Engine indication

Five procedures. Procedure 1 comes before troubleshooting any engine fault.

Procedure 1 — Establish whether the indication is real (R2)

Before chasing an engine fault, prove the instrument.

1. Check the indication against a second source where one exists — another gauge, a handheld instrument, another cylinder's probe.
2. Check the instrument — mechanical condition, power, and that it responds.
3. Check the sensing element — probe, transducer, transmitter.
4. Check the wiring and the ground. A poor ground on a thermocouple or transducer circuit shifts the reading.
5. Only then conclude that the engine is at fault.

Check — say which of the four you have eliminated. Replacing engine components because of a faulty gauge is common and avoidable (PP-D Concepts §2).

And the inverse: do not dismiss a changed indication as instrument error without doing the same four checks. A change is a finding until proven otherwise.

Procedure 2 — Measure thermocouple loop resistance (S8)

1. Isolate the circuit and de-energize.
2. Find the total loop resistance specified for that system in the manual. This number is the point of the procedure.
3. Measure across the loop, zeroing the meter leads first (GEN-A Procedures Procedure 1).
4. Compare against the specification.
5. If out of tolerance, check for the wrong lead length, a substituted lead, corroded connections, or a damaged probe (PP-D Concepts §1).

Check — state the specified loop resistance before measuring. A measurement with nothing to compare it against tells you nothing.

Failure — a lead shortened to tidy an installation. Every reading since is wrong and nothing indicates it.

Procedure 3 — Replace a thermocouple probe (S12, S9)

1. Confirm the replacement part number and lead length against the parts data. Length is calibration — do not substitute.
2. Procedure 0 from PP-F — de-energize and ground the magnetos.
3. Note the routing of the existing lead before removing it.
4. Remove the old probe; install the new one to the specified torque — a CHT probe's contact with the cylinder is its heat path, and torque sets it.
5. Route the new lead exactly as the old one was, without shortening or coiling excess unless the manual permits it.
6. Check loop resistance (Procedure 2).
7. Functional check and compare against the other cylinders.

Check — compare the new probe's indication against the others at the same condition. An outlier means the installation, not the cylinder.

Procedure 4 — Remove and install a fuel-flow transmitter or gauge (S3, S4)

1. Fuel off, system depressurized, containment ready (PP-I Procedure 0).
2. Note orientation and flow direction — transmitters are directional.
3. Disconnect, capping lines and the electrical connector.
4. Install with new seals, correct orientation, torqued per the manual.
5. Leak check with the system pressurized.
6. Functional check against a known condition.

Check — confirm flow direction before torquing. A transmitter fitted backwards reads wrongly or not at all, and the arrow is small.

Procedure 5 — Check instrument markings and manifold pressure (S7, S15, S16)

1. Get the marking values from the engine TCDS (GEN-I Procedures Procedure 1).
2. Confirm the markings on each instrument match, and that the slippage marks are aligned (AF-H Procedures Procedure 3).
3. Check the manifold pressure gauge before start: it should read close to field ambient pressure.
4. Inspect the manifold pressure line for leaks, blockage, security and correct routing.
5. Check tachometer markings and, where possible, indication against a known reference.

Check — the manifold pressure gauge must read ambient before start. It is a free calibration check and it takes a second (PP-D Concepts §3).

The checks, collected

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