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1999 STS
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Discussion Starter #1
Hey guys, so ever since I've had the car (3 yrs now) the car's temp guage would move slightly past the middle mark but then go back to normal. Never thought anything of it at the time. However, later it began to move to the 1/4 mark after the middle but again, would go back to normal again. I recently got some work done to my car including replacing the master cylinder, front left caliper and rotor, wheel bearing hub assembly, and high pressure sensor for my a/c because i was receiving the message "Service AC System." The radiator was replaced last year, however the "slight" overheating issue was happening before then. My car finally drives like a dream again, but this overheating issue has become worse.

Driving the car the other day the needle began to move past the middle as I was driving on the hwy. This usually only happened while in stop n go traffic on warmer days. I realized the needle wasnt moving back to normal as it usually does and kept climbing almost to the hot mark while my car lit up that it was turning the ac off due to the engine being hot. I pulled over and tried to evaluate the issue, checking hoses, leaks, caps, etc but was stumped. I let the car cool down and continued to my drive and the car overheats again. I pull over and let the car cool down in order to releave some pressure from the resovior tank. When I finally could remove the cap, I noticed that my resovior tank was filled to the brim. Long story short, I drained some of the coolant because I figured the guy must have topped off my fluids, and the drive home was fine. Needle stayed in the middle mark. However, today as I was driving to work, my car began to overheat as I was driving, however would begin to go back to the middle mark, but the car will shortly begin to overheat again, back to normal, then overheat again.

Im completely stumped. After doing some searching on the thread, i'm hoping it isnt the head gaskets. I've already dumped $3k on a car I bought for $4k. The caddy only has 93k on her and I really enjoy the car. I am picking up a thermostat in hopes that this will cure my issue. One question, rock auto has two AC Delco thermostats listed ACDELCO Part # 131128 for $22.89 and ACDELCO Part # 12TH11D for $9.73. Which one would I need? Should I replace the water pump while I'm at it? Any info you guys could shed on your experiences or what you have heard, I'm all ears. Thanks!
 

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White Diamond '03 DHS (with DTS floor shift)
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DO NOT start throwing parts at it. Borrow a block test kit from AutoZone or such and run the test after the next overheat episode, but be prepared for the worst. Being a '99 with those symptoms, my hopes are not high.
 

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1999 STS
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Discussion Starter #3
I figured that much. Not familiar with block tests or how to run them. I'm assuming I could figure it out. Could you tell me more about it?
 

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It is a simple kit. There is a plug type fixture that you push into your coolant reservoir opening, there is a sight glass in the fixture which you fill with a detection chemical. With the car running, you squeeze a bulb on the fixture to pull the air from the coolant reservoir through the sight glass full of chemical. If there are combustion gases present (head gasket breach), the chemical will change color. Otherwise it will not change color, pretty simple.

I just had to do the HG job on my '99 with 150k. Drop the cradle, much easier. I would only do the job IN VEHICLE if someones life depended on it!
 

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1999 STS
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38 Posts
Discussion Starter #5
Sounds easy enough. Any recommendations on where to get one or what kind? I'm sure I could find it at any auto parts store, autozone, advance auto parts, oreileys, etc?
 

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2001 Seville STS, 1990 Seville (RIP), 1972 Sedan Deville
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They don't sell them, they loan them out. Call AutoZone and ask about the loaner tools. They have a decent block test kit. All you have to actually purchase is the test fluid, ~$10.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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I figured that much. Not familiar with block tests or how to run them. I'm assuming I could figure it out. Could you tell me more about it?
==========================
first -
check the PURGE LINE -

the PURGE LINE is the only way of expelling air from the Northstar cooling system -

it is a 3/8 diameter hose that goes from a hollow bolt in the thermostat/waterpump housing area of the engine - and plugs onto a nipple on the side of the SERGE TANK -

with the engine running - there should be a gentle - constant flow of coolant - from the engine - to the serge tank -

if there is no flow - the most likely cause is a blockage in the hollow bolt -

follow the hose towards the engine - with the engine OFF - remove the hose - use a straightened metal coat hanger - or something similar - and poke it into the hollow bolt -

the coat hanger should go in about 2 1/2 inches -

once the hollow bolt is cleared - you can briefly start the engine to verify coolant flow - be sure to use some sort of container to catch the coolant so you don't make a mess -

check the hose and serge tank for blockage -

===========================
Not familiar with block tests
go here for a video on doing the BLOCK TEST -
http://www.youtube.com/watch?v=vgkMC-5Mtz8
 

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1999 STS
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Discussion Starter #8
I'm gonna try and diagnose the issue by checking the purge line and running a block test probably this weekend. I'll update this post with the results and hopefully can get this taken care of going further. *keeping fingers crossed its not the head gasket*
 

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1999 STS
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Discussion Starter #9
So... I'm convinced my car is Bi Polar ..

