My monte keeps overheating - no coolant lights came on or anything it just overheated one day on the way home from work and keeps doing it. Replaced the fans and thermostat. Flushed coolant, no leaks and it is still overheating. One dealership told me I needed a new engine, another mechanic said replacing the fans should work. No water in the oil. Could there be a leak that I am missing? Coolant levels are fine, not running low. I am trying to find the best way to fix the problem myself if feasible. And once the fans were replaced the car did okay for a while, but now I can not go very far and it redlines. I am confused at this point. 01 monte carlo ls 3.4L. Approx. 144K miles.

 

Either a blockage in the coolant system, wrong t-stat, coolant temp sensor, fans/sensors, or a bad head gasket.

 

Replacing the fans without testing them does not seem practicle. In my limited experience, I have never had a fan fail yet (knock on wood).

At any point does the temp go DOWN? Also, while it's "overheating", if you turn on your heater full blast, does it blow warm/hot or is it ambient with the outside temp?

First thing I would look into (if you haven't already) is changing the T-Stat. Even if it's not bad, if it has not been changed in many years, it's probably not bad to do from a maintenance perspective.

Also, on the 3400, there is a bleeder valve above the water pump (and I believe one above the T-State housing). While it's over heeting, if you carefull/slowly open that bleeder valve, the one above the water pump (it's on a piece of black pipe, comes straight up from the pump and has a pipe that runs along the valve cover), does air or steam shoot out? Or do you immediately get coolant?

These questiong all lead to trying to get you the best help. If you water pump is bad, it leaks coolant (and each time I've had one fail you either have a nice wet spot on the hood insulation above the pump or a puddle of coolant forms almost directly under it). I have also ran a car with a leaky pump and seen it flinging drops of coolant.

 

The 3400 is known for leaking intake gaskets and blowing head gaskets. I would not be surprised if your problem is a leaking gasket.

 

It would not surprise me either, but it's a matter of trying to properly diagnose to that step.

Over the years, I have done a lot of work on the 3100's in the N-Bodies (and the 3100 is a very close/near identical kin to the 3400). I battled hard diagnosing my '94 Grand Am with a odd cooling issue and it came down to headgaskets (the factory ones failed at 180K, changed them and still drive the car today with 207K).

I am hoping my limited knowledge and experience with the 3100s will pay off here. All about trying to collect the right data.

 

You can go to most auto parts stores and rent a coolant test kit. That way you will know for sure if there is a leak or not, and if there is a leak you will find where it is.

 

Probably the intake gasket, replaced the one on my '03 when I bought it, as it would overheat within 10 minutes of startup, then would go back down. After a few times though it was taking longer, did a search online found out about the gasket and had my garage replace the plastic stock ones. Runs fine now!

Mike

 

^X2
But mine was a 3800. The Guy at the dealership showed me the leak. it was kinda down deep in the engine and was not something i would see with a semi quick glance.

 

It is the LIM gasket bet You just look on the left side of the motor by the heads. no water n oil is good if thats it better fix it fast! can do it yourself for 60$ and a nice day! till the next 40,000 If u leave it Kiss the thing goodbye. Mine just Blew last week!

 

I feel everyone is jumping to conclusions/assumptions and not looking at the symptoms. I'm not saying the LIM gaskets are 100% out of the question, but I don't think we yet have the data to come to the conslusion. The OP did not notate loosing coolant or oil (both key signs of a bad LIM gasket). My experience with the 3100/3400, when LIM gaskets are bad:
- you loose coolant and/or oil
- you'll have coolant in the oil
- There are a couple of common areas fluids deposit
- usually a radiator pressure test will also prove helpful, as coolant will dribble out with the LIM meets the heads

From my experience, the nylon gaskets on the 3100/3400's last about 80-100K miles (for example, my '94 Grand Am with a 3100 is on it's first set of replacement head gaskets and second set of LIM gaskets at 207K miles).

I would hate to see the OP spend money and time on this if it was truly not the problem.