Using Your Engine Monitor

The devices are standard equipment or must-have add-ons aboard piston aircraft for good reason: They’re more than just a good way to lean the mixture.


I remember the first time an EIS (engine information system, commonly known as an engine monitor) saved my bacon. I was in a Cessna 210 I’d flown for many years by that time and we were high over Varadero, Cuba. I looked over at the JPI engine monitor, a retrofit on this L-model, and noticed the LED bar representing the exhaust gas temperature (EGT) for the number two cylinder was bouncing around.

Now, that EGT probe is located in the exhaust pipe somewhere four to six inches down from the cylinder head, measuring the temperature of the exhaust gases on a continuous basis as they exit through the exhaust port. That temperature, when compared with the exhaust flowing out of the other cylinders, tells me a lot about what is going on inside the engine. And what it was telling me right then was that cylinder number two was being erratic about how much power it was putting out.

Was the engine running rough at that point? Not terribly. At 7000 feet msl in relatively cool air, and at 60-percent power, it was missing just a little, something you could feel by putting both feet flat on the floor. I wasn’t okay with that, however, knowing that was Cuba below me, and with another 120 nm of water in front of me to my destination, Grand Cayman Island, a little speck of land, relatively speaking, nestled in the suddenly not-so-bucolic-looking Caribbean Sea below me.

So, what was the engine monitor trying to tell me that I couldn’t see on any other instrument? The number two cylinder was acting up and, on a fuel-injected engine, this usually means either the ignition system or the fuel injection itself somehow was compromised. I performed an in-flight magneto check. It showed all of the EGTs rising while operating on one magneto. Normal. It probably wasn’t an ignition problem. I tried increasing the fuel pressure with the aux fuel pump. That helped a little—but not much. Ah ha! The culprit most likely was a clogged fuel injector on that cylinder. I reset the mixture to keep the normal cylinders as normal as possible and completed the flight knowing exactly what to tell the mechanic in Grand Cayman. That little test and knowledge of what the results meant saved me considerable labor cost on that repair.

Interestingly, I saw the clogged fuel injector problem more than once in that engine. As I learned to pay a bit more attention to the engine monitor, however, that clogged injector began to manifest itself a little differently: It showed up as an abnormally high EGT. Why? Because I was running the engine rich of peak EGT in climb. So, when the injector clogged, the first indication of a problem was an erratic rise in EGT as the suddenly leaner cylinder approached peak EGT. As the cylinder was starved even more of fuel by the clog, the EGT fell off, as before. Again I was able to explain this in vivid detail to the mechanic at the airport where I put in for repair, and again it was a simple and inexpensive project to pull that specific injector and clean it of the minuscule bit of detritus managing to obstruct it.

Twice that engine monitor saved me from damaging my engine, even if its seven LED bars (six cylinders and oil temperature), alternator function read-out, fuel flow and “lean find” functions were rather rudimentary when compared to more-modern instruments. That alone paid for the retrofit of the gauge in my airplane. But I wanted to do more.

So I studied the greats: George Braly of General Aviation Modifications, Inc. (GAMI), famous for developing precision-tuned injectors allowing fuel-injected aircraft engines to run smoothly when lean-of-peak EGT, and Mike Busch, who wrote extensively about flying his T310R using engine monitoring systems. (These columns and articles, many of which were published before engine monitors gained wide acceptance, remain available at our online sister publication,

From them I learned how to use my monitor for much more than just leaning the mixture past the perfect 15:1 air/gas concoction and to run smoothly at economy fuel settings. I learned to avoid shock-cooling the engine on prolonged descents (it had an alarm that would flash if cylinder head temperatures dropped more than 50 degrees in a short period of time). I also learned how to diagnose fuel injector imbalances across the cylinders, exhaust port issues (leaks and bad gaskets), fouled spark plugs, detonation and preignition (see more about these last two situations in the sidebar on the opposite page).

From them I also learned that a rhythmic bouncing EGT in normalize mode possibly meant a failing exhaust valve (as opposed to that erratic jumping around, which meant fuel-flow issues). I managed to catch one before it caused engine damage. And at that point in my flying career (some 20 years in), I swore I’d never fly a high-performance piston engine without one again.

Soon I swapped rides for a carbureted Continental O-470 sporting only an Alcor multi-probe EGT instrument and the standard single-probe factory CHT gauge. People told me it was fine; a carbureted engine didn’t need a sophisticated engine monitor. But flying that way felt like flying blind.

