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99 Cadillac STS
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51 Posts
Discussion Starter #1 (Edited)
Hey Guys,

I'm completely stumped on my 99 STS. I've been reading post after post to try and figure out what my issue might be and tried just about everything.The issue all along is heat inside the vehicle. Here's a little background. this past winter I replaced the head gaskets and water pump on this car. I've checked the purge line and made extra sure it was clear and it is purging while running. I've replaced 2 Thermostat's, The car temp runs at 12 o clock then heats up to the next dash then right back down to 12 o clock again. the only time it ever puts out any heat at all is with the RPM's around 4k it will push out a little luke warm air. this after noon I decided to check if the heater core was clogged so I removed the only hose at the firewall I could and pushed air with my compressor though it and radiator fluid was shooting out of the line that was off. after that I decided to try the reverse way and it pushed fluid right back out the metal heater core connection. I hooked it all back up and took it for a drive and too my amazement I had heat! it was the same as before the heat came with RPM's but this time much much more heat and at lower RPM's (around 2-3k). After driving the car for about 5 miles it went right back to it's old self :( luke warm at 4k or so. No codes whatsoever show up. I'm not too sure about what else I might try but any suggestion would be much appreciated


Thank You,

-Rob
 

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'98 STS (RIP @ 206,xxx miles)
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4,854 Posts
....I'm completely stumped on my 99 STS. I've been reading post after post to try and figure out what my issue might be and tried just about everything.The issue all along is heat inside the vehicle....
You somehow missed the posts related to reading Diagnostic Trouble Codes (DTC). Your problem "sounds like" a HVAC door actuator problem.

The only way to know is for you to tell us which ""B" (Body) codes are set on your car. Here's how to display your codes (scroll down a little to get to your year model):
http://www.cadillacforums.com/forums/cadillac-tech-tips/55172-how-pull-codes-got-codes-warning.html
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,115 Posts
Heat comes and goes with RPM's? That points to a circulation problem to me. How's the water pump belt tensioner?
 

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99 Cadillac STS
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51 Posts
Discussion Starter #4
Will check the belt tensioner first thing tomorrow and let you know, thanks ranger you've helped me in the past with my 96 sts :)
 

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99 Cadillac STS
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51 Posts
Discussion Starter #6
Hey Guys,

Ranger: The belt tensioner is fine, raised the RPMS and the pump moved faster as expected. JimD: can you explain more on the codes? I pulled them up this morning and hit All Codes. it goes though the whole list with No PCM codes, No IPC codes, No ABS Codes and so on. do you need the Prom ID's? that's the only thing I could think of.
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,115 Posts
Sounds like you don't have any codes. What did it say after they where displayed.
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,115 Posts
If you exit without clearing them, then any codes will remain stored.
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,115 Posts
You DID Timesert the block when you did the head gaskets, didn't you?
 

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99 Cadillac STS
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51 Posts
Discussion Starter #13 (Edited)
I have a question on the Heater core. is it a single tube inlet and outlet design? or is it a Tank type design? what I'm trying to figure out is when I pushed compressed air though the heater core after that the system blew out a lot more hot air. I'll try to find a design and post the pictures of what I'm trying to describ
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Take a look at your heater core in www.rockauto.com. Click on the small picture or the (!) icon for a larger view.

Your core is a multiple tube single pass heat exchanger.
 

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99 Cadillac STS
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51 Posts
Discussion Starter #16
Can anyone suggest the proper way to flush/clean out the heater core without removing it from the car? I've heard vinegar and let it sit in for a few hours. I also heard draino but I'm not too sure on that one seems harsh.


Thanks

-Rob
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,115 Posts
Just disconnect the hoses and "flush" with water. If it's clogged so bad as to need a solvent, then it will most likely need replacement.
 
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