1989 suburban coolant temp fluctuation

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by Big6ft6, Dec 30, 2009.

  1. Big6ft6

    Big6ft6 Rockstar 100 Posts

    This is driving me crazy. After sitting outside one really cold night, my truck coolant guage started really acting strange.

    It moves around over a 20 degree temp range back and forth slowly while I drive. Doesn't seem to correlate to any driving type...like it isn't everytime I stop it rises or anything, it just seems to happen steadily whether I'm in stop and go traffic or whether I'm crusing at a constant speed.

    It just slowly climbs..just to the point to make me scared, the slowly goes back down and repeats very slowly.

    The engine is freshly rebuilt with 2000 miles on the rebuild. Cooling system is entirely new at the time of rebuild including gauge sending unit.

    I swapped the thermostat and it had no effect. Anybody every experienced anything like this?
  2. Jacqson

    Jacqson Rockstar 100 Posts

    Is the thermostat you are using around 190 degrees?
  3. unplugged

    unplugged Epic Member 5+ Years 1000 Posts

    The big question is: Is it running hot? Despite what the gauge says check the temp with a thermometer. I use a digital thermometer, but an analog kitchen type will do.

    An engine that is overheating can lose coolant and the temp sensor will not have any coolant to measure. Check to make sure you are not losing coolant. Also make sure you don't have any air in the system.

    If the truck is running in a reasonable temp range, its probably just a bad temp sensor.
  4. Big6ft6

    Big6ft6 Rockstar 100 Posts

    When I swapped in a new t-state I put in a 180. It used to be 195.

    How do you take the temperature with a thermometer? Where do you have access to the coolant? Has anybody ever used one of those laser non-contact thermometers?

    I would really like to do this becuase I could eliminate both bad temp sender and bad gauge. But I can't quite picture where I would stick a thermometer to get an accurate coolant temp reading? Please Advise!

    Last edited: Dec 31, 2009
  5. sysmex

    sysmex New Member

    Don't forget your mechanical fan. You mentioned it happens when you idle or drive at a slow constant speed. Once the vehicle is travelling at 35MPG or more you get plenty of air from the grill. If that is the original fan check it out!

    DAREDEVIL Rockstar 4 Years 1000 Posts

    Sounds like the fan clutch to me !!!! And yes ,the laser things work, just point it at the thermostat housing !!!!
  7. Big6ft6

    Big6ft6 Rockstar 100 Posts

    Hey guys yes this is the original mechanical fan, however the temperature fluctuations happen at all speeds, even highway speeds.

    I'm assuming my fan clutch is o.k. becuase it feels like a hurricane under my hood when the truck is parked and idling. It is also 15 degrees outside here in Wisconsin so I don't feel like the engine temp has any excuse to be climbing!

    Thanks for the ideas, I think I'll get a laser thermometer at harbor freight and see what I learn.

    One thing I'm wondering, would a head gasket problem cause wierd temperature swings if products of combustion were bubbling into the coolant jacket? when I bought the engine the last owner had a buddy bolt on the heads. (supposedly a local race car mechanic) I just trusted they were bolted on correctly.

    This morning on the way to work the truck warmed up normal, up to 180, paused, then continued to climb to 210, then seemed to stabilize around 210 moving just a little below and back up to 210.

    I'm completely guessing on these temperatures too, becuase the factory gauge has only 3 numbered positions.
    The left most position is 100, then 12-noon on the gauge reads 210, then the third position is 260.

    What's wierd is the 210 mark on the gague is the same distance from both the 100 mark and the 260 mark so clearly the gauge isn't labeled inearly becaues 100 is 110 degrees from 210 and 260 is only 50 degrees from 210but on the gauge they are equal distances apart ....wierd.

    I should get an aftermarket gauge and see how accurate this factory gauge is.
  8. kdr358

    kdr358 Member

    When my head gaskets went my temp gauge would bounce and do the same thing but if the motor is new that shouldn't be a problem.
  9. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    The range on the factory gauge isn't linear. That is, as you observed, that the gauge spacings are not equal distances from each other. They probably did this because once you hit 210, accuracy of the exact temperature isn't critical because regardless what the temperature is, it's too hot! So, to give you better precision on the low end of the gauge, they compressed how much one "unit" of gauge represents on the high side to give you less temp per "unit" on the low side. Hope that makes sense!
  10. Big6ft6

    Big6ft6 Rockstar 100 Posts

    What are the chances this is purely a funciton of the super cold temperatures? Is it possible that when I first turn on the engine once the coolant hits 180 the terhmostat opens and really cold water rushes into the engine and makes the t-stat close again?? But now the coolant temp rises more quickly before the t-stat has a chance to fully open so I get spike on the temp gauge? then the t-stat opens cold water rushes in and it repeats?

    I don't know...it still seems the coolant temp should never go above my t-stat temp. I pray that this is either a funky gauge issue or a funky sending unit.

    If this is a head gasket I'm going to die....what about a bad water pump? Not long ago I pulled the radiator cap off and could see a good stream of coolant coming from the heater core return hose. It seemed a pretty strong stream, so I assumed the water pump was fine, but maybe it is weak? I just bought a cheap one when I put the engine in.
    Last edited: Dec 31, 2009

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