Straight Six Running Hot

My temp gauge was stuck near low. I grounded the temp sensor wire and the gauge went full tilt so I know the gauge is operating. I replaced the temperature sensor and Eureka! I now have a working temperature gauge.

Now that the gauge is working, it appears that the engine is running hot. All my other fords had a temp gauge need that rode center or a little to the left of center.

I just got this truck in June 2020, and it looks like it has a new radiator. I can feel the pressure in the hose from the radiator to the motor when the truck is running.

I don't know where to begin troubleshooting. Any ideas how to troubleshoot this? I've added a photo and the gauge climbed even higher when idling in traffic.

Just a few questions
1 Does it have a fan shroud?
2 is it the correct fan shroud?
3 is the thermostac fan clutch locking up and pulling air through the radiator?
4 You say that it "Appears" to have a new radiator. When was the last time the thermostat changed?
5 What is the temp of the upper radiator hose while the engine is running at the temp on the guage?

Steve, thanks for the reply. To answer your questions:

It does have a fan shroud. I don't know if it's the stock shroud or not.

I don't know how to tell if the clutch is acting up but I will do some searching on this.

I don't know when the thermostat was replaced. I just got the truck in June 2020. I will look into replacing this.

I don't have a temperature gun but I'm always looking for a reason to buy a new tool.

Found some info on troubleshooting the fan clutch here

The gist: "To confirm the diagnosis, start with this simple test: Spin the fan as hard as you can on an engine that has not been started that day. If the fan rotates more than five times, you can bet the clutch is bad. You should feel some resistance and the fan may spin up to three times, depending on the ambient temperature."

I will let the truck cool down and test the fan clutch tonight.

I also looked up the thermostat. It seems like a straightforward part to replace and it's inexpensive so I'll go ahead and change it out.

Thanks again for the tips Steve!

A few things to also look at on the fan, look to see if there is oil leaking out of the shaft at the water pump, look to see if there is oil leaking from the front seal where the temp sensing spring is.

Another thing to look at is the radiator cap, if any of the rubber seals are cracked, hard or broken, replace it. Test the cap with a preasure tester. I want to say that you should have a 13 lb cap but look it up and find out for sure. If the coling system wont hold preasure, it wont cool properly.

Before you get too far into troubleshooting, I would confirm the temperature with a decent quality gauge. The factory temp, oil and fuel gauges are powered by a pulsing voltage regulator and they are notoriously approximate. With age, the approximation becomes worse... the temperature gauge on my 1981 straight six reads quite high... I have replaced the sender and I have plugged in a couple spare clusters (to test if the ICVR and/or gauge itself had issues) and it still reads high. My mechanical VDO fluctuates right around the advertised advertised thermostat opening temperature of 195*F so I know my cooling system is okay. One day I will revisit trying to get the factory gauge to read correctly (whatever that is on a gauge with no numbers or units) but the mechanical gauge will stay regardless because I refuse to put full trust into the factory electric gauge.

This may or may not be your issue, but I wouldn’t chase my tail diagnosing the cooling system based on what the stock gauge reads. Just my .02

As you saw, I wasent advocating changing parts just because the, less then accurate when new, now 30 plus year old gauge is reading a little on the high side. Instead, I was asking questions that would ether lead him to the fact that his cooling system is in fact doing it's job and the gauge is wrong. Or there is something accutaly wrong with his cooling system that needs to be addressed.

Like you, I have a similar problem with my temp and gas gauge. When it rears it's ugly head it means that I need to pull my radio and clean the ground stud that is welded to the inside of the firewall and it goes away for a couple of years.

old55pete wrote

Like you, I have a similar problem with my temp and gas gauge. When it rears it's ugly head it means that I need to pull my radio and clean the ground stud that is welded to the inside of the firewall and it goes away for a couple of years.

Thank you! I will try this!

Ford F834 wrote

Before you get too far into troubleshooting, I would confirm the temperature with a decent quality gauge.

YES!  Get an infrared thermometer and check the temp at the thermometer housing.  I doubt you have a problem.

I have mechanical aftermarket gauges on Big Blue with a brand new engine, water pump, hoses and brand new aluminum 4-core radiator - with new coolant to boot.  And with a new 195 degree 'stat my temp varies from 200 to 205.

