I have a temp sensor question. Let me explain. I have 2 engine temp sensors. The factory one in the left head. And a more modern one in the front of my intake manifold just left of the thermostat.
here's what's strange. Most of the time my car runs happily along right on 185 degrees. And both the gauge in the center cluster triggered by the sensor in the head. And the display for my EFI triggered by the newer style sensor mounted in the intake manifold agree with one another. Give or take a degree or 2. So, both temp gauges are in agreement when running along down the road.
here's the catch. So now were in stop and go traffic, sitting at red light after red light. The temp gauge in the center cluster starts to go up. Fans should kick on according to my settings, but there not. And up and up some more. Then finally the fans which are run by the ECU getting it's information from the sensor in the intake manifold kick on. Seemingly 20 -25 degrees higher than they should kick on.
So I go for a drive with the hand-held screen plugged in. (It normally sits on a shelf in the garage) And I watch the temperature of the engine coolant. Running along at 185. Both gauges agree. Sitting at the stop light or a long uphill. And I mean long. The cars gauge starts to climb. And climb. And then slowly the EFI gauge starts to rise. The cluster gauge gets up to 230. The EFI says 196. The fans kick on. Slowly the cluster gauge starts to come down as the fans are very effective. The EFI gauge holds 195. The cluster gauge gets down to 200. The EFI gauge reads 192. Slowly down it comes to 190 and fans switch off. And the cluster guage agrees.
So I buy one of those IFR thermometers. I shoot in intake manifold right next to the switch. It reads about 40 degrees cooler than the EFI hand-held reading. I shoot the Thermostat housing. It agrees with the EFI reading within a degree. Thick aluminium intake manifold is likely cooler on the outside than the inside. My aftermarket thermostat housing is not very thick and is also aluminium.
I try to shoot the area around the switch in the head but heat from the exhaust manifold is keeping me from getting any kind of accurate reading.
Is it likely that the coolant temp is much higher in the head than the intake manifold?
I replaced the sensor in the head when I rebuilt the engine about 6 years ago. But I still have the original.
would moving the EFI switch to the right head give more consistent results?
Anyone ever compare engine temps in different places on the engine as it heats up?

 

stop in traffic means water pump turning slow.

 

Granted, but in the last couple years I've had 2 water pumps. A high flow that started weeping, and now a standard replacement, no difference. None.
And those long uphills. These are very twisty roads. I'm in second and third gear most of the time. Tires are crying in the turns. RPM is up there and the difference in the two sensors is still there. Water pump speed shouldn't affect the difference in the gauges that much I wouldn't think. But waterpump speed high or low. This difference remains, but only when temperatures rise as stated above.

 

these motors have crappy cooling.
flow thru block then heads.
takes path of least resistance
look at some rear cross over pipes plumbed into thermostat housing
to direct water flow from back of engine.

 

Yes, but not really answering the questions.
if the factory temp sensor that feeds the center gauge cluster gauge reads higher then the gauge off the intake manifold. Is coolant temp. That is clearly moving coolant, really that much hotter in the head than in the intake manifold just a couple inches away?
do I need to replace sender in the head? Move the intake sensor to the other head?
What's with these differences in coolant temp just inches away? And only when temps rise?
Thoughts.

 

I have been checked various spots on my 406 with an IR gun, and its amazing how many different temperature there are. It could be the IR gun is not accurate on the various components...possibly. Its measuring surface temperature.. We do know that temperatures around the engine....combustion chamber hotter, headers much hotter.....but I am suprised at coolant temperature differences within inches.

 

Part of my issue is I purchased a cheap IR gun. And I just can't get a good temp read on the side of the head. That and the reading right next to or right on the intake manifold switch reads about 40 degrees cooler than if I shoot the thermostat housing. Not getting good info from IR gun. Looking for experienced answers.

 

Thats the readings I have been getting as well....not necessary 40 degrees, but significantly different between two very close locations.....and I don't believe it can be that different.

 

I doubt the cost of the IR gun matters. They just don't work well on highly reflective surfaces. Perhaps put a patch of matte black paint where you want a good reading? I usually just shoot the radiator hose.

I have the stock in-head temp sensor feeding my stock gauge, and it seems to track well. I was really skeptical of it, since the needle is wider than the temperature range of interest, but I've since gained confidence in the readings. It is in the left head. I don't see the spikes that you are experiencing.

I could not get the plug out of the right head (where the aux temp sensor would be for the L82 aux fan switch, for example), so I replaced my heater core outlet with a 2 wire temp sensor that is dead nuts accurate. It tracks exactly with the IR gun, and the fan control switch that is also in the front of the intake.

If the concern is that the fan isn't coming on soon enough, can you change the setting in your EFI controller? Can you add a second trigger to turn the fans on? Otherwise, hot spots in your heads at times of low RPM or high load, indicate a water flow problem.

 

I wonder if it's the mass of metal and water and the proximity to the combustion chamber that makes the sensor in the head read hotter.
I wonder what the difference would be if you put a sensor in the right side head, since it is closer to the rear of the engine.

