Brand new thermostat not regulating temp properly, overheating

Discussion in 'General Chevy & GM Tech Questions' started by RichardStarrbuck, Nov 21, 2013.

  1. RichardStarrbuck

    RichardStarrbuck New Member

    98 Chevy Tahoe 4WD 5.7 350

    My truck was running cold, was not generating heat, I pulled the thermostat to find that it was stuck open. I replaced it with a new one, filled up my coolant and went for a drive. It worked perfect for the first 20mins, I had heat and it was regulating at about 200 degrees.

    Then all of a sudden my temp gauge started fluctuating, getting really hot then cooling down, it stayed mostly around 260, it was over heating pretty bad.

    I popped the hood and saw that coolant was leaking around the main hose that connects to the thermostat housing, the hose had so much pressure built up that I could barely squeeze it with my hand. There was no hole in the hose, it just had so much pressure built up that it was pushing past the hose clamp.

    That lead me to believe that the brand new thermostat failed and was stuck closed. So I went and bought a new one, replaced it and it did the same exact thing.....worked great for about 20mins then failed.

    I also checked the 1st new thermostat I bought by boiling it in water and it seemed to be working fine.

    Im not sure what is going on. What else would cause a thermostat to not regulate properly after 20mins of driving??

    The water pump and radiator were both recently replaced about 6 months ago. There is no leakage on the inside which leads me to believe that the heater core is ok. Ive also let it run with the cap off to make sure there was no air lock.

    Im stumped.
  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Maybe the t-stat is in upside down, but, I suspect you have some air.
  3. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Possibly air in the system, in the heater core.
    Or you may have a bad radiator cap, should be allowing pressure release.
    Maybe a combination of both cap and air in the system.
  4. RichardStarrbuck

    RichardStarrbuck New Member

    I think it only fits one way. The domed end is pointing up. Even if it was in upside down why would it work normally for 20mins then fail?

    Ill try to bleed the air this weekend, Ive seen a few tutorials on doing that. Honestly I dont think thats the problem though.

    The radiator cap is just a normal cap, its not spring loaded, its not a pressure release cap. If pressure builds in the radiator it is supposed to overflow to the resivior, which it is doing because there was coolant on the wheel well just below the resivior overflow tube.

    Thanks for the tips. If I figure it out Ill post back here. : /
  5. poncho62

    poncho62 Active Member 2 Years 100 Posts

    The spring side of the stat goes toward the engine....the pointy side to the rad.

    If the stats are working right, for sure you have an air lock. When filling it, put some coolant in, start the engine and let it warm up with the rad cap open. When its warmed up, slowly start adding more coolant. As you do this, yoiu should see the coolant circulating in the rad. Slowly fill it up. Put cap on and you should be good
  6. zippstripp

    zippstripp Member 1 Year

    what condition is the coolant in? Is it clean or rusty yuck? Could be the water pump not circulating enough the move the coolant. Have seen the impellor come loose on the shaft or corrode away all together. Just an idea,
  7. RichardStarrbuck

    RichardStarrbuck New Member

    Thanks for the help guys. It was an airlock. Ive had Chevy trucks since the mid 80's and have always done most of the repair/maintenance work myself. I have never had an airlock before, its really weird how a a little air in the coolant system can completely disrupt the entire system.

    So heres what I did: For an entire day I ran my truck with no thermostat, completely took it out and let the system run free. It all worked fine, the truck ran at about 150 degrees, never over heated, as it should with no thermostat, of course I had no heat either. So in doing that I was thinking the flow is fine, there arent any clogs, no it has to be airlock.

    Now my way to fix it is probably very unconvetional and not very efficient but heres what I did. First I put the new thermostat back in, I also replaced my thermo-housing hose and clamps. The hose was not colapsing on me, but it was a little worn so I just went ahead and replaced it on general principle. So I started the truck with the radiator cap off. I let it run, it heated up and regulated normal operating temp for about 15mins. Then it started overheating, it overheated and cooled it self down a few times on its own, I was still adding coolant at this time. Then as it started to fill it naturally started over flowing because of the pressure that was building. This happened a few times, I wasted a lot of coolant, it made a mess but in the end it worked out all the air.

    Now shes running great, no overheating and I have plenty of heat.......just in time too, it was 25 degrees here in MD yesterday.

    BTW, just a little gloating/proudness, my 98 Chevy Tahoe has 311K original miles on her and still running strong. Like a rock!

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