TCM Fuel System

Fuel system inspection is an important part of inspecting Teledyne Continental Motor (TCM)-powered aircraft. It is vital to ensure the continued safe operation of your customers’ aircraft. But some mechanics aren’t performing these inspections properly. Some may not even be doing the inspection. For this month’s article, AMT sat down with Don Fitzgerald, director of training for Teledyne Continental Motors, to get tips on proper inspection and testing of TCM fuel systems.

Let’s go ahead and start off with a disclaimer. This article is meant to complement the technical information provided by Teledyne Continental Motors, not take the place of it. Always refer to the manufacturer’s maintenance manual and applicable service instructions when inspecting and setting up the TCM fuel system. Service Information Directive (SID) SID97-3 is the document that covers procedures and specifications for adjustment of TCM continuous flow fuel injection systems. The current revision of SID97-3 is “D.”

That covers my disclaimer. Now let’s look at TCM’s. If we read the first paragraph of SID97-3D, we see the following statement:

“Warning. The procedures and values provided in this service bulletin apply to TCM fuel-injected engines that have not been modified from their original type design. Refer to supplemental type certificate (STC) holder information and instructions for aircraft and engines that have been modified from the original type design.”

If you have an STC involving components or accessories to the TCM engine that affects the original type design, such as adding a turbo-intercooler or turbo-normalizing the engine, that would drastically affect the fuel system requirements and thus render the use of SID97-3D ineffective for the engine. At that point, you should refer to the STC holder’s fuel setup data.

When to perform check
Operational verification of the engine fuel system is required any time one of the following circumstances occurs:

  • At engine installation
  • During 100-hour and annual inspections
  • Whenever a fuel system component is replaced or adjusted
  • When changes occur in the operating environment

SID97-3D spells out the procedures for setting up the fuel system on TCM engines that includes pre-setup, setup, adjustment (if needed), post-setup, and flight test.

Pre-setup procedures
The pre-setup procedures outlined by TCM are an important step. Nonetheless, Fitzgerald shares that many mechanics skip over this important step. “I often find that most folks have the tendency to overlook the section in the bulletin right after the tools and equipment which is the pre-setup procedures,” Fitzgerald tells AMT. “A lot of people just jump from the tools right to the setup procedures. But the pre-setup section covers a lot of things that a mechanic should do during a good 100-hour or annual inspection. All of those items can affect the performance and the way the engine and fuel system will produce the fuel for the engine – especially in turbocharged applications.”

If you are doing the fuel system check because of an engine change or a fuel system component change, the fuel system needs to be flushed. Remove the engine-driven fuel pump inlet hose and terminate the end into a large, clean container. Operate the boost pump and allow a minimum of 1 gallon of fuel to flow through the system. If you notice contamination, locate and correct the source and repeat the fuel system flushing procedure. SID97-3D discusses a more detailed flushing procedure for all IO-240-B engines.

Verify the accuracy of the aircraft tachometer, manifold pressure gauge, and fuel flow gauge.

Remove the engine cowling. Ensure all fuel system components are of the correct part number and are installed properly.

Remove, inspect, clean, and reinstall the aircraft and engine fuel screens in accordance with the aircraft manufacturer’s instructions. Inspect the induction air filter and alternate air system for condition, operation, and cleanliness. Inspect the aircraft vapor return system for proper operation. Ensure the fuel manifold valve vent and fuel pump drain lines are properly installed, open, and free of obstruction.

Inspect all engine control rod ends for wear, freedom of movement, proper installation, and security. Inspect the throttle and control assembly link rods (where used) for correct installation, security, and wear at the attach points. Ensure all engine controls operate freely throughout their full range of travel and are properly adjusted.

Lubricate all control rod ends and fuel system components in accordance with the latest revision of TCM Service Bulletin SB95-2 and the aircraft maintenance manual.

Locate the idle speed stop screw on the throttlebody and turn it counterclockwise two complete turns. (During fuel system adjustment, idle rpm will be controlled manually using the cockpit throttle control.)
Inspect the exhaust and induction systems for proper installation, security, and leaks. Inspect all lines, hoses, and wire bundles for chafing, loose connections, leaks, and stains. Fitzgerald offers an inspection tip for the fuel lines. “The rubber grommets around the fuel lines at the standoff brackets should be checked during any visual inspection,” he stresses. “It’s very important to make sure they are intact and that the steel fuel lines are not touching the metal brackets.”

