as I was in a tight corner and suspension bottomed out, so I did special order ours with HD suspension. If I could have ordered it with the Police Package - 318 - 4bl, with the better 360 heads, and an 8 1/4" rear with 2.94 gears and a Sure-Grip, I would have! As mentioned above, the 2.2 gears were pathetic as far as acceleration, but you could cruise high speeds all day. Never had any issues with the Lean Burn system or lockup convertor. Same with the nylon gears. I imagine that unless they are very old, or have very high mileage, the "whopping" 140 HP motors really don't have enough power or torque to tear up much of anything...
April of '85 delivery was well into the 1985 model year.

If the ride height and camber weren't carefully watched, they'd scrub the inside of the front tires in a sharper turn. It was usually the right side that would run out of positive camber adjustment, which required a couple of shim in-between the subframe rails and upper control arm bracket. That became a factory field modification in 1986. They all did it, but it was the '86 and early '88 models that had the most trouble. Those things were squirrely until they were fixed. By May of 1988, they had changed the engine cradle in a few areas that seemed to eliminate the problem. I restored an '86 Plymouth squad that had received an entire new k-frame, along with the green poly biscuits. Both that, and an '89 5th Ave that I had, would hold their alignment and ride height with no trouble. An '86 Diplomat SE that I had would sag if you looked at it the wrong way. So I do believe the resolved the problem, but only 7 months until they shut down production.
 
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