Temp gauge reading strange

Discussion in 'General Maintenance/Repair Questions' started by FuzzDaddy, Jul 12, 2020.

  1. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Hi everyone!

    I still have the boat home doing maintenance things. When I run the boat with the hose muffs on the temp gauge stays at 100 with the engine running. After I run it for 5-10 mins it still says 100. I turn the key off and back to the on position without starting the engine and it shoots up between 150 and 170. If I start the boat the temp gauge moves to 100. Why does the gauge only read when the engine is off with key in the on position?
    Thanks for your time!
     
  2. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Here is a pic of the temp gauge
     

    Attached Files:

  3. Chip S

    Chip S Active Member

    179
    Jun 17, 2019
    Bordentown, NJ
    1993 Sea Ray 200 Overnighter
    1993 Mercury 150 hp Black Max Outboard
    Water from a garden hose is typically pretty cold. I've never been able to get my engine up to temperature running on a garden hose.
     
  4. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Hi Chip,
    So basically the garden hose is to cold when running through the engine. So when the engine is off the water pump stops and just heats up the water in the block on its own?
     
  5. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Should come up to temp on a hose. What T-stat is in it ?
     
  6. Chip S

    Chip S Active Member

    179
    Jun 17, 2019
    Bordentown, NJ
    1993 Sea Ray 200 Overnighter
    1993 Mercury 150 hp Black Max Outboard
    That's been my experience. On the hose my engine runs around 105 but in the water it runs around 160.
     
  7. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    I’m not sure. I didn’t take it apart to check. Since it reads with the engine off key on could it be a wiring issue? Or is it normal for the engine to spike when you shut it off? Could it be possible the thermostat Is backwards?
     
  8. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    More likely, you have no t-stat If you have a stat no water flows through the engine until it opens. So cold hose water has nothing to do with it.
     
    Chris-380 likes this.
  9. Chip S

    Chip S Active Member

    179
    Jun 17, 2019
    Bordentown, NJ
    1993 Sea Ray 200 Overnighter
    1993 Mercury 150 hp Black Max Outboard
    It's normal for the engine to spike when you turn the engine off. That's why it's recommended to idle for a short time before shutting the engine down.
     
  10. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    All good points! Also maybe I don’t have a thermostat? Is it ok to run without one?
     
  11. R-and-R

    R-and-R Active Member SILVER Sponsor GOLD Sponsor

    163
    May 9, 2018
    Safe Harbors Narrows Point - Kent Island
    2002 340 Sundancer
    8.1S Mercruiser
    Seems likely that your t-stat is stuck in the open position. Not a huge problem, but worth fixing.
    Cheap and easy to replace.
     
  12. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Engine runs more efficiently at 160. You need your oil to get hot to burn off condensation that accumulates
     
  13. Soulshine

    Soulshine Active Member

    589
    Sep 25, 2016
    Seneca Lake, NY
    '83 SRV 360 Express
    Twin Merc. 454
    Running without a thermostat means you’ll be running at far below normal operating temp.

    That can create a rich fuel mixture condition, carbon buildup in the cylinders and fouled plugs.
     
  14. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    I did notice some water in the oil
    Maybe it’s condensation from the engine not running up to temp?
     
  15. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Yup, get a stat and a gasket. Easy fix.
     
  16. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Well I just pulled the thermostat housing and bam!!!! No thermostat! I’ll pick one up tomorrow. I know I’m not in the clear with the water in the oil but I sure hope installing a thermostat helps or solves the problem!!
    Job well done everyone.
    Thanks for the help!
     
  17. Soulshine

    Soulshine Active Member

    589
    Sep 25, 2016
    Seneca Lake, NY
    '83 SRV 360 Express
    Twin Merc. 454
    You have to wonder why there’s no thermostat in there.

    The reason I mention this is my last boat ran so cool when I bought it that the gauges didn’t register anything at all. They tested as functional though so I looked further and found no t-stats.

    Soon after installing them I had overheat issues caused by obstructed manifolds. The manifolds were snotty looking and replacement was planned from the get go.

    The PO took the t-stats out to keep it cool.

    The carbs were leaned out to compensate too. It seemed like just doing the manifolds was too much trouble for him so he employed many layers of repairs to avoid it.
     
  18. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Oh boy. Maybe I’ll get lucky. I’m glad to hear that be be my issue with water in the oil though. Are there different temp thermostats or only one? I’ll get a thermostat and see what happens. What are the normal recommend temps running with a thermostat?
     
  19. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    Usually 140-160°
    Depends on the gear of your motor. Check your motor number against the mercury online parts Manual.

    For salt water cooled 160° max as the salt crystallizes more easily at temps higher than that.
     
  20. FuzzDaddy

    FuzzDaddy Member

    37
    Jul 8, 2020
    2002 Sea Ray 230 Signature
    5.7 Mercruiser
    Thank you very much for all the info. Since I mentioned I had some water in the oil which could be because of not having a thermostat, how much condensation could be caused by running at lower temps without the thermostat? I’m probably making a big deal about nothing but I thought I should ask. I would like to know so I can keep a eye on it.
     

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