Weird Temperature Fluxuation

Discussion in 'The Big Chill' started by FLGS400, Jan 28, 2022.

  1. FLGS400

    FLGS400 Gold Level Contributor

    Ok, so... I searched really quick for this and didn't see it anywhere, so I thought I would check with my Buick family and get some opinions. Lately, I've noticed my AutoMeter mechanical temperature gauge doing some strange, random things. I seriously doubt it is a problem with the gauge.

    Sometimes when warming the car up, the gauge will go from 120 (lowest reading) to 200+ really quick, then drop to 160-170. I don't find this part too odd. When driving though, as long as I'm idling at a stop or when I'm on the throttle, it stays between 160 and 185. When I let off the throttle, like coasting up to a red light or stop sign, the temp will shoot up to 200+, then drop back to 160-185 again when I stop or get back on the throttle.

    Car has the original 400 in it (rebuilt 20K miles ago), mild KB cam, Performer intake, Quadrajet by Cliff Ruggles, 32 degrees total timing (12 initial and 20 mechanical in by 3000 RPM), 10 degrees vacuum advance on ported vacuum, new fan clutch, 160 Stant thermostat, and Cold Case radiator. The stock fan shroud is still intact and the stock fan itself is in great condition. I have yet to change the water pump.

    I'm thinking that the thermostat is cycling closed and causing the temp to shoot up really quick like that. I just hate to take it apart again, this will be the 4th time (first 2 times I had leakage problems around the bypass hose area). I just find it strange that the temperature only shoots up like that when I'm coasting.

    Anyone else have this issue? If so, how did you fix it?
     
  2. TrunkMonkey

    TrunkMonkey Well-Known Member

    Some gauges that have fast reaction/highly sensitive will do that.

    Temp from cold start, thermostat is closed, block heats fast, temp rises to about 10-15 degrees above thermostat rating, and it takes a minute for it to fully open. Water starts flowing and the temp drops to "normal". It may then do smaller version of the same thing over the next several minutes as the coolant flows through the radiator and the cool water takes heat from the engine.

    The "coast" temp rise is often due to the heat soaked coolant and the rpm suddenly drops, and the pump slows, to the coolant has a bit of hydraulic back pressure. The slowed pump acts as a hydraulic "snubber" and that is also normal.

    As long as temps don't get over 220-225 for more than a few seconds, I would not worry about it.

    Another thing could be air trapped, do you know how to "burp" the cooling system?

    (Normally, a good cap and proper overflow tank filled about 1/2 to 2/3 full will take care of elimination of air after a couple heat soaked drives.)

    Just keep an eye on things.

    (you could run a combustion gas test on the cooling system to rule out head gasket, cracks or other issues, if you still have concerns)

    A lot of cars that are running water, or have a bad pressure cap can mimic some of what you have. If you have another cap to try and see if it makes a difference.
     
    FLGS400 likes this.
  3. FLGS400

    FLGS400 Gold Level Contributor

    Thanks TrunkMonkey, I didn't think about that. I put a new cap on it with the radiator, just can't remember the brand. I know it was not the one that came with the radiator. I'm also running 50/50 mix of green antifreeze and distilled water.

    I could snag the cap off of my Malibu and try it, tomorrow.
     
    TrunkMonkey likes this.
  4. TrunkMonkey

    TrunkMonkey Well-Known Member

    I use Stant "saftey lever" caps.
     
  5. FLGS400

    FLGS400 Gold Level Contributor

    I have to run some errands tomorrow and may take the Buick (if it's not raining). If the cap from the Malibu works out, I'll just swing by O'Reilly or Advanced and see if they have one of those.
     
    TrunkMonkey likes this.
  6. FLGS400

    FLGS400 Gold Level Contributor

    Well, the radiator cap from the Malibu did not make a difference. I may pick up a 13 lb. Stant Safety Lever type cap and see if that helps. I noticed yesterday that I'm also getting the coolant seepage again, building up on the lower front portion of the intake manifold (Performer). I may have to pull the thermostat anyway and reseal for the 4th time...
     

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