In recent weeks, engine maker CFM instructed several airlines including American and Southwest to inspect a total of 25 engines on their grounded Boeing 737 Max fleets. The order follows a March 26 incident involving a Southwest Airlines 737 Max 8 plane where pilots had to shut down an engine shortly after taking off from the Orlando International Airport.

Sources say the investigation into that incident revealed signs of "coking" around at least one fuel nozzle on the 737 Max's CFM Leap 1B engine. Coking, a byproduct of combustion, essentially involves a tar-like gunk that can build up. According to the FAA, coking can restrict fuel flow to some nozzles and causes greater amounts of fuel to flow into nozzles empty of the tar, thus creating a dangerous and uneven heating in the engine.

Those initial findings prompted Southwest Airlines to inspect 12 engines from its fleet of 34 Boeing 737 Max jets parked in Victorville, California since the worldwide grounding. Southwest said in a statement its maintenance team completed those inspections on April 10 and has submitted the results to the engine maker, CFM.

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The inspections were described by a source familiar with the situation as "exploratory in nature."

On March 26, a Southwest crew was ferrying a 737 Max 8, without passengers, from Orlando to Victorville for long-term storage. Shortly after takeoff, pilots experienced what was described at the time as a "performance issue" with the No. 2 engine, leading to an engine overheat. The plane returned to Orlando and landed safely and the incident is still being investigated.

Southwest Chief Operating Officer Michael G. Van de Ven said, "The working theory on that particular airplane was that there was coking around the fuel nozzles and it created a variance in the hotspots and cold-spots in the engine."

Southwest CEO Gary Kelly told an analyst he believes this is a "break-in issue" with the new CFM engines.

"It's not unusual for an engine to have some break-in things happen and the engine for the most part has performed in line with our expectations, especially with the fuel efficiency," Kelly said. "It's a great quiet ride and it's a good engine."

American Airlines said it "conducted a borescope inspection of three aircraft engines, at the request of the engine manufacturer, without any findings." CFM requested American change nozzles on two engines despite the lack of findings.

Engine maker CFM is a joint venture between GE Aviation and Safran Aircraft Engines.

"We're being very proactive with all the LEAP-1B operators during the grounding to ensure a smooth reentry into service when the time comes," GE Aviation Spokesman Perry Bradley said in a statement. "The activity centers on providing proactive guidance and recommendations to properly preserve the engines, complete any outstanding inspections and to pull forward any scheduled maintenance activities or Service Bulletin recommendations."

This is the second maintenance issue to make headlines for Boeing recently. During a shareholders meeting in Chicago Monday, Boeing admitted an indicator light that could have alerted pilots to incorrect Angle of Attack (AOA) sensor readings that trigger the MCAS anti-stall system to override pilot controls was supposed to be standard feature but was not operational on all 737 Max jets "as intended."

This admission came as a surprise to federal regulators. The failure to activate the AOA disagree light on all 737 Max aircraft will now be part of a review that began on Monday focused on the approval of the plane's automated flight control systems.

The faulty AOA sensor readings and MCAS system are being investigated as the causes of two Boeing 737 Max crashes in recent months, an Ethiopian Airlines flight in March and an Indonesian Lion Airlines flight in October, that resulted in 346 total deaths.

An earlier version of this story did not include the addition of the statement from GE Aviation Spokesman Perry Bradley.