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'99 SLS (RIP) '99 STS
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Discussion Starter · #1 ·
I've had a '99 STS for about 6 months now and it runs great most of the time. However, it has quite the thirst for dex-cool and is running through the stuff fairly quickly. The car runs fine at speed but if its been running for a while, the temp creeps toward the red but has yet to actually hit it. Once the car is moving again it cools back down and is fine again. I previously had a '99 SLS and loved the car aside from the head gasket issues and kind of know what to expect there, so I'm skeptical of the STS's issue being in the head gaskets. The symptoms have pointed to the idea that it could be an intake manifold gasket going bad and so I am wondering (and hoping) if that seems more likely than a head gasket issue. Any advice would be greatly appreciated.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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There is no coolant or passage in a Northstar intake manifold. It is completely dry.

The only places for the engine to use/lose coolant are the head gaskets themselves (extremely large amounts of discussion and testing on this in here, Deville, and down in Engines: Northstar performance), the water pump (driver's end of engine, driven off the front
intake cam extension), the water crossover (the manifold across the transmission end of the engine), the radiator, the heater piping at the firewall (right) side of the engine, and the coolant reservoir.

Many ideas in Engines and Technical Tips.

You might want to get a real GM/Helm Service Manual from eBay or www.helminc.com to study up on the Northstar engine.​
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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6,405 Posts
What you describe engine getting hot(ish) with stationary car sounds allot like a fans issue, either the fans or the relays or the fuses.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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What you describe engine getting hot(ish) with stationary car sounds allot like a fans issue, either the fans or the relays or the fuses.
Yeah, that ^^^^

Use this picture for reference to "hot".........and I stuck your fan circuit down in my first post.
 

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03 STS
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using coolant, running warm? its cool in indiana now. do fans even run during normal driving at this time of year? if you run defrost, does compressor kick on, and fans run than? don't have flow chart to know that. i think compressor will not run below 40F?
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #6 ·
The temp made it up to about 245 based on the temperature picture after running on the interstate for close to 3 hours, it went from normal (12 o'clock) to that 245 in the time it took to wait at a drive thru, I then got the care back up to around 45 mph and the car cooled down within a mile, and proceeded to tell me to check the coolant level. Both fans run fine.
 

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03 STS
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low coolant will cause high temps. but where is coolant going? maybe a leak. cracked radiator sides. steel coolant lines. heater core hoses. my car will cruise at 204f all day. come to stop and it bumps up to 215f. get on hwy and it drops to 190f than will go right back to 204f. i have never been over 220 though.
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #8 ·
Well, its been in the shop twice to be checked for leaks, none have been found which leads to the idea that its getting burnt up in some fashion. And considering the coolant loss, the coolant system holds at a normal pressure at all times.
 

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i'd put money on head gaskets... get an exhaust gas test done to confirm ... i can't remember the link to the kit, but many parts stores will rent or sell you one fairly cheap
 

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1997 Cadillac STS (BOSE w/AUX In, Massage Seats, URPAS, "Z")
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Is not HG guys, it cools down when it has air flow (exactly when the engine is under load to pick up speed), if it was the HG it won't cool down, but rather clime higher.
Check your fans, check your coolant level.
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #13 ·
A have not checked any of that but will tomorrow now. Is there anything special that needs done to check the condenser or the purge line? As N*Caddy stated it does cool off when under power, and it does so fairly quickly. And now for a rather dumb question, I am assuming HG means Head Gaskets in this case, rather than Hg as in Mercury since the Head Gaskets were such problems on the 4.6L Northstar.
 

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2010 DTS
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88,476 Posts
To check the purge line just start the engine and pull the 3/8" line off the side if the surge tank near the top. If there is coolant flow the line is clear as it should be. For the condenser, just be sure it is not full of dead bugs, thus restricting air flow. If it is, just blow it out with compressed air and then flush it with a garden hose.
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #16 ·
Alright well I checked the purge line and it had coolant running thru but I hit it with our air compressor anyway. When I blew air back into the pump it would not take air but would force the the air back out the purge line; I'm guessing this happened because of the water pump not a clog. The condenser seems to be fine. And on my drive back to school today the car ran fine but I had to stop to add Dex-cool about half way thru, after adding before I left (100 miles between). I'm going to test for Head Gaskets soon at this point, however if there's any other ideas I'd appreciate it. And I've also heard Dex-cool is really corrosive and was wondering if anyone has tried any other antifreeze in the Northstar.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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No matter what you have heard, DEX-COOL is not "corrosive". As with any other engine coolant it needs to be changed periodically - while GM recommends 5 years, we seem to like 3 year intervals.
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #18 ·
Alright thanks for that on the Dex-Cool, and she's burning thru enough to probably go thru a gallon jug every 500 or less miles.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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Alright thanks for that on the Dex-Cool, and she's burning thru enough to probably go thru a gallon jug every 500 or less miles.
Mmmmmmmmm........That not good, Kemo Sabay.....
 

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'99 SLS (RIP) '99 STS
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Discussion Starter · #20 ·
Yeah I'm going to go talk to a mechanic at the Cadillac Dealer tomorrow also, see if there are any other possibilities but I'm thinking its destined to be the HGs, anything I can do while the block is torn down to help prevent future problems with them.
 
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