98 Grand am cooling system problems - overheating while idling -- low coolant light on, but has coolant.

Hi everyone. First time post here, so I'm hoping I'm in the right place.

I have a 1998 Grand Am SE 4 door. It has the 3100 motor in it. I am NOT a car person, so I don't know too much about the engine or the cooling system, so please bear with me here.

I have only had one problem in the past which was a broken gasket seal that caused a leak and never had a problem since until now.

I drove approximately 5 hours and I saw my low coolant light come on. I stopped immediately and checked and I was out of coolant. Filled the coolant and continued driving an hour and a half and parked the car for the night.

The next morning, all my coolant was on the ground. I took the car to a mechanic and they did a pressure test and determined that my heater core was bad and I was losing my coolant from the core. Instead of replacing the heater core at this time (450.00?!) I opted to just bypass it for now.

I took the car back to my hotel and noticed that there was no more coolant leak and I thought I was ok. However, now the low coolant light does not go off. Worse, when I am parked somewhere for even a short period of time, the car starts to overheat, getting close to the 240 mark before the redline. When I am driving, it is perfectly normal, no overheating at all.

I checked and the fan does come on even when I am parked, so I don't think that is the cause. It seems very strange to me that multiple parts would go back all of a sudden, and at the same time. If anyone has a suggestion, I would appreciate it, I'm not really able to pay a mechanic at the moment for hours of diagnostics, only to find out my problem hasn't been solved again.

Thank you.

You said the low coolant lamp was staying on, is the coolant actually low? It's possible you may have another leak (internal) like a lower intake gasket. If your heater core was leaking that bad it would not hold enough pressure to show another leak. Have you checked the oil for signs of contamination(milky color)?

If low coolant light is on, but the reservoir is full, tap the sensor in the reservoir lightly with the handle of a screw driver.....they tend to stick, after they are refilled....

When you say bypassed, did you mean the two hoses were joined together with tubing/clamps or blocked off(they better not be)....

Also, if the car lost a lot of coolant, it's possible you have an air pocket in the engine.....if it was not properly burped after the repair and refill....

Have you checked the lower intake gaskets for an external leak?

The sensor tend to stick. Look at the overflow tank, you'll see some wire going to it, that's where the sensor is. just tap with a screwdriver handle or something, it should make the float rise back.

Have they filled it with water? or a proper mix of coolant? Water only will boil!

Yes, it was filled with coolant. I will try what you both suggested in regards to the sensor, and see if that changes the light status or not. No, I have not checked for an internal leak, lower intake gaskets? I tried to explain I know next to nothing about cars, so I am assuming this is probably something I would have to have a mechanic look into?

Ok, so if I do manage to get the low coolant light off, how do I go about solving the big problem, the engine overheating when idling?

It seems so strange to me that I would get all these problems all at the same time. Busted heater core, low coolant warning, and overheating, all at once?

And yes, the heater core was bypassed correctly, the hoses are connected to each other, not blocked off.

I don't know anything about burping the system... They didn't do anything that I know of other than putting the coolant in after they drained it when bypassing the heater core.

So they didn't purged the air from the system... no wonder why you have problems... Probably have a few air pocket, which prevent the coolant from reaching some zone at low flow (idle).

I never tried to bleed mine (I have a 2.2), but I keep hearing that the 3.4 is a pita to bleed proprelly.