PIN 8D3791 - Caterpillar

All 3400 Industrial, Marine, And Generator Set EnginesSome carbon build-up (deposits) on inlet valves, pistons and in inlet ports (openings) is normal. Reports show that not everyone knows what is normal build-up and what is not normal.
Some engines have been found to have carbon build-up in the inlet ports. These engines did not show a decrease in engine performance. Also, it is normal for the amount of carbon build-up to be different from port to port and valve to valve. Illustration 1 shows a typical example of normal carbon build-up on inlet valves.
Illustration 1. Typical example of carbon build-up on intake valves.Engines with direct injection (DI) will normally have more carbon build-up on the top land area of the piston than engines with precombustion chambers (PC). This is a result of the differences in the combustion procedure. Also, in comparison to the PC pistons, the DI pistons have more clearance between the top land and the cylinder wall. Illustration 2 shows a typical example of carbon build-up on a DI piston.
If carbon build-up looks to be more than normal on your engine, the areas that follow can be checked to make sure engine performance will be kept at an acceptable level.
1. Check the amount of inlet air restriction. See the Testing and Adjusting Section of the Service Manual for your engine for the correct measurement. Some possible causes of air restrictions are:a. Plugged (dirty) air cleaner or soot filter.b. Crankcase fumes in the engine room or compartment (can cause a build-up in the air cleaner filter).c. Restriction in inlet air lines.
Illustration 2. Typical example of carbon build-up on pistons.2. Check the ports of the inlet manifold. They must be dry. If they are wet, the turbocharger compressor seal is probably damaged.3. Make sure 6N7174 Shields have been correctly installed on all valves in 3408 and 3412 Engines. Operation of these engines without shields can cause carbon build-up on inlet valves.4. Check inlet valve stem and valve guide diameters. The acceptable diameters are shown in the chart. Too much clearance will permit oil to come down the inlet valve guides. 5. Check the crankcase pressure. Make sure the fumes disposal tube is not closed or decreased by a restriction. The fumes disposal tube must be removed when the crankcase pressure is checked.6. Check to make sure the thermostats (temperature regulators) operate correctly. See the Testing And Adjusting Section of the Service Manual for the correct opening temperatures. Low coolant temperatures can increase carbon build-up.7. Check the oil consumption records for the engine. If oil consumption is high, it is possible that the piston rings are worn or damaged. This will permit more than the normal amount of oil to get past the piston rings.8. Make sure the oil used is a Service Classification CD oil. Also, check to make sure the Total Base Number (TBN) for the oil used is acceptable with the percent of sulfur in your fuel.