Mercruiser 8.1s Reving up and down

Discussion in 'General Maintenance/Repair Questions' started by wakerider09, Aug 14, 2020.

  1. wakerider09

    wakerider09 New Member

    Mar 26, 2014
    Windsor, Ontario
    340 Sundancer(2005)
    T-VD 8.1s Horizons
    Hi all, I have a 2005 Merc 8.1s that idles a little rough, but fairly ‘normal’... when I rev it up just a little it revs up and down up and down up and down.... pretty consistently.... for example 900rpm to 1200rpm; 900rpm to 1200rpm over and over.

    I replaced the IAC valve and it made no difference.... I’ve had a mechanic test the following and all check out good:

    compression test - OK
    Fuel pressure test - OK
    Spark test - OK
    Injector drop test -OK
    Fuel filter - changed

    The engine itself is fine, what am I missing? Could it be the Throttle position sensor??
  2. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Had a similar issue with my 502. Replaced my injectors for the fix.

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