: '99 STS Temperature Problem ?

05-22-05, 01:20 PM
I have a '99 STS w/ 96000 mi.

Recently I have noticed my temperature gage moving. It usually stays straight up, but recently it has been moving about half way to the next mark and then after a short period it will move back to staright up. Yesterday it moved all the way to the next mark and after ahort period it moved back half way. This all happen when driving on the Interstate. Around town it very seldom moves.

Any ideas as to what is going on?


05-22-05, 05:56 PM
i would maybe drain the coolant and add 6 GM coolant supplement pellets to the radiator house and add new coolant, 50/50 dex cool and distilled water. see if that helps.

05-22-05, 07:40 PM
Welcome to the Forums, SCSTS!
You should expect at least 3 posters here advising you to replace your head gaskets, ignore them until you have examined and tested everything
I posted an earlier thread, myself having similar problems. Of course, several "experts" advised me to replace the head gaskets, one of them even advised against "throwing money" into the diagnosis because "certainly" it has to be a head gasket problem.
As it turned out, my radiator was somewhat clogged. Sure glad I didn't pull the engine as advised and still had an overheating issue after replacing the head gaskets!

(Turns out my engine is a fairly new "Crate" engine, installed by the dealer and has less than 30K on it, but the dealer had been changing the Dex-Cool in this car every year and adding the supplement each time, must have helped clog the radiator.)

Search through the forums using "overheating" as the search term and you should find a multitude of posts concerning issues similar to yours.
Some things that come to mind: Crossover coolant line clogged, radiator clogged, water pump pulley belt bad, Water pump pulley belt tensioner weak or stuck, water pump pulley belt contaminated with silicone or other slippery substance, small leak in radiator or heater hose, bad thermostat, bad pressure cap, cooling fans malfunctioning, etc.
These cooling systems are very fickle and seem to require exacting standards to perform flawlessly. Please thoroughly check your cooling system components, maybe even find an independant Radiator shop to troubleshoot your problem, they are least likely to want to replace your head gaskets being as they typically aren't set up to do major repairs.
My radiator shop guy said that I was only one of many "Head Gasket" jobs he has diverted since the Northstars came out. Sure, head gaskets are a common problem, but even simple cooling system issues seem, to me at least, to be more common.
Hope you get this resolved soon.
Please post back with your results.

05-22-05, 10:56 PM
Definitely check the water pump drive belt and the small tensioner for the water pump drive belt to make sure it is tensioning the belt and not frozen due to corrosion or something. The water pump drive belt is on the rear of the engine off the intake camshaft on the left hand (front) bank so it is pretty easy to get to and check.

Make sure the system is full of fresh 50/50 DexCool/distilled water.

The thermostat may be getting lazy after all the miles/years. They are a maintenance item and do not last forever. At 100K they don't owe you a dime so I would replace it anyway as a maintenance item. It is also very easy to access so replacing it is a simple matter and is very easy when you are replacing the coolant anyway.

05-23-05, 06:15 PM
Thanks to all for your responses.

I will try them and let you know the verdict.

05-23-05, 06:16 PM
Thanks to all for your responses.
I will try them and let you know the verdict.

05-25-05, 10:45 PM
You guys were absolutely CORRECT.

Had the thermostat replaced and sure enough the mechanic told me it was bad and showed me where it had been sticking.


05-26-05, 07:37 AM
That is GREAT news!
Please take the time to inspect your entire cooling system though also if you haven't already, at nearly 100K I'm sure it could use a little attention. Even a very minor leak in a hose at the clamp could cause a lot of headache. The Factory "clamps" are, in my opinion, not a good thing. I would replace them all with good quality standard hose clamps.
I changed mine before getting it fixed and discovered evidence of leaking around a couple of them as well as both upper and lower radiator hoses getting "soft".
Also, as in the above posts, change your coolant, add the supplement, new pressure cap, etc.
While you're at it, carefully scrutinize your water pump belt and tensioner. The belt is only a few dollars but failure could be catastrophic! One of the first things I noticed on my '97 was that the belt was stretched so that the tensioner was nearly "bottomed out" to the stop tab. This was at only 30K+! (Probably didn't install a new one during the change).
Be sure to keep a very close eye on your entire cooling system at all times, periodic checks should be a habit. I had gotten so accustomed to just driving my Caddy that I never even checked the coolant in the tank until my initial overheat! Never again, gonna check it at least twice a week now.
Just take a little time and spend a few extra bucks now and it may save your Northstar and give you another 100K+!