Air in Fuel
This procedure checks for air in the fuel. This procedure also assists in finding the source of the air.
- Examine the fuel system for leaks. Ensure that the fuel line fittings are properly tightened. Check the fuel level in the fuel tank. Air can enter the fuel system on the suction side between the fuel transfer pump and the fuel tank.
- Install a 2P-8278 Fuel Flow Tube (Sight Gauge) in the fuel return line. When possible, install the sight gauge in a straight section of the fuel line that is at least 304.8 mm (12 inches) long. Do not install the sight gauge near the following devices that create turbulence:
- Elbows
- Pressure relief valves
- Check valves
Observe the fuel flow during engine cranking. Look for air bubbles in the fuel. If the engine starts, check for air in the fuel at varying engine speeds. When possible, operate the engine under a load and look for air in the fuel.
- Elbows
- If excessive air is seen in the sight tube in the fuel return line, install a second sight tube at the inlet to the fuel transfer pump. If a second sight tube is not available, move the sight tube from the fuel return line and install the sight tube at the inlet to the fuel transfer pump. Observe the fuel flow during engine cranking. Look for air bubbles in the fuel. If the engine starts, check for air in the fuel at varying engine speeds. When possible, operate the engine under a load and look for air in the fuel.
If excessive air is not seen at the inlet to the fuel transfer pump, the air is entering the system after the fuel transfer pump. Proceed to Step 5.
Show/hide table
 | |
| Illustration 1 | g00578151 |
2P-8278 Fuel Flow Tube (Sight Gauge) (1) A steady stream of small bubbles with a diameter of approximately 0.1588 cm (0.0625 inch) is an acceptable amount of air in the fuel. (2) Bubbles with a diameter of approximately 0.635 cm (0.25 inch) are also acceptable if there is two seconds to three seconds intervals between bubbles. (3) Excessive air bubbles in the fuel are not acceptable. | |
- Pressurize the fuel tank to 35 kPa (5 psi). Do not use more than 55 kPa (8 psi) in order to avoid damage to the fuel tank. Check for leaks in the fuel lines between the fuel tank and the fuel transfer pump. Repair any leaks that are found. Check the fuel pressure in order to ensure that the fuel transfer pump is operating properly. For information about checking the fuel pressure, see Testing and Adjusting, "Fuel System Pressure - Test".
Show/hide table
 |
To avoid personal injury, always wear eye and face protection when using pressurized air. |
 |
Show/hide table
| NOTICE |
|---|
To avoid damage, do not use more than 55 kPa (8 psi) to pressurize the fuel tank. |
- Proceed to Step 6 if an infrared thermometer or an exhaust temperature pyrometer is not available. Start the following procedure with a cold engine. Take all measurements within the first five minutes of operation. The temperatures of the exhaust manifold ports should be within 20 °C (68 °F). An exhaust port that has a low temperature may have an injector that is leaking air.
- Start the engine and operate the engine at low idle.
- Measure the temperature at the center of the number one exhaust manifold port and at the center of the number six exhaust manifold port. Compare the two temperatures.
- Measure the temperature of the number two exhaust manifold port and the number five exhaust manifold port. Compare the two temperatures.
- Measure the temperature of the number three exhaust manifold port and the number four exhaust manifold port. Compare the two temperatures.
Note: Injectors that are downstream from a leaking injector may measure cold but not leaking air.
- Remove the valve cover and operate the engine at low idle rpm.
- Check each cylinder sequentially from the number six cylinder to the number one cylinder. Use a screwdriver to move each fuel injector rack (1) to the fuel-on position. Watch the sight tube in the fuel return line. An increase in the amount of air in the fuel indicates that the injector is leaking air. Also note the amount of increase in fuel injector rack that is required to cause a cylinder to knock. An injector that is leaking air will require the fuel rack to be advanced more toward the fuel-on position. If the source of air can be isolated to a particular injector, remove the injector. Ream the injector sleeve if the seat area for the injector is pitted. Replace the injector sleeve, if necessary. Reinstall the injector. Ensure that the injector is properly seated. Refer to Tool Operating Manual, NEHS0738, "Using the Injector Seating Tool " and Tool Operating Manual, NEHS0610, "Using the 128-8822 Tool Group on 3114, 3116, and 3126 Engines with Mechanical Unit Injectors (MUI)". Also, refer to Video Tape, LEVN4032, "Troubleshooting Guide for the 3100 Family of Diesel Engines" and Video Tape, LEVN4732, "Injector Seating Procedure". After the injector is reinstalled, check for air in the fuel.
Show/hide table
 | |
| Illustration 2 | g00598910 |
(1) Fuel injector rack (2) Fuel-on position | |
- If the source of air can not be isolated to a particular cylinder, remove each injector. Ream all injector sleeves or replace all injector sleeves. Replace an injector only if the injector cone is damaged. A burned O-ring or a missing O-ring on the end of the injector does not indicate a faulty seal. The O-ring should not be replaced.
Caterpillar Information System:
3500B Engines and 3500 Engines with EUI Option Electrical Power Supply
3003, 3013, 3014 and 3024 Engines for Caterpillar Built Machines Rocker Shaft - Assemble - 3003 Engines
G3606 and G3608 Engines Actuator (Hydrax) - Install
Instructions For Correcting Speed Surge In 3406C PEEC III Truck Engines{1408} Instructions For Correcting Speed Surge In 3406C PEEC III Truck Engines{1408}
G3606 and G3608 Engines Actuator (Hydrax) - Assemble
Electronic Modular Control Panel II (EMCP II) For PEEC Engines Indicator for Overcrank - Troubleshoot
Electronic Modular Control Panel II (EMCP II) For MUI Engines Indicator for Low Coolant Level - Troubleshoot
G3606 and G3608 Engines Actuator (Hydrax) - Disassemble
Electronic Modular Control Panel II (EMCP II) For MUI Engines Indicator for High Water Temperature - Troubleshoot
G3606 and G3608 Engines Caterpillar Ignition System (CIS) Module - Remove and Install
Electronic Modular Control Panel II (EMCP II) For PEEC Engines Indicator for Emergency Stop - Troubleshoot
3114, 3116 and 3126 Industrial, Marine and Generator Set Engines Flywheel
3114, 3116 and 3126 Industrial, Marine and Generator Set Engines Flywheel Housing
Electronic Modular Control Panel II (EMCP II) For PEEC Engines Engaged Starting Motor - Troubleshoot
Electronic Modular Control Panel II (EMCP II) For PEEC Engines No Engine Shutdown - Troubleshoot
3500B Engines and 3500 Engines with EUI Option Analog Sensor Supply
3126B Marine Engine Cylinder Head Valves
3500B Engines and 3500 Engines with EUI Option Cat Data Link
3126B Marine Engine Piston and Rings
3500B Engines Electronic Control Module (Engine) - Remove and Install
Converting a Four Cylinder Air Compressor Air Supply From a Pressurized Air Supply to a Naturally Aspirated Air Supply{1803} Converting a Four Cylinder Air Compressor Air Supply From a Pressurized Air Supply to a Naturally Aspirated Air Supply{1803}
Electronic Modular Control Panel II (EMCP II) For PEEC Engines Erratic GSC Operation - Troubleshoot
3114, 3116 and 3126 Engines Alternator Charge Rate Is High
3114, 3116 and 3126 Industrial, Marine and Generator Set Engines Belt Tensioner