Page 90
TROUBLESHOOTING
ENGINE MISFIRES — NO CODE(S)
Possible Cause Correction
1. Broken or leaking high-pressure fuel lines. 1. Check for fuel leaks. Repair as necessary.
2. Poor quality fuel, or water or dirt in fuel. 2. Drain fuel from tanks. Replace fuel filters and fill tank with
MACK-specified diesel fuel.
3. Air in fuel system. 3. Check fuel system for air leaks. Repair as necessary. (Air
generally gets into the fuel system on suction side of the fuel
pump.)
4. Low fuel supply pressure. 4. Check to be sure there is fuel in the fuel tank. Check for sharp
bends or kinks in the fuel line between the fuel tank and the fuel
transfer pump. Also, check for clogged suction pipe (in the fuel
tank) or a plugged fuel suction hose. Check for air in the fuel
system, and check the fuel pressure. If the pressure is lower than
specified, replace the fuel filters. Inspect fuel return check valve for
free-moving poppet. If still low, replace the transfer pump.
5. Improper valve lash adjustment. 5. Check adjustment. Correct as necessary.
6. Worn camshaft lobe. 6. With valve lash properly adjusted, check rocker arm movement. If
not within specifications, replace worn parts.
7. Valves not seating properly. 7. Remove heads, recondition valves as required, and reinstall
heads.
8. Defective fuel injection nozzles or unit pump. 8. Note: The following test will register a fault in the ECU which may
be cleared after test is completed.
Make sure all EUP terminal wires are connected and tight. With the
engine operating at low idle (625–675 rpm) and using V-MAC
Support Software connected to the chassis, perform the cylinder
cut-out test as outlined in the Support Software Manual, 8-333. If
the cylinder is firing correctly, the engine sound will change. If a
cylinder does not change the engine sound, that cylinder is not
firing correctly.
Note: With step 5 software, shorting out the terminals is the only
way to determine if a cylinder is not firing correctly. However,
starting with step 7 software, you must not short out the terminals
on the EUPs. The use of the cylinder cut-out test is the only
recommended procedure for determining if a cylinder is not firing
correctly. With step 8 software, you can stop the engine by
shorting out the terminals on an EUP.
If a cylinder is not firing correctly, determine if a fuel pulse is
present in the high-pressure injection line by touching the line
firmly with a screwdriver about one inch from the EUP.
Note: When it is difficult to determine if the problem is the EUP or
the nozzle, first switch the EUP with that of an adjacent cylinder
and recheck for a pulse in both affected cylinders. If the problem
follows with the EUP, then the EUP is at fault. If the problem
remains with the original cylinder, then the nozzle may be at fault.
Compare the pulse felt with the EUP shorted vs. not shorted. If a
normal pulse is detected when the EUP is not shorted, the problem
may be in the injection nozzle or the engine valve adjustment. First
check engine valve adjustment. If OK, repair or replace the nozzle.
If no pulse is detected, replace the unit pump for that cylinder.
Note: A tachometer that senses injection-line pressure can also be
used to check if pulse is present (use J 39638 Tech Tach or
equivalent). If an engine rpm is recorded on the tachometer, the
problem may be in the injection nozzle or the engine valve
adjustment. If no reading is obtained, replace the unit pump for
that cylinder.
9. Cylinder head gasket leakage. 9. Check for visible signs of leakage, coolant in the oil, or traces of oil
in the coolant. Use a compression tester to check each cylinder.
Replace cylinder head gasket if necessary.
10. EGR valve stuck open. 10. With the engine running and EGR gases flowing, unplug the EGR
valve. The valve should close. If not, replace the valve.
5-111.bk Page 90 Monday, July 10, 2006 2:26 PM
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