help!!! replaced the thermostat and now the engine is overheating

**88burb** · 10-25-2009, 05:17 PM

So I replaced the old 195 termostat with a brand new one from autozone and now things are going haywire. I should mention with the old thermo the engine never hit 210 ever. When we put the new one in it warmed up as usual but then it overheated so the thermo didn't open. Then we turned it on after it cooled off and the thermo immediatly opened and it went down to the first hash mark. just sitting there it bounced between the first and third hash mark. But when I went to drive it overheated quickly with the heater on. Now i'm stumped as to what is happening. I changed the thermo because my heater wasn't heating up much at all, I thought because I took off the broken AC comppresser. An ideas as to what is wrong with my truck?

**unplugged** · 10-25-2009, 05:46 PM

Welcome to the site. It helps if you take a minute to let us know what truck/engine/trans/accessories/mods you have so we can be of more help.

So it wasn't overheating and you changed the thermostat, and now it is overheating?

Check to see if the tstat is working. Take the tstat out and drop it in a pot of boiling water.
It should open.

One common mistake is putting the tstat in upside down. The pointy end faces up.

You took the AC compressor out? Do you have a truck with the serpentine belt or the older v-belt?

If you have the serpentine belt, did you route it to make sure the water pump is turning in the right direction?

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**vncj96** · 10-25-2009, 10:59 PM

I would start with the correct install of the tstat, and even though it is brand new they can fail easily and quickly. Was this a guess that the tstat was a problem and maybe it wasnt the fix, you may need the radiator and coolent lines power flushed. Or your radiator is failing or you could have a blown head gasket, there can be many reasons for your symptom

**brianj** · 10-26-2009, 11:01 PM

It also sounds like you might still be air locked from opening up the cooling system. Definitely check the thermostat is installed correctly and you have the right rotation first, and if they are, check to make sure the coolant is actually flowing. Sometimes these things can be a real pain in the **&! to get the air out of, and the pump just sits there and cavitates.

**Mr Dawg** · 11-02-2009, 11:04 PM

I just had that same problem today. First I had the water pump replaced and it idled fine. Drove a couple blocks and it was back to overheating. I called my mechanic and he said he had a 5.3 sierra that did the same thing and after the customer replaced the water pump, t-stat, and fan clutch it turned out to be the radiator. Swapped out my radiator and its back to running normal now.

**vncj96** · 11-03-2009, 08:57 AM

you can also start the truck up and then remove the radiator cap and that can help get some of the air out of the system, if there is any, you will see some bublles popping up.