I have a '79 280GE that had an '83 om617a swapped in it years ago. I haven't had the truck long but after driving it found a strange fuel related problem. I've confirmed with the previous owner that it's had the problem for years.


Symptoms:


With 2/3 to full tank of fuel the motor runs great with no issues. Once it gets below 2/3 (approx) it runs the same while it's cold, but once it warms up it starts to lose power under load, like going up a slight incline. It acts like fuel starvation.


This is very consistent and predicable as soon as the fuel gets below a certain level and the motor is warmed up. The motor swap was done a long time ago, they used the original fuel tank and the lines look like they're connected correctly. I've run the fuel all the way down near empty and the symptoms don't change once under the level that it occurs.


How should I go about diagnosing? I have a fuel pressure gauge and was going to start by checking the lift pump. Any other suggestions. I'm a newbie to diesels.


Dan