Trying to find the year of my Goold, Shapley & Muir S.N. 6788 Speed 400 H.P.6 hit and miss engine

road speed?
flat, uphill or down hill?
what load is on the truck.?
d-400 is 1-1/2ton truck
Max GVW: 14,000lbs that huge truck had 318 gas engine only.
did someone put in the
Cummins V6 engines 354ci six cylinder Perkins Diesel engine. back then vast choices of engine /transmissions were offered
and all are rare.
most of these back then, me on the road too a loaded truck was going 40 to 50 mph; under even small hills.
the speed would be limited to max diesel RPM. then.
I drove dump truck then and 15 speeds, 3x5 boxes.(crash box)_

things that could be wrong.
compression leak (Piston rings or valves )
fuel (fuel air mix)
ignition (spark plug lead or dispack)
battery or alternator power
spark would be my first though as this needs to be working properly for full range. weak spark will run but will over tax the engine at speeds. if the engine is running fine until a certain speed then this would be affected in affected flood the spark. can als be the alternator if the battery is not charging correctly.

The engine coolant temperature is high. Your other Codes are for missing cylinders on cylinder 1,2,3,and 4. Get the Codes cleared and start over. Changing the plugs and wires,etc could have generated the missing cylinder Codes.

Autozone,Advance, and Oreilys will scan for free.

Check your engine for an IDLE SPEED CONTROL SOLENOID, which may be in the intake. Or a fuel pressure regulator.

A lot of times people work on the spark side and forget the fuel injectors. They can cause a miss.

You may check the Air sensor on the engine.

Often as an engine ages and parts wear out rhythmic oscillations can cause a obvious and repeatable noise. Even when new, hitting a resonant frequency (for my personal car holding at 1500-1600rpm causes a terrible noise and vibration) If you're only experiencing sound as a result of the speed it is probably not a major mechanical issue. However, I have had a car before, about the same year as this one, where as it tried to shift gears at about 40mph into fifth gear the engine would bog down, detect low engine speed at prepare to downshift, and would repeat this process. Solution to that was cleaning the fuel injectors (which will run you about 50$ most places.)

VW Bugs use a tiny accordion looking thermostat that is attached to shutters on the fan shroud. When cold the thermostat pulls the shutters closed to impede the flow of cooling air. When hot the thermostat expands and pushes the shutters open to allow airflow over the engines cooling fins. Changing it is a piece of cake and in fact much easier than the original answer. It's done from the bottom of the engine. It's the only accordian looking thing under the car. You will need to remove a piece of cooling tin on the bottom of the motor to get access to it so in addition to a flashlight and screwdriver bring a 10mm wrench. (Now would also be a good time to get a couple cans of brake clean and use them to clean off the oil and dirt on the bottom of the engine and cooling fins). If you have further questions or need more instruction I strongly reccomend you get a used Muir Manual from Amazon and thumb through the section on cooling. In my opinion VW Bugs should have been equiped from the factory with the Muir Manual.

ENGINE MISSING AT ROAD SPEED CAN BE MANY THINGS.FUEL FILTER CLOGGED FAULTY FUEL PUMP. OR BAD FUEL.BAD SPARK PLUGS AND WIRES.LOW OR UNEVEN ENGINE COMPRESSION.FAULTY IGNITION COIL.BAD DISTRIBUTOR CAP AND ROTOR IF EQUIPPED.VACUUM LEAKS AND LEAKING EGR VALVE.

EASY!!! Bad wiring or bad terminals or loose wiring or even a bad ignition

might have to order that part from a dealer, also the goold ol salvage yard is a cheaper alternative