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Just sharing this. I am pretty sure some of you(s) encounter that before, so please comment.
IROC-Z 1989, LB9 TPI, 5speed, low milage, bone stock, Upon data logging, I noticed a consistent pattern for CT. Temperature would increase at a steady rate and suddenly flatten around 150°F and slowly increasing to operating temperature thereafter. The faster the car was moving the longer it would take to warm-up, specially in cold weather where large CT swings were observed. At idle, the pattern was less visible. All pointed to a thermostat opening too soon or a faulty ECM CT sensor.
The dash gauge reported accurate temperature as it was matched with the ecm CT readings. They moved in tandem. Also, an infrared thermometer gun was used to confirm CT temperature. All look accurate. Fan kicked in at commanded temperature. So the CT sensor appeared accurate.
I removed the old thermostat (OEM) and confirmed it was a 195°F. Visually it looked fine. Coolant was clean as it can be and passages were not clogged.
Of course, all you can read everywhere is the "hot water test" bla bla bla.
Well I did just that and the old thermostat was opening at 195°F!!!!! It actually appeared pretty accurate and reacting to temperature changes. Water temperature was double checked with 2 thermometers.
I ordered an OEM thermostat and installed it. Problem solved!!!!!
The only difference between the old and new thermostat was the spring rate. The newer one was harder to open with finger pressure.
QUESTION IS: can the hot water test not tell the whole story? Can the thermostat behave differently when under pressure?
DEFECTIVE 195°F
NORMAL 195°F
Last edited by SbFormula; Oct 12, 2023 at 06:13 PM.
I dont really have an answer to your question, but I'm not surprised by the outcome. I have always been suspicious of the hot water test. Sounds reasonable in theory, but it doesnt really duplicate the variables possible when the thermostat is installed. Years ago I tried multiple hot water tests on some coolant passage fan temp sensors. Hot water test indicated they should work correctly, but they definitely were not and a new sensor did fix the problem.
I dont really have an answer to your question, but I'm not surprised by the outcome. I have always been suspicious of the hot water test. Sounds reasonable in theory, but it doesnt really duplicate the variables possible when the thermostat is installed. Years ago I tried multiple hot water tests on some coolant passage fan temp sensors. Hot water test indicated they should work correctly, but they definitely were not and a new sensor did fix the problem.
