235 Chevy Thermostat Shoots up to over 240 and then back to 180 Deg

Discussion in '1947-1954' started by mockoski, Nov 1, 2019.

  1. mockoski

    mockoski Member

    Jul 14, 2014
    Hey guys,

    I recently installed a heater in my 53 Chevy Pick up. I drained the radiator, and installed a new 180 deg thermostat.

    Since then, the org. gauge shot up over 220, and eventually broke the gauge. I installed a temporary gauge and see that when I first run the engine it shoots up past 240 degs and then will shoot back down to right around 180. While driving it will go up to about 220 before the thermostat opens up.

    I first thought it might be an air pocket, but I did burp the system ( at least I thought I did.. by leaving the cap off until I got some fluid coming out of the top radiator then placed the cap back on). But the temp gauge still shoots before the Thermostat opens up.

    Also, the thermostat that I purchased already had a pre-drilled hole in it (with a little brass jiggly thing in it), so I thought that hole would be enough. Perhaps another hole?

    Anyone have thoughts about what would be causing the thermostat to not open up until the temp gets much hotter then it should?

  2. Chiro

    Chiro Member

    Sep 3, 2006
    Long Island, New York
    Could just be a bad thermostat. I had some issues with one on my flathead V8. Take the thermostat out and test it on the stovetop with a pot of water and a thermometer. Replace it as needed. The pre-drilled hole with the jiggly thing is adequate to burp the system. I drilled 1/8" holes in my thermostats for the flat V8. Replaced them both recently.

  3. RidesWithYah

    RidesWithYah Member

    Jan 10, 2011
    Lexington KY
    Stovetop test the thermostat (as Chiro said) is a good idea;
    you could also try running the engine without the thermostat and see if it behaves the same.
    I have read of some people enlarging that little bypass hole, but can't swear to the results.
  4. The Red Truck

    The Red Truck Member

    Dec 25, 2019
    Out in the shop
    I just finished a coolant/thermostat swap last week on my '52 235, for good measure it's been a while. Thought I'd chime in.

    My preferred method to refill coolant is to fill block via the thermostat housing before placing thermostat. That way I know the TS is wet. Typically TS's will not operate (well) with steam. They like to be submerged. This is particularly important on cars that I have that have remote reservoir caps.

    I typically drill a 1/16" hole in flange to help things along. Then go so far as installing TS side of upper hose and filling from hose to insure both sides wet. If I have plenty of beer in the fridge I'll even let rest overnight and/or pressure test the system to help get things sorted out/check for leaks. I've found that pressurizing on/off will push bubbles to different places on hard to bleed systems.
    In your repair I would have even used a funnel to prime the heater hose to make sure that system was confirmed full. Run until TS opens using a IR gun, check level again tomorrow after a full cool down.

    If none of that works I'd start checking coolant routing with a IR gun for possible flow issues, run heater wide open when you see it start to rise to see if your getting heat and when, that should help some because doesn't sound like coolant getting to radiator soon enough. TS not opening or opening late or even see water pump impellers eaten off causing flow issues.

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