Hello. I have an 84 f250 with the 5.8L HO. I got this truck this summer. I live in northeast ND, and when the weather turned cold, I noticed that the heat wasn’t working. So, I figured I’d start by fixing the temp gauge. To do this, I replaced the coolant temperature sensor, because when I disconnected it and jumped the wire to the negative terminal on the battery, the gauge looked to work fine.

once the temp gauge was working, the engine would never get up to proper temp, so I assumed the thermostat was stuck open. I replaced the thermostat.

Now, with the thermostat replaced, the heat in the cab works well, but the temp gauge reads likes the engine is overheating even though it’s not. I’ve been flushing the radiator, so it just has water and cleaner in it. To see what temp the coolant/water is at, I opened the radiator cap after driving for 20 minutes. I stuck a thermometer in there with the truck running and it only read about 125F at the hottest part I could find.

any ideas on why the temp gauge reads so high even though the actual temp isn’t that high? And why does the water in the radiator only read 125F.

I even bought a second coolant temperature sensor. Both duralast tu25.

I’ve added pics of the gauge as well as the thermometer inside the radiator.

thanks

 

Mine reads low just a hair over the line at the "N" and like you I replaced the stat and sender 2 times as I forgot if I did the stat and thinking the sender was bad.

Also like you I got the same sender from the same place each time and I wonder if the senders are the issue?
Are you sure the sender is for a gauge and not a light? Me I am not 100% sure and when I think of it may go to a different store and get a replacement sender.
Dave ----

 

Because it's a piece of junk factory gauge system that tells you nothing but "Normal". I always put aftermarket gauges on any engine I care about. Oil pressure and temp gauges.

 

Yeah, I hate that it’s mostly just ‘normal’ with no numbers.

Do you install aftermarket gauges that go in the dash cluster, or do you mount them somewhere external?

 

Im pretty sure it’s a coolant sensor. Duralast tu25

 

I just mount them under the dash in the panel that comes with them. If you want to make it a project later, you can make your own dash panel or mount them in the dash where ever you want.

 

Regarding the original post, does that temp look normal? The thermometer is a few inches down inside the radiator? I feel like I can tell when the stat opens and closes, because of the water flow.

 

You are correct about the waterflow. You can also tell because the upper hose will be cool, when the thermostat is closed, and hot when it's open. 125F in the top of the radiator is plausible, but the temp will vary a lot depending on where you take the reading. And do you remember what temp thermostat you installed? Original is something around 192. The store will usually sell you a 195 which is perfectly fine. But you have the option of a 180 and I think even a 160 degree thermostat. I have run the 180 before with no problems.

 

The temperature you recorded is probably just fine. You are reading on the cool side of the radiator. Hot coolant from the engine, typically around 180F, enters the opposite (passenger) side of the radiator. The hot coolant then moves sideways through the core, where airflow extracts heat. You want to see at least a 30F drop in temperature from the hot to the cool side. The temperature drop can be much greater on a cold day, which I believe you're familiar with in ND.

For an accurate idea of what the thermostat is doing and the factory temperature gauge is seeing, you'll want an infrared thermometer. No whining, they are cheap nowadays and very worthwhile for any mechanic. Aim the spiffy laser pointer at the thermostat housing or the top of the radiator tank on the passenger side. The value you see should be close to the rating of your new thermostat.

 

Please see my previous reply about getting an infrared thermometer, and where to take the temperature measurement. There is no guarantee your new thermostat is accurate, especially with the Fling Dung brand stuff peddled by the discount parts chains. I have found thermostats to be one of the items most likely to be bad from stock. OEM, quality aftermarket, cheap aftermarket, it doesn't matter. I've learned the hard way to always test a new thermostat before installation (not just Ford, but other vehicles, too). With an infrared thermometer, you can test the thermostat already installed to make sure it is reasonably accurate.



If the infrared thermometer shows the coolant temp is within range (close to the rating of the new thermostat), you will need to investigate the indication system. But please oh please oh please confirm the actual temperature first, before loading the parts catapult (Pull!) in hopes of a fix. For the moment, you don't even know if the actual coolant temperature is high (note the stellar heater output) or the indication system is inaccurate.

I see you have already met Dave. Here at the Stock Gauge Anti-Defamation League (SGADL), our trained counselors are working hard to implement a customized treatment plan for him. Unfortunately, he keeps chewing through the restraints and escaping the facility. Without further intervention, we fear he may turn to a life of crime to finance his addiction to $15 Chinese gauge sets.

