Ok, so Iv had this 1994 Chevy 3500 dually with a Detroit 6.5 Turbo (non EGR) for about 3 years now and i have always had this problem. The truck starts right up and runs perfect. If i stay below 60mph and under dont go over 2k rpm for 30sec then its fine. Pulls hard, idles great, perfect running truck. If i drive over 60mph (2k rpm) for more then 30sec the truck goes into an odd "limp mode". I say odd because you can still drive it everywhere just fine but it acts like wide open throttle is only 50% throttle... You can even drive it down the highway at 70-75mph all day (If you dont hit a long enough hill) When it goes into this "limp mode" you can feel it. Its like a light switch that kills 50% power. No SES or any kind of light on the dash, shifts fine, lockup works perfect everything seems perfect except the little gremlin that throws it into this limp mode. I have replaced every known sensor with new GM parts to no avail. The only sensor that made any kind of difference was the MAP. This is where it get weird. I have replaced the map sensor with 4 different brands including GM and i seem to get 2 outcomes. 1- Full throttle until 60mph then permanent limp mode until i shut it off and wait 15mins, or, 2 -part to full throttle for 3-5secs then limp mode but if i let off and get back into it, then it comes out of limp mode but right back in after 3-5sec of full throttle again. I pulled just over 7000lbs from VA to CA with the truck like this and managed but its very aggravating not having 100% power on the highway. I own a small auto shop and i am no stranger to diesels by any means but this one has me stumped.
A quick run down on the truck

-Truck was originally a HollyRock Diesel built truck in the early 2000s,
-Full 4" exhaust out two 8" stacks, no cats, mufflers, nothing.
-Injection pump replaced around a year ago along with new BWD injectors, lines, ect.
-New PMD relocated with a FSD heat sink and a #9 resistor replaced same time as IP.
-Turbo replaced with a GM4? turbo (if i remember right, GM5 maybe)
- around 6-8lbs of boost and stays pretty consistent.
-Transmission is a 4L80E built by a very reputable shop in VA about 2yrs ago
-Lift pump, filter and fuel lines were replaced ( had a fuel leak problem when i bought it.)
-Full set of gauges and i keep a close eye on EGTs
-Vac pump is gone along with the old WG diaphragm.
-Truck uses stock boost solenoid controlled by the ECU to regulate boost but rather controlled by PSI then VAC. Just like a wastegate on a turbo gas engine. Works flawlessly.
-Duramax fan and dual thermostats installed
-EGTs stay "low" around 400-600f when at highway speeds cruising. Full tilt maybe 1200-1300 peak with a big hill and heavy load but i watch it very closely.
-Minimal black smoke on hard take-offs

Anyone have any idea where to point me? The truck is perfect other then this one issue and i have about pulled all my hair out trying to find the cause. This truck has done the same thing since i bought it even before all these parts were replaced... Im down to the point of replacing the ECU but that is a big $$$$ gamble im not sure if i want to bet on.

 

The last sentence clinched my response.
Mine did same after 4"exhaust and better air filter/intake piping.
limp mode was caused by ECU from boost pressure exceeded factory spec.
got a new/reprogrammed Ecu and pmd relocate from Heath. After install I
felt as though I had a new truck.
​​​​​​spend the $. It felt good enough to pull 19k@12lbs boost. across 7 sisters @45mph over the top, last trip I was going 15mph at top of hill.
BUT factory gaskets won't like being abused. but it was time to refresh top side anyway. I spent more $ on my like new feeling 157000mile '98 K3500.
I rebuilt leaking Gm8 turbo installed the new ARP studs, S & S heads, AES cold air intake and custom piped charge air cooler w/electric fans.

 

I have a similar limp mode problem on my 1994 K2500 6.5, although I get the SES light. When troubleshooting the OBD1, I get a code of 78, which is a wastegate solenoid. I've replaced this with a brand new Borg Warner unit, but still get the same code and light. I've also replaced the lift pump and oil pressure switch, which are both performing and testing as they should I should mention that my problem occurs at 71 MPH on flat straitaways...unsure what RPM because I have no tach installed, but it happens immediately upon reaching that speed and latchs the light. If I'm on a hill, it happens well below 71 mph, so I'v e sure it has to do with the RPMs and pressure. Could this be an ECU unit? I suspect my problem is overboost, but not sure. ANY suggestions or tests I can do to fix this? I've been troubleshooting this for a month, it is my father's truck, and he has had this limp mode problem for over 16 years. No mechanic has ever been able to fix it. I should also mention that I do not get the 12 code when checking the OBD...it goes straight to the 78 code and this is the only one that repeats.