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A few thoughts from a guy who has worked with these since they were new... The pump and PMD are cooled by excess fuel flowing through the injection pump, and returning to the tank. If the fuel lift pump located in the frame rail, more or less under your left heel, quits, or gets weak, or the oil pressure switch or other circuitry that powers it (setup varies by model year)fails, the engine may still run reasonably well, but the IP and PMD won"t get the cooling benefits of the excess fuel. Also, the fancy plastic "beauty cover" over the top center of the engine only holds heat in, and makes the IP and PMD run HOTTER. Even with bad luck, and a weak fuel pump, 12,000 miles from a PMD is REALLY below average. Are you SURE the PMD is failing, or is there a wiring harness or connector problem you are temporarily "fixing" when you change out the PMD"s??? Also, take a close look at your PMD"s. There may be different versions out now, but traditionally, if you look at the backside, there will be 2 oblong "pits", with power transistors in them, each covered by a plastic insulator. Once you pop the insulators out with a pick, the power transistors are exposed. They are held down with two 1/4" nuts. On probably 75% of "failed" PMD"s, these nuts have worked loose, from a combination of thermal cycles, and vibration. This causes an poor connection between the collector (case) of the transistor, and the connections on the circuit board behind it. I remove the nuts, install a new "star" lockwasher and a tiny drop of locktite, and TIGHTEN down the nuts. On one of my pickups, I have about 40,000 miles on a PMD "restored" in that manner, with NO problems. Using an extension harness, the FSD can be mounted behind the LH headlight, where it gets a constant flow of air while the pickup is moving. You can buy a harness, or simply make your own, by cutting the plug off of the existing harness, with a couple off inches of wire left on it, and solder extension leads to each wire, IIRC, somewhere between 30" and 40". The "jobber" heatsinks are pricey. I have made several of my own by using surplus finned heat sink material bought on ebay. (About $20.00 for four good-sized pieces.) All that is needed is to accurately drill and tap the 4 FSD mounting holes, and a couple of holes to mount the heat sink assembly with. I take a FSD that has had the transistor mounting nuts Loctited and tightened, omit the insulator caps, then fill the "pit" around each transistor ALMOST full of silicone dielectric grease. Then, I coat the mounting face of the FSD with the GOOD silver-bearing computer CPU heat transfer compound, and bolt it in place. Then, I thoroughly clean off any extra heat transfer compound that has squeezed out, wiping the area clean with solvent. Then, I put a bead of clear silicon sealant around the area where the FSD and heat sink meet, to prevent the possibility of any moisture creeping in. In several years, with a half-dozen or so of these setups, we have not had a single problem with the FSD"s.
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