I'm collecting parts to install a C5 electric fan assembly on my tired L-48 and am contemplating sensor locations.
I've considered using the right cylinder head vs. the intake.

 

move the fan sensor to the other head and do the same test....your heads are most likely the cast iron ones which will hold heat and the intake is aluminum which will pull heat and disappate heat faster. you will get better performance from your motor if its cooler longer so switch the sensor so its starting the cooling cycle sooner than later.

 

my fans are sensed in the right cylinder head and its pretty much the same as the center gauge says. the aluminum intake will be acting like a giant heat sink pulling heat out of the coolant as it passes by.

 

My heads are Edlebrock aluminium. My intake is also Edlebrock, single plane torker from the 70's. When I had just a thermo switch running the fans I didn't see this. Just strange that both sensors agree below 190 and then don't above that. Is it one bad sensor. Or is.it location? Can't find a good way to get a good reading on the head. Shooting the thermostat housing agrees with computer reading. Lowering the settings on the ECU any lower would have my fans running full time. And this clearly isn't a good answer nor is it needed.
perhaps the only answer is to drain the coolant and move the ECU switch to the right rear head location. And see what happens. Was hoping this temp gun would give me answers. But no way do I get a good read on the left head near the sensor. Do I have a faulty sensor. And which one?

 

Is the sensor for the center gauge original or a replacement...if its a replacement it may be wrong. I had this same issue. I bought a fully mechanical gauge. You can boil water in a pot and drop the sensor in so it isnt touching the side or bottom and use a real thermometer to verify the temp. Put that in the right side head and compare the 3. I ran that for a while. Also had an accurate one i put right in the upper hose for temps.
Lots of ways around this.
And you can verify your ir gun on the water surface as well. The upper radiator hose is a good surface, chrome or very shiny surfaces are not for a cheap gun

 

I would probably do what someone said, use pass head for the fan control.

 

If you really want to find out, then just swap the leads to the 2 sensors and get another set of readings and a temp gun reading.
I'm thinking one of your sensors is "wrong".
I have both of my sensors, 1 for gauge, and 1 for EFI, on either side of the thermostat outlet. They both track appropriately. Before I dialed in my gauge with an adjustable resistor it would always read low.
I suspect that your sensor going to your gauge is incorrect. Just for funsies I bought a suggested 3/8" sensor to see how it would read..... I could dial it in to read the same as EFI at 175°-180°, but it would deviate considerably as the temperature went up.
Let me find that post..... but I suspect the sensor for your gauge may be incorrect.......gimme a sec.

 

Here is the story where I tested a suggested Duralast TU-66 sensor in my 68. I dialed it in to read accurate at 175°-180° along with the EFI readout. It deviated considerably (similar to your experience) as the temperature went up. Pictures are included. You can read the rest of the thread for more background info:

https://www.corvetteforum.com/forums...post1603572711

 

Doc, I would not expect that much difference in the coolant temp from the head to the intake, it's moving fairly rapidly through both. I have found more than one inaccurate center cluster temp gauge before and, for some odd reason, they always read too high when they are wrong. I've had the same issue on a couple of C2 temp gauges as well. Latest was on my '65. It acted exactly like you're saying yours does. I was constantly in fear of a boil over in slow traffic, according to the gauge, but never seemed to get the symptoms of a hot running engine. Finally changed out the gauge and it is now in line with my IR readings and all is fine. They aren't that expensive to replace. If I had to guess (and I am) I'd replace the gauge with a new one.
Greg

 

Thanks for all the comments guys.
first off I can't just swap wiring from one sensor to the other. While this sounds like a great idea, one is a modern sensor with a proper weather sealed 2 wire connector. The other is the 70's style that grounds through the head and has but on wire clipped on.
Also, inspite of high gauge readings but only when going up. The car does not show any signs of over heating. (Still, high gauge readings are unnerving).
pointing the IR gun at the thermostat housing seems to agree with the EFI sensor. Pointing the IR gun at my highly polished stainless steel flex type upper rad hose reads lower! Sorry, I don't run a black rubber upper rad hose.
when I rebuilt the engine about 6 years ago I replaced the temp sender in the head with a new one I purchased from one of the Corvette vendors. I replaced it "Just because".
at least 2 people have reported that these after market senders can read good at 180 but off when temps go up. Which I hadn't heard before I asked this question. I am certain that although I have moved 8 or 9 times in the last 6 years I have that original sender somewhere.
So I am going digging for that original sender that goes in the left head.
I don't wish to move my EFI sensor if that won't solve the issue. As everytime you open up that cooling system your going to make a mess. And then there's the rewiring part and I want the wiring very tidy so that's a project in itself.
so with information at hand. I'll try the factory sender in the head first and if that works I'll only make a mess once.
I'll keep you all posted.

 

That' s a good plan.
My old original sender is installed right now. I installed an adjustable resistor from Willcox (no longer around) and it works like a dream and tracks with my EFI. Don't toss your sender and/or gauge if it reads low. I think you can order an adjustable resistor from other vendors now..... mine read low for decades before I decided to address it when Willcox came out with their adjustable resistor. Worked great.