My eldo has overheated twice now. Both times it overheated I was climbing some substantial (for NH anyway) hills. Coolant was literally "boiling" and overflowing out of the resevoir. I recently changed a cracked radiator and DID NOT install the coolant supplement.

Could the lack of the supplement cause me to overheat?

Can someone help me with the best approach to diagnosing the problem before I start randomly replacing things?

I see no evidence of water in the oil, the car runs fine (with this exception). In the old days I would automatically replace the thermostat but I thought I'd ask for advice before diving in.

Thanks in advance,
Bob

The lack of the supplement can't be the cause of overheating. It's just leaking preventing thing.
I hope the radiator you install recently is new or at least the good one, not clogged. Also, did you refill the system with mix 50/50? Did you just refill or flashed? Hope you just refill...

I bought a new radiator. I refilled with 50/50 mix. I'm not familiar with the term "flashed" as it relates to coolant. After installing the radiator I put 50/50 into the reservoir. As a note - I ran from February thru May fine (of course those are winter months).

Sorry, I meant "flush".

Did you replace the thermostat? I just watched my friend replace one this weekend on a car that was over heating. It was stuck in the open position causing it to overheat. It wouldn't collapse...

Did you replace the thermostat? I just watched my friend replace one this weekend on a car that was over heating. It was stuck in the open position causing it to overheat. It wouldn't collapse...

You mean in the closed position, right? Stuck opened thermostat should not be cause of overheating.

Yeah something like that. He's the mechanic. All I know is that the car was overheating and this was the cause of it.... He said on GM cars you should replace it once a year.

Take everything he told you with a grain of salt. I never changed the thermostat and it works fine. It is his job to find something wrong with a car, otherwise he can loose his job! :D :D :D

I didn't mean the thermostat can't broke. But to change it every year...

Check the water pump belt tensioner to be sure it is applying enough tension and not slipping. Check or replace the cap to be sure it is still capable of holding 15 lb. of pressure. Check the purge line that runs from the water pump to the surge tank to be sure it is flowing and not plugged.

F.Y.I. The Northstar will not leak oil into the crankcase when a head gasket goes due to the design per Bbobynski.

Could the lack of the supplement cause me to overheat?



No, but you need to get some in there after you fix your problem. Check to simple stuff like Ranger said. Then I would definately go at that thermostat.

Thanks to all for your help. It turns out the thermostat was toast. I replaced the thermostat and all is well. Overheating that wasn't the head gasket!

Congratulations! It sounds better than having head gasket problem! :)