Originally Posted by Diesel911

CAT Fuel Injection Pumps compared to a BOSCH Fuel Injection Pump about as much as a Gasoline Engine Compares to a Diesel Engine. They have a lot of the same parts but the parts are way different.

Some CAT Fuel Injection Systems in the Past were made to be replaced like that. The Fuel Injection Elements were calibrated at the Factory to put out the amount of Fuel Specified. On some you could use a depth Micrometer or Dial Indicator to set up the parts in the Main Fuel Injection Pump Housing.

If they did not take apart the whole Fuel Injection Pump and inspect the Bearings and Governor Parts and replace the worn ones they cannot say that they Rebuilt the Fuel Injection Pump in the Field.

When I worked in a Fuel Injection Shop and I was Calibrating a Fuel Injection Pump I was amazed how little movement it took of the Element Fuel Adjustment the make a big change in the amount of Fuel Quantity that the Elements delivered.

The other think is (this is related to your use of the Heat Gun ) that especially on a used Fuel Injection Pump getting the Idle fuel quantity. correct compared to the correct full load fuel is difficult. Because, at idle the Fuel Injection Pump turns slower and the Fuel has more time to leak past worn Plungers in the Elements. So often the idle fuel quantity is more erratic than the more important full load fuel quantity. The full load fuel quantity is the most important; an erratic idle is a nuisance but does no harm.

The way this relates to the use of a Heat Gun to measure the Exhaust temp is that you need to do this while the Engine is under some sort of load. In big Engines they do the with a Pyrometer in each of the Exhaust Manifolds; sometime they Pyrometer port/s is built into the Cylinder head.

Last comment on CAT Fuel Injection Pumps. Despite the fact that some of that on some CAT Fuel Injection Pumps you were supposed to be able to replace the Elements without calibrating the Fuel Injection Pump. We used to Calibrate the ones that were small enough for our test stand to handle and quite a few of them were not within specs.