I. Turbocharger - Basics and Troubleshooting

II. Fuel Injection - Basics and Troubleshooting

A. General Overview

The function of an aircraft turbocharger system is to maintain a desired manifold pressure at a given throttle setting regardless of varying conditions of ambient air temperature and pressure. The system consists of a turbocharger, a bypass valve or wastegate, a pressure controller, and a direct acting absolute pressure relief valve.

The discharge pressure of the turbocharger compressor is regulated by controlling the flow of exhaust gases through the turbocharger turbine. The exhaust gas flow is modulated by diverting excess exhaust gas through the wastegate. The wastegate is actuated by pressurized engine oil that first flows into the actuator via a restricting orifice or capillary tube, then it flows out of the actuator to the controller, then it dumps into the engine crankcase. When the controller senses insufficient compressor discharge pressure, a poppet valve in the controller moves towards the closed position raising the oil pressure in the actuator closing the bypass valve and forcing more exhaust gas through the turbocharger turbine. This raises the turbine/compressor shaft speed and the compressor discharge pressure. When the controller senses excessive compressor discharge pressure, the opposite action occurs.

B. Maintenance

Regular inspection intervals should include inspection of the turbocharger housing for condition and security. The housings should not have any bulges. Some cracks may be allowable. Limitations on cracks can be found in the Airesearch overhaul manual. Inspect the impellers for nicks. RAM recommends that the turbochargers receive a soak of the oil seal at least on an annual inspection to eliminate any coking of the seal.
See Maintenance Tip Save-A-Turbo

Inspect the wastegate butterfly valve for warpage and security, and inspect the butterfly shaft bushings for condition. Inspect the linkage for freedom of movement and the arm spring for security. Lubricate the wastegate butterfly shaft at each oil change per RAM Maintenance Tip Turbocharger Wastegates.

Inspect the pressure controller for evidence of oil leakage. Inspect the control arm bushing for wear. Inspect the plunger roller for freedom of movement.

Since the majority of suspected turbocharging system problems are the result of an induction or exhaust leak, leakage inspection of the intake and exhaust system should be included in the maintenance of the turbocharging system. See RAM Maintenance Tip Manifold Pressure and Prevent Exhaust Leak Damage. Do not forget to inspect the air filter canister and alternate air source for integrity.

On some installations, RAM installs an inline strainer in the oil supply to the wastegate that should be changed at each oil change.

C. Troubleshooting - Bootstrapping

As stated earlier, leaks in the intake and exhaust system are the primary cause of altitude related manifold pressure problems. The integrity of these systems cannot be determined by a visual inspection, they must be pressure tested. RAM Maintenance Tip Manifold Pressure gives a detailed procedure for the pressurizing of these systems. If properly followed, ninety percent of the manifold pressure problems can be repaired without unnecessary replacement of the turbocharger, wastegate, or controller. It can be embarrassing to explain to an aircraft owner why an expensive turbocharger did not fix the problem, nor can it be returned. On the other hand, your service will be well remembered after you repair a perceived major problem by tightening several clamps, saving everyone valuable time and money.

Cessna defines bootstrapping as "The unstable manifold pressure condition that occurs when the wastegate closes at high altitude under low RPM operation." In other words, it is the point that the manifold pressure begins to fall off when the RPM is reduced. The wastegate has fully closed and the turbo is not spinning fast enough to produce desired manifold pressure. The Cessna maintenance manual has a detailed procedure to determine if the engine falls within their bootstrapping limitations. Refer to the engine section under "Turbocharger Operational Flight Check Procedure." If the engines pass this test, the system is doing all it is designed to do.

If the air induction system is not getting all of its air from outside the cowling, then the engine is operating in simulated high outside air temperatures. The performance will be decreased to the point that bootstrapping may occur at an altitude lower than it should. The transition duct from the air box to the air scoop often deteriorates over time. Holes develop in the duct, thus allowing hot air from inside the engine cowling to be used for combustion. The same phenomenon happens if the alternate air door is open. Be sure the alternate air door is completely closed. Check to ensure the transition duct and air box do not have holes. Confirm the filter cover to the air box fits so snuggly there are no gaps or leaks.

Cessna T206, T210, and P210 aircraft are very sensitive to the hot air being ingested into the engine. One major problem is with aftermarket intercooler installations. Be sure there is no air leakage between the air box and skin junction. All air for combustion should be coming from outside the cowling. Look carefully for wear at the air box magnets and the alternate air door hinge. Wear will allow the door to crack open. Even worse, it can open fully.