If the indication system needs some love, there are some simple steps we can walk you through for troubleshooting. But please first get a good temperature reading at the radiator inlet (upper hose, passenger side) or thermostat housing and report back. It wouldn't surprise me if you got a bad temperature sensor, and the second one came from the same defective batch. If you ever suspect a new part is bad from stock, make sure to get a replacement from a different source. I don't care to discuss how I learned this.

 

Sorry to say, inflation has hit them also. I try to buy the ones where the needle moves all the way around (270 degree versions). This gives you a lot of needle movement and spreads the numbers out for a easy indication. But I sometimes settle for the ones that sweep less like the factory ones do.

The ones below are $48 on Amazon.

 

Gauges work on 2 principles.
5-6 volts supplied by the IVR (instrument voltage regulator). Not sure where its located on trucks, usually on the back of the cluster. Looks like 9 volt battery connection.
10 to 90 ohms input reading from the temp sensor. I see a lot of complaints of new ones not being accurate.

Fuel gauge is the same.

 

If it was the IVR then all 3 gauges, fuel / oil / temp would read high in his case or low.
We cant see the oil gauge and guessing he has 3/4 of a tank so it would be just the temp gauge he needs to trouble shoot.

As a test he could swap the oil & temp wires and see if they do the same thing, oil read high and temp normal.
That would show a sender is bad.
One day I need to do this to mine as the temp gauge reads low but I am ok with that as I have tested my system and know the temp is normal.
Dave ----

 

Ok. So, I did some testing and the results aren’t making a ton of sense to me, so I’m hoping you can provide some guidance. FYI, I recorded the process of taking temps and attached the video…in case you want to watch that first.
I was using one of those funnels that attach to the radiator fill neck/hole. Here’s the process I went through. I filled the radiator with coolant, bled the heater core, squeezed the upper radiator again and again to get the air out. When the thermostat opened it burped out a bunch of air and filled with more coolant. I, again, bled the heater core and squeezed the upper radiator hose incessantly with both hands. Air kept coming out, so I let the truck idle for around 1/2 hour. Once it seemed to have little to no air, I removed the funnel, and put the radiator cap on. Then, I went for a ten mile drive where I topped out at around 50 mph. After getting back, I recorded these temps(again, this part is in the video). All temps in F.

stat housing just under where the hose attaches 120 degrees

engine block about an inch from where the stat housing bolts up 165 degrees

Upper Rad hose close to stat housing 112

mid rad hose 128

rad hose close to rad 145

rad next to hose inlet 164

rad by cap 116

bottom of rad on drivers side 112.

I have master force infrared thermometer. In the video, you can see that I hopped around and took more measurements after these measurements.

Also, it’s a 192 thermostat from the duralast housing, gasket, and thermostat kit.

The truck drives well. I’m not sure if these measurements are normal. If they are normal, I believe we just need to figure out the indication system. The rest of the gauges work well.

 

The numbers look fine, if anything a little on the cool side. Some surfaces, especially hoses, don't always accurately show the internal temperature as the rubber acts as an insulator. The values on the metal surfaces certainly don't show overheating, and you've got a healthy temperature drop across the radiator.

Time to move on to indication. Dave should be along shortly to admonish you to give up troubleshooting a very simple system and install aftermarket gauges. If you choose to give in to the dark side, we at the SGADL recommend you wear chainsaw chaps to protect your legs. We've had too many horror stories of unsuspecting drivers accidentally slicing off their legs on those unsightly gauges hanging below the dash. Here at the SGADL rehabilitation facility, we have taken the lead fitting prosthetic legs to the injured, but we'd rather those services were no longer needed.

To troubleshoot the indication system, ​​​​keep in mind the oil pressure, fuel quantity, and temperature gauges are the same internally. The only difference is the label on the face. The different senders all operate on the same range of 10 ohms = H and 73 ohms = L. You can use test jumpers and swap connections between the oil pressure and temperature senders. Run the engine briefly and now your oil pressure gauge will display the signal from the coolant temperature sender, and the temp gauge will display the signal from the oil pressure sender. This will quickly isolate whether the gauge or sender is at fault.

If you confirm the sender is at fault, please see post #10 above for sourcing a replacement.