Okay, so I don't necessarily think it's related to the spark plugs I changed. However, based on some more research. I think it could be one of these things.

1. I've read some posts that there could be a possibility of an air bubble in the system. However, my car recently started having this problem, so I think I bled the coolant system properly. I don't know why all of a sudden, a bubble would form in the coolant system.

2. I was thinking maybe a possibility of a bad thermostat but if the thermostat would be bad wouldn't that trigger a check engine light or at least store a code in the system? My car currently has neither and doesn't overheat so I'm debating if I should try changing it out.

3. This is what I'm tempted to try but based on some threads on here I believe my car has a coolant temperature sensor and I think it may have gone bad. I believe the Denso part number is 1931000. I think this is where I should start tomorrow but I'm open to suggestions still.

 

Check for a faulty auxiliary fan.

 

If you're talking about the radiator fans both of them seem to be working, I checked again this morning.

 

It has two of the sensors. A poor connection or minor damage to the wiring can cause a gauge to be jumpy. One is for the ECT the other is for the gauge. I believe the one near the thermostat housing is for the ect that reports to the ecm. The other in in a tube like housing in the lower radiator housing. Show the same p/n. 89422-33040

 

It has two of the sensors. Usualy its a poor connection or minor damage to the wiring when the gage is jumpy. One is for the ECT the other is for the gage. I beleive the one near the thermostat housing is for the ect that reports to the ecm.

Do you think it's a better idea to start with swapping the sensor or doing a coolant change again? I think you might be right on the sensor portion. Found this video last night and it doesn't look too hard to change.

 

If the coolant is good I'd just catch it in a clean container and put it back in. I don't think the coolant is the issue since the car is running properly.
Use Denso or OEM only, no mazon or Ebay. Toyota's don't like after-market electrical parts.

 

Ah okay, perfect. I'll see if my local parts department has the part in stock or if AA has a Denso one nearby.

Thank you.

 

Napa carries Denso as well.

 

The p/n you found is correct, p/n 89422-33030)
The one I gave you is for the 2020 Venza. My bad!

 

No worries, thank you for letting me know.

 

So small update

I tried to change back to my old spark plugs but the temperature gauge still fluctuates while driving. I tried to check the radiator fans, and both seem to be working at different temperatures. I will try changing the coolant again and order the Denso Coolant Sensor I'll update this when it comes and hopefully, the sensor will solve my problem.

 

Changing good coolant isn't gonna change how the gauge reads.
Did you dis-connect and then re-connect the ECT sensor and see if it changes anything.

 

Like sdspeed suggestion is to reset the ECM by disconnecting the negative battery terminal for 5 minutes and put it back on. Also you may want to check for head gasket leaks. Take off the radiator cap when engine is cold preferably first start in the morning. run the engine until the thermostat open. If there are air bubbles from the radiator, it is a sign there is a problem with the head gasket. Did you experience any overheat before? an overheated engine always damage the transmission too. the only way to check is look some burn smell from the transmission fluid. also check your water pump weep hole if there is any small leak. Another thing to look out is a stuck thermostat. Check also for engine vacuum leaks.

Run diagnostic test thru Toyota Techstream. Look on Ebay and it's cheap.

 

Changing good coolant isn't gonna change how the gauge reads.
Did you dis-connect and then re-connect the ECT sensor and see if it changes anything.

I haven't had a chance to disconnect the sensor and reconnect it yet but I can always try to do that sometime to this.

ECM by disconnecting the negative battery terminal for 5 minutes and put it back on.

I'll try and disconnect the battery and leave it for some time too this week. I hope it's not a head gasket as that can be quite expensive and out of my expertise. The car itself doesn't necessarily overheat it's just the gauge not reading properly. You can drive it for hours and the temperature gauge will fix itself a lot of the time to stay in the middle. But every once in a while it will just randomly jump. I'll look into replacing a thermostat as well. I'll double check for vacuum leaks and while I don't have a fancy scan tool I can also look into techstream if necessary.

 

Does anyone by chance know the OEM part number for a thermostat? I was planning on picking this up from RockAuto however there's a whole list of thermostats and it ranges from 169F all the way up to 192F. I'm not sure if 169F, 180F, or 192F matters, but I'm just curious to know what the OEM requirements are if anyone knows.

More Information for AISIN THT019

 

P/N 90916-03129
Opening Temperature
179.6 F

 

Is there a gasket I am supposed to replace with the Thermostat? I noticed on RockAuto that some of them said including gasket.

 

Gasket p/n 16325-28010
They're sold separately.

 

Quick question

I notice when the engine is fully cold and you take off the radiator cap there doesn't seem to be any coolant in the radiator however after running the engine for some time and letting it cool down and then taking off the cap you can see the coolant in the radiator. The reservoir is currently near the low side but sometimes fluctuates as well is this normal?

 

Today I tried changing out the engine coolant temp sensor (I used a Denso one) and drove the car around, but the temperature gauge still keeps fluctuating. Would changing out the thermostat be a good idea? I can't seem to get any heat from the system currently. It constantly blows cold air only. Not too sure where to start.

 

Okay, so another small update:

I was able to get my hands on a fancier scan tool. After scanning the system there were no DTCs besides my passenger side AC servo motor that failed a while back which is causing my AC delay. Is there a specific way to determine if a thermostat is bad or not?

I tried to see if I could find some data in the fancier scan tool that shows anything related to the thermostat and I found Coolant Temperature and Radiator Coolant Temperature readings but what is considered a normal operating temperature reading in Fahrenheit?

 

After looking more into the scan tool here are some of the values I found.

When I went to the section labeled air conditioning and selected read data it showed “Engine Coolant Temp” at 58.28F but after driving around for 20 minutes while the gauge was still fluctuating it caps around 194.99F.