D. Manifold pressure falls off in climb

Induction and exhaust system leaks are the most probable cause for the manifold pressure to drop in climb. Normally the manifold pressure can be regained by advancing the throttle until the throttle travel runs out. If this is not possible, then there is cause to suspect leakage. Leaks, whether in the intake or exhaust system, cause the controller to send a signal to the wastegate actuator to close so that the manifold pressure can be maintained, but the wastegate is already fully closed. As the plane continues to climb, the turbo cannot overcome the leaks; therefore, the manifold pressure falls off. If a leak check does not reveal any leaks, inspect the condition of the wastegate butterfly. It may be warped not allowing it to close completely, or the butterfly could be binding. The pressure controller could be defective; however, components are generally the least likely factors involved in this situation.

E. Manifold Pressure Fluctuations

Pilots sometimes report that their engine is "bootstrapping." What they are actually experiencing is a manifold pressure fluctuation. Be sure to debrief pilots thoroughly so that their problem is properly described. You don't want to waste time chasing the wrong problem. The most helpful and cost effective thing you can do is to go fly with the pilot to witness the problem yourself. Watch to see what happens, and in what sequence. For example, it may be reported that the RPM is fluctuating - when it fact the manifold pressure started fluctuating first; thus, it started driving the RPM.

Many manifold pressure fluctuations can be the result of a bad pressure controller or wastegate. It may be hard to pin point a specific component. Try lubricating the wastegate per RAM Maintenance Tip Turbocharger Wastegate to see if it helps. When lubricating the butterfly shaft, it is more effective to actuate the butterfly back and forth. If it squeaks when actuated in this condition, then you can be assured it is probably sticking when it's hot. Thus, it is a candidate to be replaced or overhauled. If lubricating the wastegate does not help, then you can suspect a bad controller. However, don't totally dismiss the wastegate because it may be beyond servicing with lubrication.

The wastegate can be pressure tested to see of it is closing and opening at specified values. Use an air regulator tied into the inlet port of the wastegate. The outlet port should be capped. It should begin to close at 17 psi, and be fully closed at 41 psi. It should start to open again at 30 psi, and be full open at 6 psi. These values are general values. You should consult the Airesearch Manual for the specific value of the wastegate you are testing. If the butterfly chatters when actuated, it is binding when hot and is the probable defective component. Unless you have a test bench for the controller, about all you can do is back flush it to remove any foreign material that may be under the poppet valve seat. Use a cleaning solvent (such as Mineral Spirits or Varsol) in a pressure pot hooked to the outlet port of the pressure controller. Disconnect the wastegate supply oil line and direct it into a bucket to catch the oil and solvent. While back flushing the controller, be sure to actuate the throttle - which will actuate the poppet valve.

If the manifold pressure follows the airspeed, i.e., the manifold pressure increases immediately upon airspeed increase, then the most likely defective component is the wastegate.

If optionally equipped with an inline strainer to the wastegate, clean the inline strainer. RAM suspects that the primary cause of wastegate and controller problems is foreign matter in the oil. Foreign matter will typically be captured within the inline strainer.

F. Torque up split

If the pilot reports that the engines are not accelerating together, check to see if they are rigged alike. Holding the throttle levers together, accelerate the engines to 30 inches of manifold pressure. If the manifold pressure split is below 30 inches, the mechanical linkages are not properly rigged. They should be rigged so they match up side by side. If the manifold pressure split occurs above 30 inches, then the problem is in the pressure controller. Testing and adjustment of the controller is in the Cessna Maintenance Manual Engine Section under "Variable Absolute Pressure Controller Adjustment - Low Pressure Setting."

Occasionally, a pilot will report that the engines will not achieve rated manifold pressure on the take off roll. After troubleshooting the normal probable causes for this problem, take a look at the divider in the turbo wye pipe. Sometimes a portion of the divider will break loose and wedge itself in the inlet of the turbo housing - blocking the exhaust flow to the turbine wheel. This situation can also cause the manifold pressure to fall off in climb, or cause bootstrapping.

G. Troubleshooting Chart

The following troubleshooting chart is designed assuming that the mechanic working on the engine has accomplished routine procedures. This chart of probable causes and probable corrective actions is to be used as a supplement while also using the applicable Cessna and CMI troubleshooting charts.

A. Maintenance

Regular inspection intervals should include a thorough search for any signs of damage and an inspection for fuel leaks. If you suspect a leak, place the mixture control at "idle cut off" and operate the electric fuel pump on high and observe the area under actual fuel pressure. If the suspected leak is in the manifold valve or nozzle area, then the mixture will need to be set to the full rich position. Important areas to check are the air reference chamber in the injection pump and the manifold valve atmospheric vent.

The areas around the nozzles should be inspected for fuel dye stains. Leaking O-rings on the nozzle will produce stains on the nozzle and cylinder head. Nozzles should be removed and cleaned at least every 300 hours, or more frequently if environmental conditions dictate.

Essential to proper operation and longevity of the fuel injection system is proper lubrication. The throttle shaft should be lubricated at least every 100 hours. Link rod pins should also be lubricated with a drop of lubricant at each hinge point.

