Date: 202106
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: 10
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Powerplant Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Troubleshooting; Time Pressure
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Fuel Issue; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate; In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Gate; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; MEL; Procedure; Staffing
Primary problem: Aircraft

Narrative: All events through pushback were accomplished without note. During pushback; a puddle on the ramp aft of the normal oil spot was noticed behind the #2 engine. Puddle was not present on the walkaround. A stream of drops along the ramp was also noticed while the #2 engine was run. After five minutes the fuel leak had not stopped and we towed back to a gate. Maintenance was called; ran several tests; and two engine runs. The leak dissipated as the tests were completed. No fuel appeared to be leaking while engines were running; except for one within limits from the heat exchanger. Forms were completed to indicate it. Aircraft pushed and started again. No leaks were noted.Shortly after takeoff/cleanup; Captain announced a split in the thrust levers. Autothrottles were used for takeoff with VNAV (climb page 2L adjusted for 202 knots at 1;000 feet AGL). Looking at the engine instruments; I noticed the #2 thrust lever about three knob widths ahead of the #1. #1 N1 was at commanded power with the #2 N1 showing the white commanded power bar of about 95%; but the engine only producing about 88-89%. Further advancement of the throttle lever produced no increase in power. Vertical Speed was selected and we reduced our climb rate to bring the power back. As the power came back; the throttle synced and power matched. VNAV was reselected and as the power returned to full climb; the split reappeared with the #2 indicating a commanded increase without the engine delivering the power.As this was being discussed; the center tank fuel pumps were turned on (750 pounds in the center tank). Immediately after the pumps were turned on; the power matched and all indication were normal. As soon as the center tank fuel was exhausted; the split returned. Crew leveled at FL 270 and elected to return to ZZZ. As power was less than full climb at cruise/descent; and no abnormal indication were present during these flight conditions; no QRH Checklists were run. The rest of the return to ZZZ was completed without note. Engine performed as expected from level off to shutdown.Contract Maintenance expressed an increased level of calls from aircraft coming from the paint shop; fuel leaks were common with these aircraft.  Would recommend a Maintenance individual check and run engines prior to returning the aircraft to the line.


Second narrative: Origination flight on an aircraft that had just come out of the paint shop. Preflight was uneventful with no fuel drips or puddles noted. We pushed five minutes early. On push back #2 was dripping fuel and did not stop after a short time and engine was shutdown. After consultation with Maintenance Control; we returned to the gate and called Contract Maintenance. Contract Maintenance did four motoring tests as directed by [company's] Maintenance. With each test; the dripping became improved. Then an engine run was completed with decreased dripping; but not to a small enough level to allow Dispatch. Contract Maintenance then opened the cowl and did an engine run to determine location of drip. After that run; the component dripping was identified and the dripping had decreased to an amount that allowed MEL. MEL was completed. This was about a 2.5 hour process. Ground Ops and Operations were very helpful with accommodating the Passengers as well as they could. I had given periodic in-person updates to the passengers. Finally; we pushed-back and the engine starts were uneventful without any drips. Uneventful taxi. We took off on Runway XX with a headwind 20G30. On push up of thrust; #2 lagged a bit and I pushed the thrust lever up to get closer to match and continued my crosscheck.  As we approached V1/VR I noticed #2 still lagging by a few percent and pushed up the throttle some more. Normal on centerline liftoff and climbout. After liftoff; I informed the F/O  (First Officer) that we had a split throttle situation. We cleaned up on schedule and began to evaluate the situation (again the aircraft had no adverse flight characteristics). With the aircraft cleaned up and climb power set; we observed that although the #2 demand pointer was correct; the #2 N1 lagged approximately 2-5% N1. Once our climb rate was reduced in Vertical Speed and at a slightly lower climb thrust setting; the engine was normal and tracked normally with the demand pointers and thrust lever position. To this point we had continued to follow ATC instructions to keep it simple. We found that the engine reacted the same with the autothrottle On or Off. Also we had 750 pounds of fuel in the center tank; so once established in the climb we turned the pumps on and immediately #2 N1 returned to normal operation at full climb power. When center tank was empty and pumps turned off; the malfunction returned. With this information we leveled off and informed ATC that we would probably need to return to the airport. While level we reconfirmed no other abnormal conditions or switches out of place. Also at level flight in this situation there were no conditions that warranted a QRH Checklist although it was discussed. F/O and I discussed the situation and decided that without the aircraft able to achieve full climb thrust on Main Fuel pumps only; the safest choice was to return to the airport. F/O flew and talked to ATC; while I informed flight attendants and passengers of need to return to the airport for safety reasons and gave them our ETA. I called Station Ops on VHF and informed them of our need to return; get a gate; and asked them to contact Dispatch. We also confirmed via ACARS with Dispatch the need to return due to engine unable to achieve full climb power (or something like that). The F/O did a very good job and stayed ahead of the aircraft preparing for the briefings; descent; and Approach Checklist. We referenced the Diversion Checklist to ensure we had covered the suggested items. We accomplished a normal descent and asked for a long visual approach to increase probability of a stable approach and landing without the need for a thrust needful go-around. F/O flew stable approach and landing with normal braking. No emergency was declared; no special handling requested; and a normal weight landing occurred. Uneventful taxi-in and shutdown. Flight was canceled and the passengers accommodated as best as Ops could. Debrief with the flight attendants yielded that although disappointed in their flight being cancelled; they were appreciative of the Safety of turning back. Immediate notification with XXXXXX was done after engine shutdown with Dispatch; Supervisor; Maintenance; and Chief Pilot.The Location X crews both above the wing and below the wing were very familiar with the Maintenance trouble shooting of engine drips on starts because they have seen it more the once from aircraft coming out of the paint shop.  An engine run after paint shop would have found this problem before a revenue flight was disrupted.  Maybe this may be a prudent practice--an ounce or prevention may have saved a pound of Customer frustration.  Perhaps a faster crosscheck by myself might have allowed a reject option in the 70-90 knot range; but even then there was no decrease in power and I perceived the N2 engine thrust close-enough to N1 engine thrust to ensure a safer option than a medium speed reject.


Synopsis: B737 flight crew reported an air turn back after 3 knob throttle split developed in climb with a lack of power on an engine following a 2.5 hour maintenance delay for a fuel leak issue.