RELAY GP 3471934 - Caterpillar

and 3116 Commercial EnginesThis article replaces the article with the same title on Page 1 of the December 1995 issue.
When evaluating crankcase blowby, Caterpillar recommends measuring the Flow Rate of the blowby fumes from the breather, using the 8T-2700 Blowby Indicator Group. This instrument measures blowby volume flow rate in units of cubic feet per hour (ft/hr), or in liters per minute (L/min). The approximate blowby rate for new 3114 and 3116 Engines is 1.2 ft/hp hr or, 0.5 L/hp min. For example, a 100 hp engine would be 1.2 X 100 = 120 ft/hr (0.5 X 100 = 50 L/min). Blowby is considered "excessive" when it exceeds twice the normal "new engine" value.
Crankcase pressure measurements are less reliable as an indicator, and high crankcase pressure may be caused by something as simple as a partially plugged breather.
Many servicemen may assume that excessive blowby measurements always indicate piston ring and cylinder bore wear. However, there are other possible causes that should be checked. Some of these causes follow:
1. One possible cause is the intake manifold gasket. It is possible for the gasket to leak turbocharger boost pressure into the valve mechanism area, which exits the engine through the breather as "blowby". This can be checked by removing the valve cover, and inspecting the inlet manifold gasket for signs of deterioration.2. Another possible cause of high blowby is a ruptured fuel ratio control diaphragm which can leak boost pressure into the governor and into the engine. The fuel ratio control diaphragm can be checked by disconnecting the boost line at the control, and using an FT 1906 Tool Group (or equivalent) to pressurize the control to 103 kPa (15 psi). If the diaphragm is defective a pressure loss will be observed.3. If an engine is equipped with an air compressor, worn rings in the compressor or a defective unloader valve could cause excessive blowby. To determine if the air compressor is the cause, observe blowby while the compressor is cycling (building pressure) and unloading (bypassing pressure). If the blowby figures fluctuate, check the air compressor for dirt and/or unloader valve problems. If the air compressor air inlet line is connected to the engine air intake manifold boost port and uses charged air, it will be necessary to disconnect the line from the inlet manifold port to determine if the air compressor is causing high blowby.