B. Adjustments

1. When Performing Adjustments

Minor adjustments of the fuel control unit will typically be necessary throughout the life of the engine. The most common adjustments needed are for idle speed, mixture and injector pump un-metered pressure and metered fuel flow.

While making fuel adjustments, keep in mind that the engines should be warm; however, you should try to avoid heat soaking. As the temperature under the cowl increases, the aneroid valve is affected due to expansion of the aneroid unit. Heat soaking produces a gradual decay of the performance of the engine and will reduce the metered fuel pressure. So keep adjustment and checking to the shortest possible time to avoid heat soaking. Geared/GTSIO 520 series engines are especially susceptible to heat soaking. Short test flights are preferred to extended ground runs. This will give you more accurate numbers and it is not as hard on the engine.

RAM has modified the CMI recommended setup procedures for fuel injection systems on TSIO-520 and GTSIO 520 series engines. We believe we have developed a setup procedure which is more suited to the use of an electronic fuel management system. The physical adjustments are the same as recommended in the Cessna or CMG service manuals (Maintenance M-0), but are set at different values (SID2006-01 Series IV, VI, and VII only ). Special instructions pertaining to the GTSIO-520-N engine are in Continental Motors (CMI) Service Bulletin M89-10. On fuel systems that do not have a fuel discharge regulator, RAM sets the cruise climb fuel flow to the specified limits allowing the take off fuel flow to be slightly higher than the specified limits.

2. Un-metered and metered fuel pressure

It is imperative that the un-metered fuel pressure is correctly adjusted in order to proceed with any troubleshooting procedures. Make sure that you are using a calibrated gauge for checking the pressure and that the fittings and hoses used to connect the gauge into the system are the same inside diameter as the pump outlet fitting, otherwise false readings can occur. It is desirable to have the auxiliary pump pressure and the engine pump un-metered pressure approximately the same; however, sometimes it may not be practical to accomplish this especially on bonded-wing aircraft.

On aircraft with bladder type fuel tanks (typically the aircraft with tip tanks) the auxiliary pump pressure can be adjusted per the Cessna Maintenance Manual. On the bonded-wing aircraft (typically the aircraft without tip tanks) the auxiliary pump pressure can be determined by the way the un-metered pressure reacts to the aux pump switch being turned "on" and "off". When turning the aux pump to the "on" position if the indicated un-metered fuel pressure decreases the engine driven pump pressure is too high. If it increases the pressure is too low.

The engine driven pump pressure can be adjusted so that there is minimal change to un-metered pressure with the aux pump "on" or "off." Also, after each adjustment is made, run the engine up to clear it out and recheck pressures. Keep in mind that changing the un-metered fuel pressure will change the idle mixture. Changing the idle mixture may change the un-metered pressure slightly; therefore, it is good practice to recheck both when making adjustments.

3. Idle speed and mixture

The adjustments for idle speed and mixture are straightforward. A clockwise rotation of the idle speed adjustment screws will increase speed. Conversely, a counterclockwise rotation of the idle adjustment screws will reduce speed. On a direct drive TSIO-520 engine, a clockwise rotation enriches the mixture with the idle mixture adjustment, and a counterclockwise rotation leans the mixture. On a geared GTSIO-520 engine, the idle mixture adjustment is opposite. On a geared engine, a clockwise rotation leans the mixture, and a counterclockwise rotation enriches the mixture. Adjusting the mixtures will sometimes change the rpm; therefore, set your mixtures before setting the idle rpm. Check the mixtures twice. Check them with the aux pumps "on." Check them again with the aux pumps "off".

4. Starting a hot engine

What commonly happens is that the heat from the engine, upon shutdown, vaporizes the fuel in the lines. The injection pump cannot supply fuel in its vapor state fast enough at engine cranking speed to provide adequate fuel for starting. The situation typically goes from one extreme to another and the engine becomes flooded. The secret is in using the electric fuel pump to purge the lines and fill them with fuel. Consult your aircraft POH for hot start procedures.

C. Troubleshooting

1. Rough idle

Low un-metered fuel pressure will cause rough idling. The natural tendency for correcting this is to enrichen the idle mixture. However, the mixture would then be too rich at fast idle during taxiing. The correct procedure is to adjust the low un-metered fuel pressure to the specified limits then adjust the idle mixture to the specified limits. If the low un-metered fuel pressure is too high the symptoms will be a little different. The engine will start fine when it's hot or cold, but will idle rough. Excessive low un-metered fuel pressure will cause the engine to quit on roll-out (just as it did when the low un-metered fuel pressure was too low) only this time it will quit due to an overly rich condition. The only satisfactory solution is proper adjustment of the low un-metered fuel pressure to the specified limits, followed by proper adjustment of the idle mixture.

