3116 and 3126 Truck Engines Problem Engine Misfiring Or Running Rough

Probable Cause(s):

* Cold Outside Temperatures
* Air In Fuel
* Injector Problem
* Hydraulic System Problem
* Water In Fuel
* Low Fuel Pressure
* Throttle Position Sensor/PTO Problem
* Valve Clearances
* Head Gasket
* Mechanical Failure In Cylinder

1. Cold Outside Temperatures:

NOTE: It is normal to get white smoke while cranking and for a very brief period after starting the engine. For operation in cold and very cold temperatures, a winterfront cover or radiator shutters are recommended.

a. Check to see if the inlet air heater is inoperative. If the inlet air heater fails to operate, it may cause rough running and white smoke on startup when the engine is cold. Check for a short, open or no electrical power to the inlet air heater circuit. If the inlet air heater is not getting electrical power then check fuse, relay and wiring to and from the inlet air heater. For further information, refer to Problem 32: Inlet Air Heater.

b. Using an electronic service tool check for logged or active diagnostic codes. Refer to 3100 HEUI Diesel Truck Engine, SENR6566, for Inlet Heater Driver Open Circuit or Inlet Heater Driver Short Circuit.

2. Air In Fuel:

Check for air in fuel by installing a 2P8278 sight glass on the return line. If access to the rear of the engine is restricted, install the sight glass on the return line at the tank. Check fuel transfer pump fittings and joints for air leaks. Check the unit injectors and sleeves for combustion gas leaks. Remove each unit injector and use a magnifying glass to inspect the base of the sleeve. The unit injector may also show evidence of leaking combustion gas.

NOTE: Refer to Disassembly And Assembly, SENR6553, for the proper procedure for unit injector removal and installation.
3. Injector Problem:

a. An injector may fail to operate properly, causing a misfire in that cylinder. Some possible causes; debris in the oil supply, incorrect removal of an injector, mechanical pressure on the adapter section, mechanical pressure on the solenoid, or the solenoid coil may develop a shorted or open circuit.

NOTE: There is no bench test (“pop test”) for HEUI injectors available for field testing. The only test for injectors is “on the engine”.
b. Using an electronic service tool check for logged or active diagnostic codes. Refer to 3100 HEUI Diesel Truck Engine, SENR6566, for Cylinder Cutout Test.

4. Hydraulic System Problem:

a. Check dipstick to be sure oil level is normal.

NOTE: Using an electronic service tool check for logged or active diagnostic codes. Refer to 3100 HEUI Diesel Truck Engine Electronic Troubleshooting, SENR6566 for electronic problems and for the hydraulic troubleshooting test.

b. Check for air in the system. After opening or working on the high pressure hydraulic oil system, there may be air remaining in the system. To purge the air, crank the engine for thirty seconds at a time at least three times. Allow the starter to cool off for two minutes between each cranking period.

c. Check for leaks in the high pressure line, fittings between the hydraulic pump and the oil manifold, and inside the valve cover.

5. Water In Fuel:

Condensation may occur in fuel tanks, typically while standing overnight in cool or cold ambient temperatures. The use of a primary filter/water separator is recommended. If water is found in the fuel, it will be necessary to drain fuel tanks, lines and filters and refill with known good fuel of the correct grade, and replace fuel filters.
6. Low fuel pressure:

a. Check fuel pressure with a gauge. Fuel pressure at low idle should read about 400 to 435 kPa (58 to 63 psi), and during normal operating conditions under load, should read between 400 and 525 kPa (58 to 76 psi) measured at the inlet to the secondary fuel filter. Fuel pressure to the cylinder head fuel gallery should be as above, less the pressure differential across the secondary fuel filter. Pressure drop across the secondary fuel filter is typically at least 35 kPa (5 psi) with a new filter. As the filter accumulates deposits, the pressure differential will increase.

b. If fuel pressure at the unit injectors drops to approximately 69 kPa (10 psi) it is possible that misfires will occur. This would typically be a “rolling misfire” that affects all cylinders at random, rather than a consistent misfire on a given cylinder.

c. Check fuel filters and fuel lines for restrictions or plugged. Check for fuel transfer pump or unit injector sleeve failure. Check to see if the orifice check valve is loose, missing from the fuel return line, or if the wrong orifice was installed. If sleeve or O-ring damage is found, replace with new parts as necessary. Refer to Special Instruction, Using the 122-7093 Sleeve Replacement Tool Group, SEHS9120.

7. Throttle Position Sensor/PTO Problem:

NOTE: Momentary loss of the throttle position, cruise control or PTO signal may cause a “speed burp” or brief drop in engine rpm. Intermittent signal loss may be perceived as an “unstable engine” problem.

Using an electronic service tool check for logged or active diagnostic codes. Refer to 3100 HEUI Diesel Truck Engine, SENR6566.

NOTE: Throttle position, vehicle speed, brake and clutch switches, and other PTO and cruise control component operations can be viewed in the Display Status screens on the electronic service tool.
8. Loss of Compression:

Check valve clearances and make adjustments. Valve clearances set too tight may cause poor sealing and loss of compression. Refer to the Testing And Adjusting section of this service manual. Low compression can also result from a blown head gasket, or cracks in the cylinder head/engine block.