SID97-3D outlines the setup process for performing the system test. During this step, the mechanic disconnects specific metered and unmetered fuel lines from the engine and attaches them to corresponding hoses and fittings going to the Porta Test Unit or to test gauges. The throttle control is positioned in the full open position and the mixture control is moved to full rich. The boost pump is turned on and all air is bled from the test unit and hoses using the instructions on the Porta Test Unit. If using gauges, the fittings are loosened at each gauge to bleed the lines of any air. (Hold the gauge at or slightly above the height of the fuel system component during the bleeding operation). Operate the boost pumps only long enough to complete the purging process. Turn boost pump off and verify that all lines, hoses, and fittings are secured and torqued and that there are no fuel leaks. Ensure that the test hoses are routed clear of the exhaust system and are adequately supported the entire length to avoid inaccurate gauge readings.

At this point, TCM stresses an important warning. “Make certain all fuel has drained from the induction system prior to attempting engine start. Failure to do so could cause hydraulic lock and subsequent engine failure.”

Install the engine cowling or cooling shroud for ground runs.

System test
Now you are ready for the fuel system test. Due to limited space, we don’t have the luxury to discuss the complete procedure. The test procedure is pretty straightforward. Start the engine and allow it to warm up and stabilize. You then will note the test gauge readings at idle and at full power (1,500 to 1,800 rpm) and record both the actual readings along with the specified readings on the operational test form on the last page of SID97-3D. The readings you will be recording include engine rpm, manifold air pressure, unmetered and metered fuel pressure, fuel flow, EGT, TIT, cylinder head temperature, and oil pressure and temperature.

If all readings are within the limits spelled out in SID97, the test is over. You can shut down the aircraft, disconnect the Porta Test Unit, re-connect all the fittings and leak check them, and re-cowl the engine.

If any of the actual readings are outside the specified limits, you will need to adjust the fuel system. Refer to SID97-3 for specific adjustment instructions. All adjustments are made with the engine shut down. Also, make all adjustments in small increments. Here is an overview of the fuel system adjustment procedure.

The first step is setting the idle rpm fuel pressure, also called unmetered fuel pump pressure, and is spelled out in step 18 on page six of SID97-3D. The adjustment is made at the low pressure relief valve. Turn the adjustment clockwise to increase idle rpm fuel pressure and counterclockwise to decrease pressure. Tighten the lock nut, re-start the engine, operate it at 1,500 to 1,800 rpm for 15 seconds, then retard the throttle back to idle rpm. Repeat this step until pressure is within the specified limits. Fitzgerald offers a tip. “You’ll want to set your low end low. What that does, it leans out the engine. So in step 19, when you do your fuel air mixture rise, that will force you to adjust that in such a way that will slightly enrich the engine again, and gives you better part throttle enrichment.”

Next, with the engine operating at the specified idle rpm and unmetered fuel pressure, slowly move the mixture control from the full rich position toward the idle cut-off to check the fuel/air mixture. A rise of 25 to 50 rpm should be obtained. A rise greater than 50 indicates the mixture is too rich. A rise less than 25 indicates the mixture is too lean. Adjust to within limits in accordance with SID97-3D.

Last, adjust the full power metered fuel pressure by turning the adjustable orifice screw clockwise to increase fuel pressure, and counterclockwise to decrease fuel pressure.

After all adjustments are made, go back through and re-check all readings. “The mechanic needs to go back and re-check all readings because one adjustment will affect the other,” stresses Fitzgerald. “For example, full power will affect low end, and low end will affect the full. So they need to go back and re-check them again.”

Fitzgerald shares a test indication that would be a telltale sign for contaminants in the fuel system. “Take a look at table 3 in the service bulletin. The IO-550-N, at full power, 2,700 rpm, the metered pressure should be in the range of 19-21.3 psi. If you adjust it all the way up to the high end of 21.3, the unmetered pump pressure at that same rpm is in the 28-32 range. In other words, you should be close to 32. The metered and unmetered should track together. If they don’t, it’s because there’s something clogging the system somewhere. So, if you adjust it up to 21.3, and find out that the unmetered pressure is all the way up to 40, then you’ve got a problem. You need to stop with the procedure and flush the fuel system out to try to purge it of contaminants until you can correct the problem.”

Once all parameters are within limits, you can shut down the engine, disconnect the test equipment, re-connect the engine lines and leak check them.

Having trouble with the fuel system setup on your TCM engine? Fitzgerald stresses that you don’t have to struggle. “If mechanics are having trouble with some of this, and they can’t seem to get the system right, give us a call on our toll-free number at (888) 826-5465. We have our mechanics and IAs in there to answer questions. After all, we are standing by to assist you because you are the first line of service to the pilots, therefore YOU are our customer.”

Teledyne Continental Motors has a DVD titled “Continuous Flow Fuel Injection Set-up” that goes through the fuel system setup. AMT is hosting that video on its website this month. If you would like to see the video and learn more about TCM fuel system setup, go to