I have a 02 Monte LS 3.4(77,000miles) that is overheating. Figured it was the thermostat so changed it and it still overheats. Can be driving along and it shoots up 240/260 in a second and some times it seems if I gas it that it starts to cool back off fast. Both fans appear to be working and it has plenty of coolant and is not leaking, any advice as to what to try next?

 

What ratio did you use for coolant/water? Also, after changing did you bleed the air out of system? Also, is your waterpump functioning properly? If the bearings are worn out, or just wear in general the pumps blades might not be spinning right. Have you pressure checked the system for leaks/ clogs?

edit: my buick had a similar problem. I added too much coolant to water and it ran pretty hot when it sat. The coolant needed to be drained and re-mixed.

 

Make sure the coolant is a 50/50 mix. Also make sure you bled ALL the air from the system.
If you suspect a leak, grad a radiator pressure tester from the auto parts store loan-a-tool and pressure test the system.

At your mileage, you could be experiencing the beginning of a failed lower intake gasket (but not enough info to confirm).

 

It's so strange only does it like in the mornings or when car has been sitting a few hours without running.

 

get that system tested. If giving it gas helps it cool off, it might have air in the system.

 

I would guess that based on your mileage and the age of your car you have the famous 3400 LIM or Head or Both gasket problems.

 

Going to test it today. If it's the LIM or head what gasket kit do you guys like to use?

 

Based on your description, first thing to ensure is all air is bled out of the system.

For LIM gaskets, get the Fel-Pro aluminum gaskets (if you put ANY nylon gasket in, you're just taking junk out and putting junk in).
And if you had to do head gaskets, again, I like Fel-Pro.

RockAuto.com can also be a good source for the parts (and search MCF, RockAuto is a sponsor and has a coupon code listed).

 

Thanks, we will bled and then pressure test before doing anything else. Thanks for all the help.

 

I had the exact same problem last year and when I took it in my catalytic converter was acting up. Since yours is at 77000 miles you should be able to have it fixed for free at the chevy dealerbut you might check first cause the reason it was bad was the the head gasket on the rear had burst and it leaked antifreeze in to the converter causing it to get a thick coating inside. HoPe this helps