To further diagnose my symptoms, on the way to work this morning the car got to normal temperature as usual, but slowly the temp needle began to creep. Got to the 5/8 mark then would creep back to normal, back up again, to the 6/8th mark, then back to normal... repeat cycle... However it would never get past the 3/4 mark.. After work, I topped off some coolant (COLD) to the middle mark of the reservoir tank and continued my trek home during rush hour traffic. The car begins to overheat ... Slowly creeping past the 5/8 mark, to the 6/8 mark.. then PAST the 6/8 mark close to Hot.... "Engine Hot - A/C OFF" .. I pull over and let the car cool down SOME... Relieve some pressure from the reservoir cap, slowly, let the car cool down below normal temp .. screw the cap back on and continue my trek home.. Again.. car begins to creep past normal and I immediately pull over.. Release some pressure and when I remove the cap I see that the coolant is boiling over from the cap and is to the brim... This is where I'm assuming the purge line could be my issue ... With the cap off, the coolant begins to drop down...

This is where the bi polar issue comes into play.. The car stayed at 12 o clock basically my entire 25-30 min ride home, until I finally got maybe a mile away from my house and the needle crept maybe a millimeter passed normal... It stayed there ... Did not creep any further... I pull in and let the car idle in my driveway.. The needle goes close to the 5/8 mark but not quite... then back to normal...

WHY is the overheating so inconsistent !? It drives me nuts !... I'm letting the car cool all the way right now so I can check the purge line.. Correct me if I am wrong but basically let the car cool all the way... Let the car idle and pull the purge line from the reservoir tank and see if coolant flows steadily ? .. If not, then it must be blocked... I also read that the line could be blocked further down towards the throttle body, is this correct? Would you recommending removing the purge line entirely and attempt to unblock any clogs? I plan to do the block test but wont be able to until the weekend ( got surgery thursday ) and will be out for the count until the weekend probably... Ill keep updating as I figure this out. Im open to any suggestions or inputs.
 

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White Diamond '03 DHS (with DTS floor shift)
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Correct me if I am wrong but basically let the car cool all the way... Let the car idle and pull the purge line from the reservoir tank and see if coolant flows steadily ? .. If not, then it must be blocked... I also read that the line could be blocked further down towards the throttle body, is this correct? Would you recommending removing the purge line entirely and attempt to unblock any clogs? I plan to do the block test but wont be able to until the weekend ( got surgery thursday ) and will be out for the count until the weekend probably... Ill keep updating as I figure this out. Im open to any suggestions or inputs.
Correct on both counts. Check it after a cold start. Any blockage is almost always where it is attached to the crossover manifold aka WP housing. If no flow, rod out that hollow bolt. If that doesn't help, THEN do the block test.
 

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1999 STS
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Discussion Starter #12
^ Might just actually buy it instead of renting it from Autozone. Just gonna take it one step at a time and eliminate any possible issues before jumping the gun. You guys have been a great help seriously! Didnt get a chance to check the line tonight but I will be tomorrow after work while I have some daylight left.
 

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1999 STS
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Discussion Starter #13
Quick question in regards to bleeding any air out in the cooling system. I forgot to mention that during my drive home from work with the heat on 90* and on high, it was blowing cold air, as well as overheating. I have read that this is a symptom of air in the cooling system. Now I know our special beloved northstars are quite different and don't have a bleeder screw if I'm not mistaken and what the purge line is for. Is there a way to bleed any air out of the cooling system in the northstars or am I just looking at the purge line as the bleeder?
 

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White Diamond '03 DHS (with DTS floor shift)
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Once again you are correct. The Northstar is self purging via the purge line. You (nor the system) can't bleed it if the air is being replenished via a breach in the HG which is what I fear. You have most of the symptoms, air in the heater core, thus no cabin heat. It's not looking promising.
 
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