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  1. #1
    Member
    Join Date
    Jun 2020
    Location
    Edgewater, MD
    Posts
    3

    200 EFI RPMs jumping up and down

    Hi all,
    I have a 1998 Merc 200efi- newer powerhead with less than 40 hours. Recently I have been having some issues starting and with RPMs jumping around. Not positive, but thinking the two are related.
    When starting, it seems very finicky with the throttle position...it will crank but not start when throttle closed. If I have the throttle wide open, it will crank for a second then stop (safety/rev limiting feature?) I have to start closed then quickly open er up to get it going.
    Once started it will stay running...but when I get above 2k RPMs, the RPMs start jumping all over the place. ~1000 RPMs swing in either direction without me touching the throttle. Doesnt get better once warmed up. To be clear- it sounds good at these varying RPM levels- not stumbling or dying...the issue is why is it jumping around? Also noticing a lot of blue smoke throughout...running rich?? Here is what I’ve tried so far:
    - tested for good spark
    - fresh fuel w/ ethanol treatment
    - disconnected vacuum hose from MAP sensor and started-no change
    - disconnected Intake Air Temp sensor- no change.
    - disconnected port side head temp gauge- couldn’t start
    - disconnected Throttle Position Sensor- started, but same problem was worse, more RPM swings.


    For this reason, I am thinking it is related to a faulty TPS. Perhaps this is causing the issues starting as well?? I do not have a diagnostic tool so looking for other alternatives. I saw a procedure out there for testing TPS- anyone have pictures?


    thanks,
    Pat

  2. Member
    Join Date
    Oct 2015
    Location
    Hixson, TN
    Posts
    177
    #2
    You'll need a TPI test harness and a digital VOM meter. If you don't have the OEM service manual, get one. There's probably a you tube video on this proceedure. TPI adjustment is critical to rich or lean start-up. They can go out of adjustment. The TPI for your motor is very expensive if it needs R&R. Tachometer- If you have the dual 20 amp regulators- Try swapping the grey tach wire from one reg to the other. This is how the tach gets its signal. You could also have a CDM coil cutting out but from what I read, it's just the tach or tach signal bouncing around.

  3. Member
    Join Date
    Jun 2020
    Location
    Edgewater, MD
    Posts
    3
    #3
    Thanks silveralon- I will look into the TPI test harness. On the RPMs- I don’t think it’s a tach issue, the RPMs trulY are jumping up and down as I can hear the engine revving and then coming back down. Not sure if maybe it would be any better under load, and maybe the fact I am revving in neutral is causing issues...