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They are nice, comfortable cars. Here are the weak spots.
1) nylon coated timing chain gears.
2) possible 7.25” rear axle with terrible 2.2 gears
3) lockup torque converter, though they were around 5 years by then and not as problematic as earlier.
4) Lean Burn (also called Electronic Spark Control) will get blamed for every performance issue, and people will tell you to rip it out. But the performance problems are more likely to be carburetor related than ignition.
 

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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.
ATF+(3 or 4) is much better than Dexron for anything with a lockup torque converter, which most 3 speeds got in the late 1970s. The lockup is smoother.
 
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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...
Rather than the police engine, you’d have been better served to wait on the updated 1985 318 roller engine with higher compression, a Holley carb, and the swirl heads.!Still the terrible rear gear ratio, but a better tuned engine.
 
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1983 was the last year for the old style radios (no clock, no separate ground for each speaker). The clock, if installed, was in the glove box door.

There was a great range of radios from a standard AM single speaker radio to an AM/FM/cassette. Other options included premium speakers (with amp, amp was different between electronic tuning and regular radios) and power antenna. All radios should have a silver face plate in this car.
 
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I'm curious to see the mileage difference with the EFI. Very nice to see you're taking the time and effort to maintain a classic.
I’d expect the biggest improvement will be in drivability. For example the 1987 Dakota had a 3.9 V6 with a carburetor. The 1988 Dakota had the same 3.9 V6 but with throttle body fuel injection. The EPA economy rating went down with the injection even though the 1988 was a far better truck to drive.
 

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Why did the mileage go down? New tech not yet refined?
If that was the issue, it should have went up in the following years as they refined the tech. It didn’t even when TBI ended in 1991.
My guess is they had the carburetor tuned ridiculously lean to meet tailpipe emissions. Which explains why drivability was so much better with TBI than carb, it could possibly run a little richer but still have better tailpipe emissions.
 

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An interesting system with the CB built in. I bet it and the electronic scan radio would be about equally scarce now, especially with the bright faceplate.

No, I don't use cassettes, Imperial Crown! :) Not for a good many years. But I was thinking of getting one of those cassette adapters that let you hook in a modern music player,

The whole audio system is dead at the moment, including antenna, so I don't know if the rear amp button is functional. It may take me a while to get around to this--got some bigger fish to fry first.
For 1983, the AM/FM/CB was still around but supposedly dropped from the M body (a special tri-band antenna was used on these cars so the single antenna mast picked up AM/FM and CB signals).
The "calculator button" search tune radio was replaced by a new "Quartz Lock" electrical tune radio with cassette. If you look for this radio, it is a 1983 only radio because a similar looking radio was used in 1984 but has the different connectors on the back and an integral clock.
Font Electronic device Auto part Display device Electric blue
 

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Thanks for the info, Imperial Crown! I was actually thinking after I made my post that my car doesn't have a sunroof, so that might have been for the sunroof switch. But I guess not.
Nope, the sunroof switch was overhead.
 

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Primer should be light gray. You are getting toward the era, if not a little early, for paint delamination, where the paint would peel off easily.

If the car was originally white, the top was probably dyed as well.
 

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You would likely have a ghost image on the chrome mirror where the color decal was. A good graphics shop could probably duplicate.
 
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