This page is temporary.  The Diplomat Upgrades area will eventually take the place of any info here, but until Upgrades is completed, this info will remain posted.

The is a spot behind the left (driver side) wheel skirt that is prone to rust. This is where the rocker panel and floor panel side meet. The way the wheel skirt is made, it will trap water in that area and if you get a lot of snow/salt in your area, this area might rust out. If it does, you will probably get water under your carpeting. This is an easy problem to fix and you should check this area every spring for damage.
Mike Taylor - 6/17/98

Ok, well through your site I learned that the Diplomat I have, like most, possibly has a 3 speed transmission.  It shifts 3 times like you would expect it to, But if you gradually increase speed to 50 and beyond the engine's RPM's are racing.  Now I just got a new muffler and it doesn't seem that bad because its not that loud.  But still, I'm afraid if there is anything wrong with it Maybe I'm doing it worse.  But then I read the 3 speed thing and now I don't know what to think.  My point is the Speedometer goes to 125.  That's some serious gear ratios if it's only a 3 speed.  Does yours do the same or hear of any others doing this?  It's bugging me more then anything, I mean I'm not in the Indy 500!   Oh another thing.  If I punch it, it shifts up and up fine but as I slow down the shifting points are all screwed  up.  It sounds like a manual transmission shifting down at High RPM's, I was told it could be a possible clutch fan problem.  But honestly, I don't see how.
Kevin M Krieps - 6/15/98

I know what you mean. The lion's share of the Diplomats are 3 speed autos with 318 engines. Yours is a Police Package so I assume its at least a 318.  I understand your concern about high RPM, but that's just the way they're built. It's scary, but the 318's can really take it.  My Dippy still tops out over 115MPH (with almost 300K and the engine never opened), and the revs are already screaming at 65MPH to begin with.   I guess the thinking is that you normally won't have to drive at such high speeds for extended times unless you are traveling through Montana.  Just watch the temp gauge and if it stays down (and the engine hasn't fallen out under the driveline) you're going to be fine.  Don't worry about it at all.
My tranny did have a problem though, that you'll want to watch for. When it failed, the third gear would not stay engaged.  At normal to slow acceleration you would not notice a problem through the shifts, but during faster starts the third would slide right on by and not catch into gear, or slip out when I tried to jump to passing speed.  The first time you notice this (you won't have to guess, you'll know it if/when it happens) you'll want to schedule an appointment at the tranny doctor. The longer you wait, the harder it is to remove the tranny (because of warpage of the shaft) and the more likely you'll be walking.  I haven't heard of that happening a lot though, so don't dwell on it.  Take care of your Diplomat, and it will take care of you.
As far as the shifting down problem, I haven't experienced anything along those lines, so I can't help you much. If It tried to make up an answer, I'd only be telling stories.  I don't want to try and pretend to know something I don't. I would take it to a mechanic you know and trust, and see what they say. Be sure to let me know how that comes out so that I can include the information here at Farley's Commentary.
Frank Billington

To start off on my own 1979 Diplomat four-door, I will mention a few weak points that other owners should watch for on these cars.  Although I am overall impressed by the "M" body, it does have a very few design weaknesses.
The front brake lines are prone to fail.  I have had to replace both front brake hoses about every third year.  Check yours for cracking and replace them if you have any doubts about their integrity.  After one of them blows, you can lose pressure in a hurry.  It is a pretty quick job if you have the right tools, it takes about 45 minutes.
We have had several Mopar vehicles with the 318 V8 in our family.  Watch for leaking rocker arm cover gaskets.  If it gets really bad it can drip oil on the exhaust manifold, which makes an embarrassing smoke screen. 
After about 200,000 miles it is a good idea to replace the timing chain, especially if the engine has had a hard life.  It will slip one notch and cause the engine to run horribly, and that will be your clue to do it.  If it goes two notches off, you will be making a horrible racket slamming valves into pistons, which does pretty respectable damage to rocker arms and push rods.  I've had to do that repair once on my father's 1978 Dodge Van, it's not a lot of fun.  The headache with doing this job is all of the other peripherals on the front of the engine.  When I replaced mine I had to remove the fan shroud, fan, radiator, alternator, air conditioning compressor, power steering pump, and water pump.  The housing for the timing chain also covers part of the oil pan, so working with the new gasket gets kind of tricky since it is in several pieces.  With the right tools I would say this is a six hour job.
I don't have much else to expound upon, my Diplomat has been very reliable over the past nine years, and hasn't needed a lot in the way of repairs at all.