I have a car that Heats up when idle but then become cooler when running, the gage goes up when idle but later it drop when car running. like if you driving less 25mph if will go up and then the warning light up but once your doing 40 and up its start to go down to normal. Then once you slow down to like let say 25mph it start to rise up again

HELP

 

Hi Brian & Welcome to the MCF

What year/made/model of Monte Carlo do you have ?

How many miles ? Any modifications to engine ?

Have you ever changed the thermostat ? It could `be a sticky of bad thermostat ? Is you radiator full ? Any leaks ?
Have you ever replaced your water pump ?

Please post additional information on your Monte to help member's help you > Good Luck<

 

First guess without looking at anything would be that the rad fans aren't working and when you are running slow there is not enough air coming in to the engine compartment to cool the car down, but when you speed up the more air moving through cools the motor down.

If the thermostat was stuck it would more than likely overheat at all driving speeds, maybe not so much when going fast, but would still run hotter than normal as the heat wouldn't be going through the block and into the radiator.

Does the car blow heat inside? if so, then warm water theoretically is running through your heater core which would/should negate the stuck thermostat theory.

Definitely check your radiator fluid to ensure that you have enough fluid, if you have changed the fluid did you bleed it properly? if there is an air bubble in the system it would also block flow of the rad fluid

Be VERY CAREFUL as you don't want to overheat the engine and cause some serious damage, this should be taken care of right away, you can warp the heads and possibly fry your motor.

 

It's an 2004 3400 stock and the radiator is new and fluid going to the radiator. I don't know it could be thermostat or water pump but I want to know which one should be check first

 

Are you losing any antifreeze? even a little? cause the coolant elboys could be slow dripping like mine,replace with aluminum elbows,also check radiator mine had a slight crack in plastic end cap had the end caps replaced and the core rodded and cleaned fixed it all for 260$

 

sounds like a thermostat stuck/bad though, there is a bleed screw also you have to use to purge the air from system learned the hard way!!

 

I think the wire from the antifreeze to radiator had a small leak could that be the cause of the problem

 

Just throwing this out there, I just bought a 2004 Monte Carlo LS with the 3.4 liter. Long story short the car had been sitting forever and when I got it I had the coolant system flushed when I got the oil changed.

On the way home from this service my temp gauge went nuts and I pulled over and shut it down before it could red line and throw the temp light. I let it sit for a while then got it home with madly fluctuating temperature values.

After 2 days of playing with it, I bought a new radiator cap, and a jug of dexcool. I'm guessing there was air in the system or the flush and done something because after following instructions here (I opened the thermostat bleed valve, and filled the radiator until coolant was coming out of it then closed it again) turned it on, filled it some more as the engine was running then put the cap back on.

As I watched the temp gauge, I bled out the system from the thermostat bleed valve until it seemingly had all the air out of it and I haven't had an issue since. Heather, A/C both work great and temperature doesn't fluctuate at all.

The things that I learned from this if there is air getting into the system like that it will cause exactly what your getting because in the 2 days I was playing with it I was also driving short distances and had the same result your getting.

If it's leaking find the leak, air in the system produced the problem I had and if you have a leak it could be doing the same for you. My issue was fixed when I replaced the radiator cap, and rebled the coolant system. If your coolant line is showing evidence of leaking I'd get that fixed to start and you probably need to have your system bled as well.