Turbocharged engines may employ two-way "sniffle" valves installed by the aircraft manufacturer. These valves are designed to close anytime a differential in pressure exists between the induction manifold and ambient air pressures. It is only open when the engine is at rest, or if the manifold and ambient pressures are equal. The valve is typically installed between the cylinder drain system and the overboard tube. If the valve fails to close properly, mixture problems will occur, especially in the lower rpm ranges. Excessive lean mixtures at low engine speed will result in rough and erratic idle performance. Verify the valve is causing the problem by placing your finger over the drain tube while the engine is idling. If the performance improves immediately, or you feel any suction on your finger, you know the valve is bad. Other induction and/or air reference leaks can be causing a lean idle mixture. A thorough inspection of the induction system should be conducted using the procedures in RAM Maintenance Tip Manifold Pressure.

With the low un-metered pressure and idle mixture properly adjusted, and with no induction leaks, if the engine still won't properly run under 1500 rpm then the problem may be trash under the relief valve assembly in the fuel pump. This could be a shaved piece of rubber from a hose or metal from a fitting. Sometimes the trash may be removed by using shop air to "back flush" the pump. Remove the supply line to the pump and the output line from the pump. Blow into the outlet fitting on the pump. If this does not help, then the pump will have to be removed and sent to a qualified repair station to have it repaired or overhauled.

2. Rough running engine in flight

In-flight roughness can be caused by trouble with one or more fuel injection nozzles. The engine can run rough intermittently, indicating that dirt or water has passed through a nozzle. The engine may run rough continually, indicating that a nozzle is plugged. A thorough cleaning of the fuel filters and finger screens will need to be accomplished. If this condition persists after cleaning the filters, the problem may be one of deteriorating fuel injector lines or a manifold valve. Two easy methods of determining if an injector is blocked are: (1) perform a cold cylinder check, (2) flow the nozzles.

To perform the cold cylinder check, start the engine and run it for a minute or two at 1,000 rpm. Next, using a trigger nozzle squirt bottle (with the nozzle set to squirt a stream) squirt each exhaust pipe an inch below the cylinder flange. The water should vaporize immediately upon contacting the pipe. If a pipe gets wet, then that cylinder is cold and is suspect.

To flow the nozzles you will need six (6) like size containers. Plastic baby bottles make good containers. Remove the nozzle from the cylinder, reattach it to the line and insert the nozzle into the baby bottle. Move the idle mixture and throttle to full open, turn the boost pump on high for 15-20 seconds. Each bottle should have approximately the same amount of fuel. A bottle with a marked increase will indicate a defect with that nozzle. A nozzle with a marked decrease will indicate that either the nozzle, the line to a nozzle, or the manifold port possibly has a blockage.

Blocked air reference lines, or a blocked fuel manifold valve vent, can create a symptom similar to a clogged injector nozzle by not allowing the fuel to atomize properly. Inspect the air reference tubes, the injector shrouds, and the manifold vent for blockages or leaks.

RAM recommends that the auxiliary fuel pumps be in the "low" position, for aircraft modified by MEB88-3, or the "on" position for aircraft not modified by MEB88-3 during all flight operations. Most Cessna POHs recommend that the pump be in the "low" position for take off, landing and as required. Many times at higher altitudes when the pump is in the "off" position the engine or engines will hesitate, run rough or die. This condition is more prevalent on the bonded wing aircraft than on the bladder tank aircraft. RAM believes this condition is directly related to a low head pressure at the inlet of the engine driven fuel pump. Once the auxiliary pump is turned off, gravity and the weight of the fuel are the only factors that influence the head pressure to the engine driven pump. On bonded wing aircraft, the level of the fuel is lower than the inlet of the pump; therefore, gravity and weight must force the fuel up hill. If the operator chooses not to run the pumps in the "on" or "low" position, then the aircraft should be operated at a lower altitude or lower power setting. If the engine is running rough, or dies, it can be regained by reducing the manifold pressure to a point where the engine smoothes out or resumes running. The throttle can then be slowly increased to a power setting where the engine maintains smooth operation.

Occasionally a geared GTSIO 520 engine will run rough when reduced from take off power to climb power. This usually happens as the throttle is reduced through 38-37 inches of manifold pressure; then it smoothes out again going through 36-35 inches of manifold pressure. Verify that the fuel injection system is properly set to specifications, and verify substitute 637368-19AA or -19A fuel injection nozzles are in place of the -19B or 19C nozzles. This will lean out the fuel flow to the cylinder enough that the engine will smooth out. An approved Engineering Variation Order, issued by CMI, has approved this procedure.

3. Troubleshooting Chart

The following troubleshooting chart is designed to take into account that the mechanic working on the engine has accomplished routine troubleshooting such as making sure the fuel and the magnetos are on. This chart of probable causes and corrective action suggestions is to be used as a supplement while also using the applicable Cessna and CMI troubleshooting charts.