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Cooling System/Sensor Acting Weird, A Puzzle for Experts

V

Vincenzo

New member
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[TD="class: kmessage-left"]ok, so I finished reflushing (emptying-starting for 10-20 min-filling radiator) 10 times (5 with distilled water to get all the old antifreeze out and five with 60/40 dexcool to bring it back to specs, first 2 times, chemical flush was added) purged the system the best I could, put new OEM thermostat-195 and a new radiator cap.

I just received a new OEM coolant temp sensor and compared it's reading to the old one and they both read almost the same resistance for the same temp (which was for both about 2.3 Kohms cold, and a little over 200 ohms at engine operating temperature). Then, I put little electronic resistors at the connector (with the sensor disconnected and the key at ON), and 100 ohms gave me a 3/4 gauge or about 245F temp, while 39 ohms gave me full gauge reading passed the dead dangerous zone, or about 275F. So, everything there looks fine.

I measured the voltage coming to the sensor (with the sensor disconnected), and one wire reads 5V to battery ground and 1.2V to the other wire, and then checked continuity between these two wires and ECM connectors and found their location at the connector and they were good with no brakes or loose contact or resistance (something that my Fluke shows as small fractions of an ohm).

My problem is that my engine is still, pretty much, in the middle between 1/4 and 1/2 the gauge which is about 192F, which is below the temperature that the thermostat opens, and the gauge never gets to the middle (210F) no matter what (all seasons, here in central PA it gets hot), but it does look unstable to me because it get's closer to the middle (I saw a little over 200F, once) when driving in traffic, while it approaches 1/4 (177F) when driving on a highway at night when there are no hills. That was the case for at least a year.

The normal behavor that I'm sure everyone knows is that it shouldn't fluctuate that much, and it should get about 20 degrees hotter (middle of the gauge, or 210F)

Are there any people who have ever experienced something similar?
Please help

Notes:
1. the car is a 2008 3.8 engine and has 120k miles, transmission just replaced 15k miles ago.
2. The temps of places on the gauge were measured with an OBD scan tool that was showing me the CTS reading while I was driving, that I fit in place of the GPS for about a week.
3. The cooling system was not perfectly taken care off through the years, but it was once flushed a couple of years ago at a cheap franchise and thermostat replaced, and then, in the last couple of years, green stuff was added and mixed with whatever that garage put in it.[/TD]
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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.
 
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.
 
Sven The Moparman said:
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.
Click to expand...

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?
 
SubwayGuy said:
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.
Click to expand...
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.
 


jteske88 said:
Ive actually had a issue with my harness. Jiggle the line and see if it bounces.
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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.
 
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.
 
Fivefingerdeathpunch said:
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.
Click to expand...

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.
 
The motor would have to be running pretty cold for it use extra fuel, like under 150 at all times. 180-195 is the butter zone for these cars really. If there was a change in mileage between 180 and 195 it's going to be less than ~1 mpg at the most.
 
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 :th_winking:
 


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.
 
BillBoost37 said:
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.
Click to expand...

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 :o

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?
 
your year car the fans dont turn on low speed till the temp hits 217, and high speed when it his 226. your temps are likely normal, it has aslo got hot out and your not used to see the temps so high.
 
They are not PWM on a standard 3800 motor, but are PWM on a 5.3 in the GXP because it uses a completely different computer.

Fan settings are 219 on and 212 off for first stage fan settings and 228 on and 221 off for second stage fan settings.

I could screen shot a picture for proof but that takes work.

The temps said above are incorrect but close.
 


Look at the fuse box cover, it should say cooling fan and cooling fan 2.

You aren't getting up to a high enough temp to engage the second stage of the cooling fan.
 
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
 
your temps are normal, stop asking silly questions.

a moving car runs cooler not because of the fans, but air flow from driving.
 
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