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Reference: Service Magazine; August, 20, 1990; "Improvements To Reduce Black Exhaust Smoke" on Page 12 and "New Aftercooler Gaskets Help Maintain Constant Bolt Torque" on Page 11. Service Magazine; October 16, 1989; Page 8; "New Gasket Used Between Exhaust Manifold and Cylinder Head." Service Magazine; May 20, 1991; Pages 14 & 15; "New Turbocharger Improves Performance and Reduces Smoke" on 785 and 789 Trucks.
There have been reports of excessive black exhaust smoke on off-highway trucks and tractors powered by 3500 Engines. Several changes have been made to the engines to decrease smoke levels. This article contains recommendations and a description of the changes to minimize the smoke levels.
Recommendations To Reduce Smoke Levels
If excessive black smoke has been reported, first determine the operating conditions under which this occurs. Generally these conditions can be characterized as one of the following:
- Only during acceleration
- During both acceleration and steady load
- Coming from only one bank of the engine
Smoke only during acceleration indicates a maladjusted or malfunction of the governor fuel ratio control. Smoke during both acceleration and steady load can be attributed to a number of reasons which will be discussed later. Smoke coming from only one bank of the engine indicates a problem isolated to one or more cylinders of the engine.
Smoke Occurs Only During Acceleration
Typically if heavy black smoke occurs only during acceleration, check for a problem with the fuel ratio control or check the limit setting.
The governor fuel ratio control (FRC) is designed to restrict the amount of fuel injected during the time that it takes the turbocharger to come up to speed and provide positive boost pressure in the inlet manifold. The FRC is adjusted to a limit setting which provides the best compromise between exhaust smoke and engine response. A low FRC setting will reduce smoke during acceleration, but will inhibit engine response time. Conversely, a high FRC limit setting will increase the smoke during acceleration, but will improve engine response time. As the turbocharger boost comes up, the fuel ratio control moves out of the way of the governor/fuel system linkage. Under most operating conditions other than acceleration from a stop, the FRC does not have an affect on the governor and does not effect the smoke level.
Earlier 3161 Governors used a fuel ratio control which required both oil pressure and boost pressure to arm the fuel ratio control. A leak in the diaphragm would not allow the fuel ratio control to arm and limit the fuel injected during acceleration. This would result in very heavy black smoke. The 2W4531, 7W4963, 7C3791, 7C4271 and 7C9456 3161 Governors were equipped with this style of fuel ratio control.
Later 3161 Governors use an FRC which is always armed. A leak in the FRC diaphragm will not allow the control to react to boost pressure and always limits the fuel to the adjusted limit setting. Instead of heavy black smoke, the engine would exhibit low power assuming the limit setting is correct. If the limit setting is high, then heavy black smoke would be seen. The 9Y2133, 9Y6680, 9Y7559, and 7E7249 3161 Governors are equipped with continually-armed FRC's.
Earlier 3161 Governors contained a limiter mechanism that would wear and allow growth of the limiter setting. The new governor has a new smoke limiter that will eliminate internal wear. Also, the limit setting of the smoke limiter has been reduced from 2.00 mm (.079 in) to 1.00 mm (.039 in). In addition, a stiffer spring is used against the diaphragm. The stiffer spring will cause the FRC to react slower and reduce the transient smoke during acceleration. The 7E7249 Governor contains the improved FRC limiter mechanism, lower limiter setting and stiffer spring.
Smoke Occurs During Both Acceleration and Steady Load
If smoke occurs during both acceleration and steady load operation, then further investigation of the intake, exhaust and fuel systems may be necessary.
Some improvements have been made to the intake and exhaust system to reduce the possibility of black smoke during both transient and steady state operation.
A new turbocharger, used on later 785 and 789 Trucks, has a different turbine housing which contains a smaller nozzle. The smaller nozzle enables the turbocharger to provide more boost, resulting in less black smoke.
The new 7E9492 Turbocharger Group is used on the 785 Trucks and the 7E9497 Turbocharger Group is used on the 789 Trucks.
An improved injector nozzle with a different spray angle is now used for these machines. The spray angle was changed from 150 degrees to 160 degrees to provide for optimum combustion which results in lower exhaust smoke levels. The change in spray angle was incorporated with the 7E3382 Injector. The 7C2238 Injector Nozzle Assembly with date code of "F9" or later has the 160 degree spray angle.
A loss of boost pressure is another common cause of black exhaust smoke. However, loss of boost or inlet manifold pressure is usually a symptom not a root cause. Possible causes of low boost pressure are:
- Inlet air leak between the turbocharger and cylinders.
- Exhaust air leak between turbocharger and cylinders.
- Worn or failed turbocharger.
- Excessive exhaust flow restriction.
- Excessive inlet air flow restriction.
Recent improvements to the intake and exhaust system include a new aftercooler housing gasket which helps maintain a constant torque on the aftercooler housing bolts. The new gasket contains a sheet metal core with a rubberized coating which maintains bolt torque better than the former gasket. Excessive compression of the former gasket allowed the bolts to come loose. When this happened, the gasket could fail and cause a loss of boost in the inlet manifold resulting in black exhaust smoke. The new aftercooler housing gaskets are direct replacements for the former aftercooler gaskets.
A new gasket is also used between the exhaust manifold and cylinder head. The new gasket is made of a different material that allows less compression than the former gasket. The former gasket could lose compression and leak exhaust gas. Exhaust manifold leaks cause a loss of boost, resulting in black exhaust smoke.
If there are no intake or exhaust system leaks, perhaps there are some system restrictions. Check air cleaner restriction indicators. Replace the indicator if the plunger is stiff or if it sticks. A stiff or sticking plunger could give you a lower-than-actual inlet air restriction pressure.
Replace air cleaner elements at 20 inch H20 restriction instead of the standard 30 inch.
If excessive smoke cannot be attributed to the intake or exhaust system, there may be a fuel system problem.
Check the fuel injector timing dimension. A high timing dimension (retarded timing) can cause smoke.
Also check the fuel API gravity. An engine may smoke when a fuel of low API gravity is used. Generally, low API gravity fuel will burn slower and may not be completely combusted in the engine.
Smoke Isolated to One Engine Bank
Check for a worn camshaft lobe. A worn injector camshaft lobe will result in late injection of fuel into the cylinder. Late injection timing does not allow ignition of all the fuel in the cylinder, thus expelling partially-combusted fuel out the exhaust (black smoke).
Check for a bad fuel injector. A fuel injector could be leaking excessive amounts of fuel into the cylinder. Too much fuel in the cylinder can cause black exhaust smoke. High exhaust temperature is a good indication of overfueling. Use the 4C6090 Multichannel Temperature Selector Group to measure exhaust temperatures at each cylinder. Measure the exhaust temperatures while at torque converter stall.
Check for a scuffed piston. A scuffed piston may hinder the cylinder's ability to develop enough cylinder pressure to burn all the fuel. This would result in overfueling the cylinder and cause black smoke.
Check for a bad exhaust valve. An exhaust valve that is "guttered" or not seating correctly would not allow the cylinder to develop correct cylinder pressure, resulting in partially burned fuel (black smoke). Use the 4C6090 Multichannel Temperature Selector Group, at torque converter stall, to check for excessive exhaust temperatures.