Video summary

MECHANIC THEORY TESTED: NEVER Flush a 180,000-MILE Transmission? Here’s What Happened

Main summary

Key takeaways

Product Review

Product/Service Reviewed

A transmission service using a flush machine (plus new filter and ATF) on a high-mileage 2009 Subaru Forester automatic transmission with unknown service history.

Key Setup / Vehicle Tested

  • Vehicle: 2009 Subaru Forester
  • Mileage: 178,263 mi
  • Symptoms before service:
    • Harsh shifting
    • Shuddering during upshift/downshift
    • Notably 2nd gear and 3rd gear shuttering
    • “Rumble” / hesitation feeling during shifts

Condition of Old Transmission Fluid (Before)

  • Fluid pulled from the pan was:

    • Very dark
    • Stinky

  • Pan/magnet findings:

    • Metallic debris at the bottom of the pan (sparkly / copperish)
    • Magnet had significant buildup

  • Filter findings:

    • Mesh screen filter packed with debris/particles
    • Also showed copperish metallic residue

What They Did (Service Procedure)

  1. Drain and remove pan/filter
  2. Install new filter
  3. Refill with compatible ATF:
    • MaxLife Multi-Vehicle ATF (claimed compatible with Subaru transmission)
  4. Use a transmission flush machine:
    • They pumped ~10 quarts through the system (after starting with ~2 quarts added to a “bone dry” pan)
  5. Test drive and monitor shift behavior over time

Main Features / What the Service “Does”

  • Flush machine replaces old fluid by pushing new fluid through the transmission cooling circuit/system
  • Combined with a filter change to remove accumulated debris
  • Intended to dislodge debris/sludge and potentially clean up hydraulic/valve body behavior

Pros Observed (Post-Flush Experience)

  • Initial fluid color after flush:
    • “Nice and red” (returned to fresh-looking fluid)
  • Driving results:
    • After ~60 mi: no shuttering, no harsh shifting felt during acceleration tests
    • Later up to ~130+ mi: shifting remained smooth in stop-and-go, highway, and harder acceleration trials
    • At 137.6 miles total: specifically reported
      • No more 2→3 shuttering
      • No slipping noticed during the test period
  • Smell:
    • Burnt smell reduced/changed after service

Cons / Risks Mentioned (From People and Theory)

  • Mechanic-internet caution:

    • Flushing high-mileage transmissions can dislodge debris and worsen internal wear.

  • Theory of potential harm:

    • Debris could affect friction material or valve body tolerances
    • Could cause slippage or harsh/erratic shifts

  • Explicit disagreement shown:

    • Some pros: flushing can make symptoms more pronounced if damage already exists
    • Others: flushing may be a good starting point, especially when fluid is overdue

Comparisons / Opinions Included

“Don’t flush, drain & refill” camp

  • Multiple comments/shops advise:
    • Avoid flushes on high mileage
    • Prefer drain + refill to reduce risk of dislodging sludge too aggressively
  • Logic: if it fails after a flush, it may be on the way out already—but flushing could still be the trigger.

Professional shop viewpoints (called during video)

  1. Shop A (advising against flush):

    • If fluid is discolored and there’s a shiftability problem, fresh fluid likely won’t fix it
    • It can make the issue more pronounced
    • Cautions about wasting $200–$300 on fluid with no resolve

  2. Shop B (supporting flush as a starting point):

    • “We flush lots of transmissions all the time.”
    • Since service is overdue, flush can be a reasonable starting point
    • Some cases may require a double flush (example: a 2010 GMC needed two flushes)

Numerical / Rating Info

  • No star ratings or formal scores.
  • Quantities/timeline used:
    • 178,263 mi starting mileage
    • Flush machine: ~10 quarts exchanged (plus ~2 quarts pre-fill)
    • Test mileage after service: ~60 mi, then ~130+ mi, ending at 137.6 miles
  • No reliability guarantee; outcome is “so far.”

Unique Points Mentioned (All)

  1. Dark, stinky fluid + shift/shutter symptoms suggest internal wear
  2. Flush theory: dislodges debris and can loosen/affect friction/valve body behavior
  3. Symptoms were most noticeable in 2nd and 3rd gear
  4. Internet advice often says don’t flush, use drain & refill instead
  5. Shop A: damage likely already exists; flush may worsen issues; cautions about wasted money ($200–$300 estimate)
  6. Shop B: flushing is commonly done; overdue fluid can justify flushing; sometimes needs double flush
  7. Pan/magnet: heavy debris buildup and metallic residue observed
  8. Filter: significant debris and copperish metallic residue noted
  9. Valve body/solenoids looked “good” during inspection
  10. After flush: fluid looks red; smell improves
  11. Driving test: shuttering/hard shifting stops initially and remains absent during the test window
  12. Fluid on dipstick becomes darker quickly (expected since not all contaminants removed; only 12 quarts flushed so far)
  13. Conclusion: in this case, flush improved shift quality; time will tell; flush is not ideal when transmission is already severely failing (slipping/no engagement)

Overall Verdict / Recommendation

In this one high-mileage Subaru case, the flush + new filter improved shifting and did not immediately cause slipping or harsher behavior across 137.6 miles of testing.

However, the video frames this as a high-stakes gamble: if a transmission is already catastrophically failing, flushing is likely just wasting money, and many pros still prefer drain & refill over a full flush.

Recommendation based on video content:

  • If symptoms resemble shuddering/harsh shifting with overdue dirty fluid and you can service with new filter + compatible ATF, a flush may be worth trying—but it’s not universally safe, and results may vary over time.

Speaker/Role Differences

  • Video narrator/installer/mechanic: conducted the inspection and flush test; reported results and theory; framed it as a “debunk test.”
  • Professional Shop A (called): strongly discourages flushing; believes damage already occurred and flushing may worsen issues.
  • Professional Shop B (called): supports flushing as a reasonable starting point when fluid is overdue; notes some cases might require double flushing.

Original video