Sure, I could twist the knob on the Alcor to find my hottest EGT at full-rich mixture and attempt to lean the engine based on it, but I knew from flying with the multi-probe engine monitor that the hottest EGT in climbout does not necessarily equate to the hottest EGT in lean-cruise. Cooling airflow in the engine changes with aircraft attitude and speed, and so does the hierarchy of EGT among the cylinders. My old monitor would show me what I needed to know to keep my engine properly cooled in all phases of flight. The Alcor gauge was just too rudimentary; it had to go.

By then our choices for retrofit had changed considerably. The advent of the new millennium saw multi-probe graphic engine monitors as standard equipment on new aircraft, and Avidyne was setting the stage for an entirely new representation of engine health with its revolutionary Entegra MFDs in the Cirrus SR20.

Going Mainstream
The Cirrus SR20 (and its big brother the SR22, which arrived on the scene a couple of years later) has no prop lever, and power is set using the engine monitor capability baked into the Avidyne multi-function display, not just with the throttle controlling manifold pressure and rpm. Avidyne’s E-Max system actually has a graphic showing percent of power.

With all engine monitors, EGT and CHT sensing lags somewhat behind reality. If leaning is conducted too rapidly, the monitor will sense a “false peak” EGT, as the temperatures will never have time to read as high as the true peak EGT. The correct leaning procedure is to lean quickly to a fuel flow above peak EGT, then lean very slowly, no more than 0.1 gph decrease per second, to allow for a true peak EGT capture.

Engine leaning in general with any monitor will show the leanest cylinder peaking first, followed by the richest one peaking last. In a fuel-injected engine, the spread in fuel flow between these two cylinders ideally should be no more than 0.3 gph (as measured on your aircraft’s fuel flow meter). With a carbureted engine, you may never see a spread that close; the induction system is rarely that precise.

The Entegra, as with the monitors that came before it, will pop up a warning any time a parameter is exceeded, allowing the pilot to use the screen real estate for the moving map and PFD functions during flight (instead of having a display dedicated to engine health). The downside to this setup is that a pilot who never flips to the engine systems page on the MFD during flight and relies strictly on the alarm/warning system may never notice the subtle changes in EGT/CHT that could warn of a developing problem. That’s one reason I like the dedicated engine monitoring displays. (Yes, a knowledgeable pilot can drill down into the configuration menus so that subtle changes generate an alert.)

Insight (considered the father of the light piston-aircraft engine monitor, holding the first patents in 1980) and J.P. Instruments found themselves playing catch-up to Avidyne, Garmin and Chelton, who integrated detailed color CHT/EGT graphics into their MFDs. And it didn’t take long for others to enter the game.

On the experimental-aircraft side, Grand Rapids Technologies’ robust LCD display EIS was soon complemented by its EFIS boxes, which provided color tape displays and auto-alarms. Dynon and Advanced Flight Systems, Stark (MGL) and others entered the market with options that most pilots had hardly ever seen, but all who used them quickly learned to appreciate.

By 2008, Xerion Avionix brought forth the Auracle, a TSO’d color engine monitor display, which raised the bar again with its networking ability and rapid temperature sampling. Though even the most rudimentary engine monitors of the early years provided an RS232 port for downloading and reading engine information on a computer (mostly through proprietary spreadsheet or graphing programs), the Auracle offered pilots the opportunity to see even more data.

Insight and J.P. Instruments picked up the color readout screen idea and ran with it, producing their own more sophisticated boxes in the last few years. The Insight G3 is a much smaller variant on the Auracle theme, sampling the engine’s parameters fast and furiously. Download is facilitated through a simple front-mounted SD card slot.

You Will Be Assimilated
Today, virtually all aircraft come off the assembly line with some sort of digital engine information and monitoring system hardwired in (yes, even LSAs); they’re considered a safety device. That said, there are many older aircraft out there crying for retrofit.

If you own an aircraft still sporting a simple single-probe EGT, or none at all, take our advice: Consider a retrofit. Learn how to use the new, powerful gauge in your panel: Your flight safety (and engine’s health and longevity) will be enhanced. You’ll find the retrofit will pay for itself both by saving you engine maintenance expenses (possibly a premature overhaul) and the ability to precisely and safely operate your aircraft engine in best-economy mode. You’ll also learn to love having one in your instrument scan. Trust me on this—I did.

Amy Laboda is a freelance writer, CFII-MEI and a National Lead FAAst Team representative. She holds an ATP and flies two experimental aircraft: one that’s sweet and slow and one that’s pretty and fast.





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