when driving sometimes on the freeway the normal diesel "rattle" will quite down. The motor is at operating temperature. Now that i pay more attention to it, it seems like the engine is louder at times than others, and the reason i heard it is because it was at constant rpm. No change in power or anything. i have a holley blue fuel pump at 13psi next to the tank and i am running with the stock pump inline in its original place. my fuel pressure gauge is mounted between the pumps. i do have a superchips programmer on it.
any ideas??

 

Does the diesel "rattle" get louder and softer?

Also need to know what year truck you have.

Dan

 

I bet I know. The normal diesel clatter you hear can vary with load. That said at highway speeds your RPM stays constant, lets say 1800 RPM on flat ground. No big load at all. Now say you have a hill up ahead, and your speed and RPM stays constant, what changes here is load on the engine. More load, more fuel injected to keep speed going up the hill. Same going downhill assuming speed and RPM stay the same, its just the load on the engine that changes.

 

This EXACT symptom is what turned out to be a bad injection pump on my truck.

It was like the diesel rattle would just disappear like magic, and it would instead just be a gentle hum.

I had perfect fuel pressure the whole time, and the pump still died at only 40K