When I go to the Engine live data there is a box labeled “Coolant Temp” When the engine is fully cold it’s at 58.28F. However, when driving around when the gauge is in the middle it shows roughly anywhere between 205-215F. But then the gauge randomly rises and goes to 220F-235F but then goes back down immediately to 205-215F stays there for a few minutes and then keeps repeating itself to 220F-235F again and again but the car won’t overheat at all. It’s still a problem with the readings just being off.

Checked the radiator again this morning the coolant is full and the reservoir was in the middle roughly. Both fans kick
on at different temperatures. I eliminated the choice of it being the Coolant Temp Sensor. I replaced it and it made no difference. I can’t seem to get any DTC even after driving the car around for a while and clearing the system. There’s no code for overheating or even the computer recognizing a problem with the gauge not reading properly. The coolant still looks clean considering it was changed around 5K miles ago and the brand I use was AISIN ACT-002 (Pink Coolant). I’m still experiencing no heat at all and only cold air after driving the car around for 20 minutes today.

The last thing I think that could be a problem is maybe the thermostat and replacing the gasket at this point. I am almost at 300K so maybe it has gone bad over time. I bought OEM coolant from a local dealership and am planning on using a funnel kit again to get rid of any potential air that formed over the possibly 5K. But that’s all I can think of right now and this is as much research as I have done. I’m hoping it’s not something such as a heater core as that would be way out of my expertise. I don’t think I have a head gasket problem and I’m debating if a pressure test is necessary but there doesn’t seem to be any leaks of coolant anywhere either so that’s good news for now. Will still keep updating this post and I’m not sure if I am heading in the right direction on replacing these parts but I am still open to anyone’s feedback on what steps I should take. I’ll most likely be heading to the parts department for an OEM thermostat and gasket on Monday.

 

Looking at the wiring diagram I only see one Coolant Temperature Sensor shown. If correct, then the same sensor signal goes to the temperature reading you see on the scan tool, as well as the gauge. This is a suggestion only as I'm not an auto mechanic. As a test, with the engine fully warmed up, turn off the engine and unplug the wiring connector from the temperature sensor you replaced. Start the engine and see if the gauge no longer displays temperature. This would mean you disconnected the correct sensor. Also look at what temperature the scan tool shows or doesn't. If it too doesn't show a temperature, then the one sensor controls both. Turn off the engine and reconnect the wiring connector. Use the scan tool to clear fault codes. Start the engine and monitor the coolant temperature on the scan tool. If it doesn't fluctuate, but the gauge still does, then the gauge has a problem. If they both fluctuate then a wiring problem from the sensor to the control module.

 

Thank you for the feedback.

It's a bit rainy today so I'll try to get back to you later today if the rain stops or tomorrow morning. I haven't had a chance to disconnect the sensor when the car is at an idling temperature to see if the temperature gauge still reads or not but will do it soon and report back. I did get a chance to install the new sensor and have the car at normal operating temperature but the readings from the computer still are jumping around.

Is using any sort of connector cleaner for the coolant temperature gauge sensor a good or bad idea? I'm curious to know if that can avoid looking deeper into a wiring-related problem. Wiring problems are out of my expertise but was thinking maybe cleaning the connector could help. I didn't see any corrosion anywhere when inspecting the cable and I didn't see anything damaged to the wire either.

CRC QD Electronic Cleaner 11oz

 

I have another update:

The rain slowed down finally so I tried to disconnect the coolant temp sensor when the engine was cold and tried to turn on the car and drive it around. However, when I disconnected the sensor cold and tried to start the car the car wouldn't turn on and gave me a P0118. I cleared the code and plugged the sensor back in and then the car turned on and the code didn't pop back up, but the sensor reading isn't accurate still.

 

To check the thermostat, just feel the top and bottom RADIATOR hoses for the difference in temperature. The hose where coolant enters the radiator should be hot to the touch (usually top hose coming from thermostat). The other hose should be much cooler to the touch. Any time you open the cooling system there is the possibility of an air pocket preventing hot coolant from entering the heater core.
To confirm this use the hands again to test the heater hoses for the same temperature differential.
Super quick test that requires no more tools than mother nature gave you at birth.

 

Toyota thermostats generally fail partially open. The bracket that holds the spring in place separates, then the spring can't respond to the wax pellets expansion. Gauge fluctuation is probably related to the actual coolant temperature. You hand temperature sensors cant lie to you.

 

Thank you for the tip on how to check if a thermostat is working properly. I'll try to do this early morning tomorrow.

I just wanted to make sure I understood correctly. I know there are two radiator hoses. My thermostat is located near the bottom hose. So, I should let the engine run for maybe 15 minutes or so until it reaches normal operating temperature and then I should feel the bottom hose hot, and the 2nd radiator hose near the top feel cold?

I'm debating on replacing the thermostat and gasket with an OEM one tomorrow along with changing the coolant again to remove any potential air. Do you think that could solve the fluctuating gauge problem?

 

Also, one more question

I understand when you replace a thermostat that the jiggle valve is supposed to be on top. However, is this valve supposed to be pushed in so it sits flush along the thermostat or out like this photo below?

Replacement online manual: Toyota Venza: Installation - Thermostat - Service Manual

 

Do the testing before changing anything. Could be just turn on the heat and rev the engine to purge the system. Not time to start throwing parts at this point.

 

I'll try and get back to you this evening.

 

I drove the car around for about 15 minutes, so it reaches normal operating temperature. The top radiator hose was hot and the radiator hose on the bottom connected to the thermostat was cold. It was a noticeable difference. Does this mean the thermostat is working properly?