Thread: Cooling System/Sensor Acting Weird, A Puzzle for Experts

The gauge on the GP is not a linear gauge like you're thinking. It's more of an approximation. As long as it's not overheating, you don't have anything to worry about.

Originally Posted by Sven The Moparman

The gauge on the GP is not a linear gauge like you're thinking. It's more of an approximation. As long as it's not overheating, you don't have anything to worry about.

Thank you for the hint, sir.
I actually thought about linearity for years until I discovered what you just said by mapping the scanner temp readings with the gauge positions, and the numbers I mentioned are accurate to some extent because I did not extrapolate or guess but by trying different resistor values in place of the sensor while watching the position of the gauge needle. Then, I used a digital contact thermometer in a small pot of water with a new AC Delco sensor to map real temperatures with sensor resistances.
Am I still doing something wrong? probably over-complicating things?

Originally Posted by SubwayGuy

Unless your original temp sensor was broken, you shouldn't have replaced it. Unless you got an AC-Delco temp sensor, you're gonna 99% of the time have issues. You might even still have issues with the AC-Delci one. If you still have the original one, throw it back in.

Believe me, sir, I had the same worry.
So, I did not replace the sensor. It's still sitting in the engine block. I'm not convinced yet that it's not working and that's why I didn't mess with it. As far as I know (and I may very probably be wrong), those sensors don't just shrink the range of their resistance range gradually, they either read normally or not at all. The one I bought is AC Delco. I only compared the new and old indirectly as i explained in my last reply.

Originally Posted by jteske88

Ive actually had a issue with my harness. Jiggle the line and see if it bounces.

That's exactly where I'm at now. Is that common?
I only measured the wires continuity without jiggling and it was good.
Besides, I never had fast gauge bounces when I drove, it looks like it honestly reflects the temp of the coolant going up and down according to driving conditions with an average that is strangely about 20 degrees lower than where it's supposed to be.

I'm a senior engineer and deal with sensors and control systems everyday for a living and I have this gut feeling that it's not anything electrical or electronic, but it's a mechanical thing like a water pump, vacuum leak, head gasket, ... etc.

Originally Posted by Fivefingerdeathpunch

If you had a bad head gasket, you'd see white smoke out the tail pipe and be losing coolant real fast. 3800's rarely pop a head gasket, you really have to want to it. Like over heat it many times and just run it forever at over 240-250º.

A failing water pump wouldn't put the coolant at 180º either, it would stay much higher when it's not moving because the air flow over the radiator would be cooling it off.


Just from being around these cars for so long and knowing how many other GM vehicle work, if the temp stays under 200-210 all the time there is nothing I worry about or even care about. Even my 2012 silverado, once warm it says 210 on the gauge and never moves. Yet when I have my tuner hooked up it running at 190-195 like the thermostat is rated for.

Thank you sir,
It looks like I'm newer than you with them and i'm still in the phase of worrying about the bouncing gauge or the temp being too low.
I read somewhere that running at a lower temp (if my gauge is accurate) will burn rich, low milage, not good for engine long term. That's why i'm investigating.

I had this feeling that is not solidly confirmed, that after every one of the flush rounds i did, and the more and more contaminants come out and get replaced with clean distilled water (and later dexcool), the more stable the gauge becomes. but i'm probably just trying to convince myself that all that cleaning effort was a good job

Originally Posted by BillBoost37

What you are seeing is very common and normal for these cars.

Your best test is to get an OBD II scanner on it and watch the actual temps. You'll see around 212ish that the low speed fans should kick on. But it's going to take a lot of driveway idling to get it there. Heck, I often would have to increase rpm to 2k for quite some time to heat up a car and ensure it's up to temp and the coolant system was burped properly.

Thank you sir for commenting.
I mentioned above that I did use an OBDII scanner and did about a week of driving where I was more focused on the scanner temp reading than the road I was driving on

The reading of the scanner is the same as the gauge takes it's information from the reading (resistance) of that sensor. And it was always colder than what it should be (at the middle, or around 210 degrees) in addition to it being fluctuating between 180 and 200 depending on the driving (fast/slow, highway or traffic and stop and go driving, hot day or cold night, ..etc.).

These cars don't have a sending unit that is separate from the coolant temp sensor as far as I know, so all info comes from that sensor.

I replaced the sensor with a potentiometer (variable resistor) and could change the gauge reading from the very buttom of the gauge to the very top (OBDII scanner was connected and read all temps up to 280), so the gauge and computer connections are good. It's either the sensor, or something else in the engine that causes this behavior.

I noticed that the radiator fans start as soon as the AC or heat is turned on, but I never got to the point where the fans start on their own (proving what you mentioned about how hard it is to get to operating temp in your garage).
BTW, if i keep pressing the pedal at 2000 or 3000 or even more rpm forever, my gauge will never get to more than 190. I tried holding it at 2500 for about 15 mins straight it stayed where it is at 190.

I read somewhere that the cooling fans have two (may be more) speeds. Is that thue?

If there are speeds, then is it working by the computer using pulse-width modulation (PWM) or is it only two speeds due to some wiring trick in the fan's DC motor armature winding?

If it's the latter case, then there should be two relays (one for each speed) and there is a chance that only one is malfunctioning causing the closed loop temperature control system to get messed up and bounce.

I drove, yesterday, for several hours after bringing to coolant back to around 60% and purging the system well. Using the OBDII scanner and a wireless microphone I attached to the fan to see when the work. I noticed the following:
1. It takes a relatively long time of serious city driving to bring the temp to around 210 (about 30 mins).
2. The thermostat opens at around 197 and closes at around 191.
3. Most importantly, the fans work high speed when AC or heat is on, but not without. They only work high speed at around 219 which is kind of too much unless there is a low speed operation that was supposed to start at a much lower temp in the range of 200-210.

Anyone with a 2004-2008 who knows about fan speeds?

so, I do have two relays? for each fan?

I would so much appreciate a schematic.

and that is what i'm trying to diagnose. If I drive on a freeway, my temp bounces between 188 and 194 and never higher. In city traffic, though, it bounces between 194 and 220.
I started this thread not because I have a real bad situation, but just to bust the myth (I believe) that in some cars/engines, the well designed closed loop temperature control system bounces that much.

I believe that many people believe that this is a normal situation, and I totally disagree