Question for any of our auto mechanics

Discussion in 'The Okie Corral' started by ustate, Mar 3, 2010.

  1. ustate

    ustate NRA Member

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    On my truck sometimes the engine temp gauge will be reading the normal temp and then it will drop down to where it would be when the engine is turned off, other times it will float around between the two places. I figure the gauge is going bad but I'm just wondering what I'll need to do to fix it. The thermostat was replaced about 8 months ago, is that possibly going bad maybe?
     
  2. TomZ

    TomZ Lifetime Member

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    1. Temperature sending unit (easy)
    2. Wiring to cluster/gauge (depends on your skill level, cheap)
    3. Cluster/gauge (pricey)
    4. Bad ground (skill level, 0 cost)
    5. T-stat (cheap but not likely with fluctuations)
    6. Clogged radiator (pricey)
    7. Bad coolant flow due to pump/belt (can 'o worms: acessory belt driven, cheap. Timing belt or chain driven, pricey)
    8. Radiator cap (shouldn't cause fluctuations, cheap)
    9. I'm assuming you checked the coolant level and have no more than a 60% concentration of coolant to 40% water. Don't laugh, you would be suprised how many people use 100% coolant then can't figure out why their vehicle overheats.
     
    Last edited: Mar 3, 2010

  3. zeke66

    zeke66

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    Your sensor (sending unit) for water/engine temp may be going bad. Could be in any of number of places, depending on the make/model.
     
  4. ustate

    ustate NRA Member

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    Thanks for the pointers so far....I should have mentioned that its a 1998 Nissan Frontier 4x4.
     
  5. Diesel_Bomber

    Diesel_Bomber

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    Can rule out thermostat if you have a non-contact thermometer. Get a full operating temp reading of several places around the engine, then wait for the temperature gauge to drop and take another set of readings. Or just feel the radiator hose.
     
    Last edited: Mar 3, 2010
  6. sputnik767

    sputnik767

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    My vote is sending unit or the actual gauge. My Tacoma had a similar issue, except it was with the fuel gauge and one of the instrument cluster light bulbs. Basically, very occasionally, my fuel gauge simply dropped to E, or one of the dash lights went out. The problems would fix themselves after a day or so, but banging on top of the dashboard a couple of times solved both problems. Once I did that, problems never came back.
     
  7. chris in va

    chris in va

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    Agree, it's an electrical issue. Clean the contacts for the temp sensor on the thermostat. Does it happen quickly or a slow drop to 'cold' and a gradual warmup?

    I like the suggestion about feeling the top radiator hose too, just be careful.
     
  8. erick73

    erick73

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    Air pocket maybe?

    Was the coolant system properly bled after changing the thermostat?
     
  9. elsolo

    elsolo

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    Most electric gauges are just voltmeters.
    The "sender" is a variable resistor that returns 0-14 volts back to the gauge.

    Remove the temp sender, get an ohmeter, and measure the resistance at room temp and when the sensor is in boiling water.
     
  10. KusoJijii

    KusoJijii alegacy of fear

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    What does the actual temperature of the air coming out the vents do when the gauge acts up? That would be your indicator to if this is a mechanical problem in the cooling system (low coolant, bad t-stat) or electrical (bad temp sender, wiring intermittently shorting to ground, or bad gauge)

    Usually a plugged radiator won't cause vast fluctuations it will just reduce it capability to cool making your engine run hotter and a plugged heater core will most likely just cause fluctuations in the vent temp with no other symptoms on the vehicle.

    If the fluid level is low you need to figure out where it is going, internal or external leak.
    (never remove the radiator cap when engine is hot!)
    If you have a single terminal temp sender you can check the gauge by unplugging the sender connector, hooking up a fused jumper wire to the wire (going to the gauge) and put the other end on battery ground. The gauge should go all the way down. Then move the fused jumper wire to battery positive and the gauge should go all the way up.

    *Make sure you are on the correct wire, that it is a single wire sensor, and that your jumper wire is fused with a low amp fuse (just in case there actually is a short to ground in the circuit).

    If you don't feel comfortable doing any of this, then take it to a trusted repair shop.