3.5 Overheating/Thermostat Change - Oldsmobile Forum - Oldsmobile Enthusiasts Forums

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Old 01-22-2018, 12:12 PM
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Default 3.5 Overheating/Thermostat Change

I apologize up front if what I'm entering is repetitive; but I thought I'd like to share my most recent incident with my 2000 Intrigue with the 3.5 L V6. This should be applicable to an Aurora with a 3.5 L V6 or a 4.0 L V8 as well.
The darned thing started to overheat-not exactly overheat, but having an extremely high water temperature hysteresis during operation. Temperatures would rise erratically to as high as 225 F and then drop back to the 180 F range. Fan operation was just too often taking into account the cold (40 F) relative current ambients. I suspected a thermostat issue; so I ordered an ACDelco OEM from RockAuto. This was as a result of information I had learned from other people who had used "will-fits" that fit, but didn't work. Well, the ACDelco thermostat arrived in the box and when I changed it, I noticed that although it was reportedly OEM, it "didn't look like" the one I took out. It was clearly missing a molded-on seal on the sealing portion and "looked different" in other less critical aspects. It was also identified as "made in Israel."

Cutting to the chase, I "boiled" the part before installing it. It opened as designed at approximately 180 F (using a meat thermometer to measure water temperature.) The removed thermostat could best be described as "lazy." It opened eventually, but well above the 180 F point and had to be in water at a rolling boil for a considerable period of time before finally extending. I installed the replacement thermostat following the shop manual instructions and it just did not fix my issue. Others have reported problems bleeding the trapped air in the cooling system-I merely followed the shop manual instructions with one small deviation-as written, I couldn't get the coolant to statically bleed out the bleed vent in the radiator. Covering the surge tank opening and applying a slight pressure to the overflow line with the surge tank filled to the cold-hot level readily addressed this issue and I could quickly get a reasonable flow out the bleed vent.

I then ordered an ACDelco 15-10570 thermostat/housing assembly. When it arrived, I was initially skeptical; because it came in a non-descript box and was marked "made in Brazil." However, the thermostat had the requisite molded-on seal and "looked considerably more like" the thermostat I removed. When I subsequently changed it out, I likewise found that all the casting marks on the thermostat housing matched up with the ones on the housing that was on my car. So I guess I was a little gun-shy, but once burned, twice cautious. This change-out was a success and simple. I'm now quite adept at changing a thermostat. I replaced the assembly in it's entirety, bled the system as per the shop manual using a slight pressure assist, and the car is now cooling like a champ. It warms up quickly and once warm, the water temperature hysteresis during a 40 F day is only about 5 F.

...So that was my experience and what I learned. I hope others will find this helpful.
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Old 04-22-2018, 08:43 AM
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Join Date: Apr 2018
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Default Extremely helpful

I have been tearing my hair out over here, going through pretty much the same steps you described, and reaching the same basic conclusions. I wasn't sure of any of the mess until I read this- simple, to the point, solidifies and summerizes my own woes- and I'm absolutely elated that you even listen the part numbers!...BUT ...I did a search for the therm/housing you ended up using (the ACDelco 15-10570 thermostat/housing) and it is most certainly the one I need as well, however, every website I checked said "not cureently available" or showed they longer carried or it! I mean, I can't find a replacement for this thing ANYWHERE on the fraking Internet! and I have been searching /researching myself into a migraine! Am I missing something? is there a site you might be able to recommend? I feel so totally overwhelmed by this whole thermostat hunt, like NO ONE sells the right one! They all say they fit, but, they dont! Or thier missing the gasket, or thier the world shape, size, ect....it just maddening.. Sorry if this was a little melodramatic, but I never run into dead ends like this with my Olds (and I have run into PLENTY of problems and replaced tons of parts with only minor bumps and headaches), and I need my car back in shape as soon as possible, and at this point i am sick to death of the whole "changing the damn thermostat" process, especially when it ends in frustration and more research....so any info you could give me, or any direction you could steer my quest would be amazing!
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