*You might want to grab a coffee before you stat reading
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Well, the 6.5 can be had cheaply for sure.
The problem is the castings and the electronics aren't quite up to ***** after some decent miles and years are on it.
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The 6.5 had a nasty habit of breaking cranks, cracking the main webs, cracking heads to the water jackets, and starter "ears" breaking off.
The crank is nodular iron and poorly supported by the block architecture. After a certain amount of LCF (life cycle fatigue) it is susceptible to breakage when the rest of the engine isn't up to *****. A failing harmonic balancer (rubber ring fails as they age) only hurries the process along.
The main webs tend to crack in two places: the main cap bolts and the cap registers. Cracks up to a certain point are "repairable" with a stitch lok insert, cracks in the cap registers are terminal. Unfortunately, the only way to see them is to take the pan off. You pretty much have to lift the engine to get the pan off.
A 94 could have a 599 casting or a 141 casting, maybe a 929. i can't really remember the year breaks off the top of my head. 599's are generally accepted as the best GM casting, but I've seen lots of them cracked. 506 castings (96+) are generally accepted as the worst.
The problem with GM cast blocks is they were designed as fuel efficient diesels, not powerhouses. When dodge changed the diesel game by dropping a medium duty engine (IE: 6BT) in a light truck, GM started wicking up the 6.2 to compete. As the stress went up (IE: over bore, turbocharging, etc) the reliability and parts life came down.
All the GM castings have issues, none are immune. It wasn't properly addressed until GEP (AMG subsidiary) started producing them for the HMMWV in around 2000. They had Navistar/International cast the blocks in their foundries. They also changed the metallurgy (more moly in the blocks, more chromium in the heads) added some beef to the castings (IE: the mains are .020 thicker, head cooling passages redesigned, etc) and did a bit of process refinement (IE: shot peened parts, closer assembly tolerances, machining practices, etc). These still have "506" cast in the intake valley, but are a completely different animal than the GM 506 casting. They are referred to as the "6500 Optimizer". It's just the name GEP hung on them. I have one in my truck.
The electronics are another issue. Right about 94 GM introduced the "electronic diesel". This is the Standyne DS4 pump. It's based on the mechanical DB2 and the mechanical side of the rotary pump is actually pretty good. The problem is the electronic bits hung off it. They were mainly incorporated to address the ever tightening emissions requirements. The pump driver (PMD) was the biggest offender and there are lots of fixes out there. I'm running a Bill Heath relocator kit and it has been trouble free for nearly 4 years now. Others have not been so lucky with other brands. It was more expensive than everyone else's, but Bill offers a 7 year warranty and that was the clincher for me. Turns out it's good stuff.
While the PMD is the biggest offender, the fuel shut off solenoid has failed on me (common for others too) and the optical sensor has also failed on me. The FSOS is an easy fix, the Optical sensor is not. The OS is not officially a field serviceable item. Failure means a new pump. You can replace them, but you need to know what you are doing and you need a scanner that can show you the idle fuel rate to set it right or your throttle will be all buggered up.
Oh, did I mention the throttle is "fly by wire" also? No cable. Mess up the OS and you get a freaked out throttle. Google "optical bump" for more info on how it can mess it up.
Then there's the PCM controlled/vacuum operated wastegate system on the turbo. Works great if it's in good shape. Will make you want to eat a bullet when it has problems.
The fuel system is suceptable to air intrusion when it gets old and rusty. If it gives you problems, that will make you want to eat a bullet too.
Lift pumps are always suspect. GM just seemed to make a crappy pump. Aftermarket ones are just as bad. I sidestepped the whole fuel issue by buying a raptor 100 lift pump and did 1/2 eaton pushlock hose from the tank to the filter manager on the engine. Then braided stainless from the filter manager to the IP since it runs under the intake in the engine valley.
88-95 will also have problems with the oil pressure switch. The power for the lift pump runs through it and burns the contacts because of the load. It's intended to shut the fuel supply down in case you roll the truck (IE: engine stops, no oil pressure). A simple relay system can take the high current load off the OP switch.
The 6.5 TD in stock form is pretty anemic. You pretty much have to get a new 4" exhaust on it and a decent reprogram of the PCM before it wakes up.
Cooling is another problem area. The early 6.5's came with (IIRC) a 5 bladed fan, single thermostats and water pumps not up to the flow requirements. The fan clutch in a 94 is probably pretty tired also and might have lost much/most of it's fluid. Around 96-98 GM dealt with cooling issues a bit better by installing a better fan clutch, 9 bladed fan, dual thermostats (Flow is king) and high flow water pumps.
The 6.5 is an evolutionary dead end. If you want one, you have to become the expert and not a lot transfers over to modern DI diesels. You'll never find anyone who actually knows how to fix them in a shop. Just no money in it...
That's the meat of the issues.
Now the good stuff.
The 14 Bolt FF rear end is nearly indestructible. The 4L80E is probably the best GM transmission from the era. The GMT 400 Diesel truck is probably the best start to swap in another diesel.
If it's in good shape, they make great 6BT swap vehicles. Especially if it's a "full boat" 8600 GVWR chassis. You just have to get used to swapping out front end components a little more often (IE: 6BT weights around 1200 lbs fully dressed).
You're also going to have all the usually GMT400 issues. It's still a 94 GMT400 after all....
If you want the truck, go get it. But be prepared for lots of hair pulling breakdowns or putting mucho bucks in it.
If it doesn't break down; you're golden. But never turn your back on it....
