I just recently bought this 2005 Monte Carlo, the owner said it was overheating and it was just the thermostat. So the same day I purchased, I replaced the thermostat. I drove 80 miles with no problem of over heating. Once I hit 80 miles, the car rapidly overheated. I pulled over, popped the hood and looked under the car for any leaking. There was a small puddle on passenger side behind the motor, (under passenger side door). So I let it cool off and drove to the nearest gas station and by then, it had overheated again.

I let cool, the open radiator cap, and tried to put water in, but it didnt take much at all. So I sat and let cool. I drove about a half a mile and it over heated again. So I had the car towed to my house. We replaced the thermostat again because the top radiator hose was hot to the touch and the bottom hose was cold. Bleed the lines and let run for 20 minutes without any problem or overheating. So my mechanic left.

The next morning I started the car and watched the gauge and within 5 minutes it overheated again.

I checked the water level and it is full. No leaking underneath or in any visible parts. The old owner did tell me the heat doesnt work, so i assumed a heater core and I could change that before winter, and with it being summer I wasnt gonna worry about it.

The car runs great, no check engine light, no smoke, runs smooth like a brand new car, except the overheating. I did notice the water boiling the first day within 5 minutes of starting it after my mechanic left.

Any ideas would be much appreciated. I have started the car since then and everytime it overheats the top hose is hot and the bottom hose is warm.

 

is it possible that the water pump isnt doing its job, thats my opinion especially if u replaced the thermostat and what not

 

That has been a thought of mine. But no weeping from weeping hole.

 

First, I don't know which engines GM offered in the 2005. Do you have a 3800 (with or without super charger) or a 3400??

Also, regardless of engine, do a loan-a-tool from the parts store and get the radiator pressure tester. With the car off, use that to put pressure in the system. Look and listen for leaks (since the car is off). Also, you can check the pressure tester to see if you have lost any pressure. This will help tell a few things about the problem.

 

Its a 3400

 

With the heater not working, could that be an issue? A plugged heater core? cause overheating?

 

Re-reading your first post, did you run the A/C at all? If so, the puddle of water may be from the condensation drain in the firewall (making the not part of this).

Heater cores can clog, but it's been a long time since I've heard of that, again, not impossible though. You can prove that theory real easy.... Pull the heater hoses off the firewall and get something to couple them together (and if it does not have a flare, double clamp it). Once you bypass the heater core, if your temp issues are resolve, you have solved the issue.

But I would still start with a pressure test. Air pockets and bad t-stats can cause problems with the heater blowing hot. By doing a pressure test, you will find out if you have a leak somewhere (which can result in an air pocket). If you do, then it's as simple as fix the leak.

Now, if you are not loosing pressure and no signs of leaks, other areas need explored.

Also, it may be very worth while to get a scan tool that can do a sensor read from the diagnostic port under the dash, ensure that the reading from the temp sensor is fairly accurate to you gauge reading. You might be chasing the wrong issue.

 

Well I just changed the radiator cap, and felt the hoses going in and out of the heater core, they are both warm. so I guess its time to try the pressure test. Once is cools down Ill bypass anyway and see if it helps.

 

Also when I took radiator cap off, it DIDNT over heat, but once I put the radiator cap on it does!

 

Did you get this resolved?