Here is a DTAC case describing pilot injection:
Solution Number: 69498
Solution Summary: 4045, 6068, 6081, 6090, 6125 6135 Pilot Injection cycles cause diesel rattle or rap
Publication Date: Feb 4 2009
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**Paper copies of solutions may not be the most current solutions**
Complaint or Symptom :
The engine runs quiet at startup and then develops a diesel knock, rattle, miss, diesel ping or flutter.
Problem or Situation :
Tier 1 & Tier 2 engines:
The Tier 1 and Tier 2 engines (4045, 6068, 6081, 6125 engine models) with HPCR fuel injection system have pilot injection when they are started. Pilot injection acts as a pre-injection to aid in combustion during cold-start conditions, thereby reducing white smoke at startup.
The engine will sound different when the pilot injection switches from on to off.
Technicians have described the transition from pilot injection as sounding like a rattle, miss, knock, or ping.
For tractors, an additional variable was added after reprogramming for the PIP 03RW281. If the engine ever drops below 750 rpm, pilot injection will turn on. This may be noticed when starting from a stop at low idle. If pilot injection turns on under the 750 rpm and it is objectionable, you can adjust the default starting gear to a lower selection on your tractor.
Pilot injection is controlled by three variables:
Time
Coolant Temperature
Load
Pilot will turn off when one of the three variables is met:
Pilot turns off when the time from start up exceeds 10 minutes.
Pilot turns off when the coolant temperature exceeds 167°F or 75°C.
Pilot turns off once a load is applied to the engine.
Tier 3 Engines:
The Tier 3 engines (4045, 6068, 6090 & 6135 engine models) pilot injection option is always active. On Tier 3 engines the pilot injection will cycle on & off whenever the engine is operating. The customer will most often notice a flutter in the engine at low idle without a load.
Solution :
This is a normal operation of the engine. No repairs should be made. This pilot fuel function is utilized to reduce white smoke during start/warm-up.
To verify if pilot is on or off, use Service Advisor or DST and refer to the Pilot Injection parameter in the Data List. The display will say either "On" or "Off."
Additional Information :
Ping, Rattle, Rap, Pilot, Tier