When was the last time you adjusted the valves on it?

If they've never been done, that might be where at least some of it went.

Any 3406 CAT engines I had were real fussy with valve settings. They called for .015 and .030 and they didn't mean .014 and .029. Fuel mileage would suffer by a few tenths.

Kevin

OP’s 06 will have hydraulic lash adjusters.

I pulled the cover on my 01 at 150k…. Thought there was one getting loud, nothing was really even worth adjusting. Of course one came into work a week afterwords and I could hear the same noise. Forget what “normal” is sometimes.

I’m not a huge additive guy by any means but I’ve seen the CRC diesel fuel therapy do some pretty amazing stuff on the common rail trucks (03 up).

My VP pickup I use 2 cycle oil in the fuel (pint to 35gal)…. Gets so hot out here the newer fuels thin and you can set p0216 pump timing failure codes. Got this tidbit from Cummins.