At a loss - She's running colder then normal.
 

Hello all, I have a stock 98 ls1 T/A non ram-air, ac belt removed.

My coolant temps have always run between 190* and 215* that is her happy place. A few weeks ago the coolant temp would not pass 180* and the heater was only warm, not hot, so I replaced the thermostat and coolant temp sensor, both delco parts. After refilling and bleeding the air out (I did not do a full flush) shes back in her normal operating range at idle, fans kicking on and off properly.... now for the fun part

When I turn on the heater (much hotter air now) the cooling fans kick on as they should, but the water temp drops back down to 180* and does not heat back up unless I turn off the heater. With the heater running and the temp gauge at 180* everything in the engine bay becomes to hot to touch. Turn off the heater, coolant temps rise back up and stay in normal range. If I let her heat up without turning on the heater nothing in the bay becomes to hot.

Basically... it seems like the thermostat is not opening when the heater is turned on. I did cycle the heater a few times when I bled the air out and have gone through the air bleed procedure more then once.

I am stumped and its driving me nuts! Please help, any and all assistance is most appreciated.

heater core sounds like its clogged up

If the heater is blowing hot air the core is not plugged.

It sounds to me like the heater is able to take more heat from the motor than it produces at an idle.

What happens when you drive it?

I went for a ride this afternoon, it was 50* and sunny out here. water temp did not even get to 180* only 170-175* If I let it warm up to 210* before I drive, the water temp just drops down to the 170-180* range as I go. I felt the motor after my ride and she was not hot to the touch, just warm. Also the upper and lower hoses are warm/hot and pressurized to the point that it feels like they would pop off if I squeezed them to hard. It seems like if there is air running over the radiator she will not get to temp at the sensor.

I've owned the car for 5 years or so and drive it year round. She has never done anything like this.

Are you positive you put a stock temperature thermostat on? When I first got my car the previous owner put a cooler thermostat on and when I would drive on the highway in 50° weather my car would throw a code saying the engine coolant temperature was below threshold or something like that (it couldn't get up to temperature).

Hey JB, the new thermostat was stamped 87* which is 188.6* (thanks google) I got it from rockauto. I had to rotate the thermostat 180* in the housing because the air bleed hole was at 6 o'clock, other then that I did not see any issues with it.

Yeah that's probably the correct temperature then. Not sure which one you purchased as Rock Auto has a lot for sale, but overall I'd say if there are no codes your engine is probably fine. You could always test the thermostat by putting it in a pot of water and heating it up to about 190* to see if it opens.

I actually did that before I installed it, the original one worked fine too. An odd thing I noticed when I replaced it... I removed the hose from the housing expecting the radiator to drain and nothing came out. I really thought the rad would have started to empty out of that lower hose.

This is the new stat info - ACDELCO 12T84D {#19137262} Professional
Temperature: 187F; Includes housing and seal; w/ Original water pump requiring integrated thermostat and housing.; OEM Recommended Temp

First off I want to say thank you to the few who have responded so far, I do appreciate it....

Does anyone else have any idea why she would be acting this way? Its going to start getting cold here next week and not having proper heat while driving because the engine isn't staying hot will not be fun.

What's the condition of the water pump and serpentine belt? While I don't know how a malfunction in either of these would cause the temp to be lower, it's worth taking a look at.

Hey xny, The water pump is stock with 66k miles and the belt is a gatorback I put on 4 or 5 years ago that has probably 5k miles on it. I agree that an issue with either of these would/should cause an overheat condition. Hell, every other issue I can think of would cause an overheat.

If your hoses are really presurized it sounds like that could be the radiator cap not functioning properly and not bleeding off the expanded coolant.

You might also get a laser thermometer and test the temp in various places on the engine while running and not running the heater and see how much difference you are getting. Cause it almost sounds like there is still air in the system or maybe getting vapor locked somehow because of the cap, i dont know, Just brain storming.. ��

Bad temp sender? Causing the ECU to run the fans early... maybe?

I'm going to do a full flush in the next few days, I am replacing the main hoses and cap then. I already replaced the temp sensor and the fans are working normally. Totally putting a laser thermometer on my christmas list.

Harbor Freight one works adequately for this kind of stuff for me,, I've tested it against a much more expensive one and they were within a degree of each other at 212 and 800 degrees.

FWIW I have a jeep that turned out to have 2 temp sensors, 1 for the ECU and 1 for the Gauge. they didn't do that in this case did they?

Not that I'm aware of, everything I've read says there is only the one.

Thought I would post an update....

There is no change. At idle it still takes twice as long as it used to for her to get to temp - 210* When I drive it the temp drops to 170-180* If I let it idle to temp and then turn on the fans via heat/ac the temp drops to 170-180*

I have flushed the system, cleaned out the reservoir, replaced the upper and lower hoses, cap and the thermostat(oem) a second time. The temp sensor(oem) is also new. The original and first replacement therms were not stuck open. There is strong pressure in the system at 210* My gas mileage also seems to be suffering. Heat blows hot/normal at 210* and warm at 170* which makes sense.

I'm going to pick up a laser therm like Lefty and pdx suggested, but it will be a bit before I can get one.

Sigh/Grumble.

The are three most likely causes.

1) A defective thermostat that stays open. This allows the coolant to flow thru the radiator and actually overcools the engine.

2) It could also be a defective gauge ot sensor.

3)The other cause is a defective coolant fan relay. If the radiator fan runs constantly it can also overcool the engine.

Where is your coolant temp sensor located?
Don't trust the sensor in radiator only... add a temp sensor to rear of passenger head (typical hot spot) to read water temp at the heads. Any cheap gauge will work for diagnosis I'll lend you one if local to Houston.

Could be a lot of things, I'd focus on getting 2 sensors at the radiator and motor to check for a wide variance. Then be sure the radiator core is clean and not full of junk.

70T/A - I replaced the therm twice, the original and first replacement were not stuck open. I replaced the sensor with no change and the gauge is working normally. The fans are also working properly. Everything else I can think of would cause an overheat, hence my frustration.

imma - The stock sensor is in the drivers head and is fairly accurate on the 98's. I appreciate the loan offer, but I am just outside Chicago. I agree that checking the temp at different points needs to be done, I'm going to get one of those ir temp guns when I can.