In addition I've put a very accurate voltage regulator in place of the original ICVR, which means my factory temp gauge is very stable.  I just walked in from a 160 mile drive and the factory temp gauge was glued on the M of NORMAL for the whole trip.  But if I wanted it on R or A that would be easily done by lowering or raising the voltage on the regulator - although that would also change the oil pressure and fuel level gauges.

Basically I'm saying is that the factory gauges are decent with a good ICVR powering them.  But even then they are approximate.  If you want precision get a mechanical aftermarket gauge.  Mechanical because most of them have a 270 degree sweep and you can read them pretty easily.

Still, with all of this being said, he still needs to figure out if his cooling system is doing it's job or not. There does need to be some trouble shooting to make sure everything is operating with in what it was designed to do.

Agreed. When I put in a 195 T-stat into mine, my temp readings when driving around town tended to be a bit past mid-gauge... so this might be normal. IR thermometer would be the way to go if there is a concern.

I'd add that it's best to put a piece of masking tape on any bright metal surface before measuring with a non-contact IR thermometer.
Reflective surfaces effect emissivity.

As said you need to check just what the cooling system temp as that factory gauges are not great.

He would only have a clutch fan & shroud if the truck has AC.
Non-AC trucks don't use a clutch fan and only have a small top of fan shroud so the fan will not eat you if the motor is running.
If he does have a full AC shroud the fan blades need to be half way or a little more inside the edge of the shroud.

Now I have just the reverse of you.
My temp gauge only get s up to the 2nd line on the cold side.
2 new 195* stats, new radiator and hoses, new sender and all grounds are good as I made sure when rebuilding the truck. The oil & gas gauges work great.

Now I do have AC, new condenser and full shroud I had to come up with.
But I am using a non-clutch 5 blade fan and have a lot of air flow thru the front grille using the paper test.
Even the IR gun give a good temp so it has to be a gauge issue so I am not going to worry much.
Oh the gauge will go up a little more when it is 100* out and I have the AC on and taking back roads home from work but still no where to the center.

First you need to see what the system temp is.
Air flow thru the grille at idle, does the paper stay to the grille?
Being this happens at idle it looks like an air flow issue.
AC condenser plugged with dirt?
Does the lower hose have a spring inside it? Check when motor is cold.
Dave ----

I’m glad my cooling issue is a HOT topic.

I’m ordering an IR thermometer and will post my findings.

My truck has an aftermarket AC system. I will post pictures when I get the thermometer.

I got my thermometer and took some readings.

I spun the fan and the fan is stiff and doesn't rotate easily by hand so I think the fan clutch is okay. There is coolant leaking from the heater core. It's not a major leak that has a large impact on the coolant level. I do notice some vapor in the dash vents.

The reading off the radiator hose of 159 is the one to use.  The one off the engine of 251 is probably getting some heat from the exhaust system, but it doesn't really matter as we are concerned with the cooling system.

But the 159 reading is lower than I would have expected.  Was your engine really/fully warmed up?  If so, I would have expected it to be close to whatever thermostat is in there.  And, unless that reading is an error, your engine certainly isn't overheating.

And your fan seems like it is working well.

So, if you had driven the truck and it was fully warmed up, then I'd take a few more readings on the upper radiator hose and thermostat housing.  If they are consistent at ~160 I'd assume there's a 160 'stat in there.  Then you have to decide what you want for a thermostat.  Personally I like the factory 195.  But I'd want to know that the readings you are seeing are consistent with the 'stat.

Then you can troubleshoot the temp gauge, knowing that the cooling system is "right".

It sounds like the thermostat is ether stuck open or is too low a temp as it should be around 195 by the book.

You might want to give a thought to changing the heater core as well. If you remember what I said about the radiator cap earlier, it is supposed to hold a certon amount of pressure in the cooling system. I am pretty sure that yours needs to be 13 lbs, but you need to look that up and be sure. In any case, the cooling system needs to be able to hold that amount of pressure to do it's job.

For what it is worth, I had normal temp readings until I changed my sensor to an aftermarket one. The gage now reads just a little less than yours. I do not believe I am running hot but will be delving further into it.

You could put it in boiling water and quickly ohm it out.

Then follow the ohms as it cools, using the IR gun to determine 210, 200, 190, 180, 170, 160F

Resistor combinations at the sensor end of the harness would allow you some idea of '200 = the high side of M" "190 = the low side of M"
or whatever....