There was an old thread from 2018 where someone had the exact same issue but they never did a followup. I have an 85 F150 with a 300, New Process 4 speed 4x4 that is having overheating issues. New water pump, Thermostat, Sending unit, checked ignition timing, fantastic compression test results and it will idle all day at the lower end of the temp scale. The moment I try to move it, it overheats. Like within 100 feet overheats. It is very very odd. The upper radiator hose gets hot so it's pushing water and the heater pushes hot air out the vents so it's not vapor locked either. I'm kind of at a loss. I guess this is why people should post follow ups when the solve a problem.

 

Well that is a strange one
First is this overheating going by the factory temp gauge or aftermarket gauge with numbers on it?
If factory temp gauge, where does it run when not overheating and then when overheating?
Did this just start and if so what changed or has always done this or the truck is new to you and no clue other than the water pump, Stat & sending unit?
You say as soon as you start moving what if you dont move but just raise the RPM to say 2500 will it do it then?
When doing this test do the paper sheet test. Take a sheet of paper and hold it to the grille at idle does it stay in place when you let go or gets pushed way?
Raise the RPM to 2500 and do the paper test again does the paper stay or gets pushed away?
Maybe the fan was installed backwards and is pushing air forward and not pulling air back?
Does the truck have AC and if so factory or dealer installed and a fan shroud?

You may want to get some real temp numbers using a IR temp gun. You can get a cheap one at HF and it will give close enough numbers to know what is going on.
When cold remove radiator cap and make sure level is good. Start the motor and let it run. You can do the air flow paper tests above.
When the top radiator hose gets hot do you see flow in the radiator? How does the lower hose feel cold or warm?
If you can feel the front side of the radiator (non-AC) or motor off back side, feel the radiator core, top to bottom and side to side you should feel the same temp across the core if not and you see flow you have a bad radiator and needs to be replaced.

Report back your findings
Dave ----

 

For anyone that come cross this post and has installed a new stat and has problems bleeding the air out, dont know why you would?
But if you drill a 1/8" hole in the flat part of the stat that will let air through and not hurt how it works.
When installing put the drilled hole at the top.
Dave ----

 

Oh just a thought does the oil & fuel gauge work right when the temp gauge goes to "over heating"?
Dave ----

 

We pulled the engine to replace the clutch and rear main. The engine is a 1993 was rebuilt about eight years ago and has been very carefully maintained and as you can see in the photos there is almost no sludge formation. Figured while it was out replace the timing gear, front seal, all the various pan/cover gaskets, thermostat, water pump and sending units. It's the factory gauge and all the other gauges read fine. When idling it sits on the "o" of Normal. The radiator is only about a year old. It has factory A/C but as I had it out of the truck I was going to convert to R134 so it's not reinstalled yet. I bled the air out of the system by filling with the top heater hose off and filled until the coolant came out the top of that. Fan wasn't removed from clutch so that orientation didn't change. Did a compression check while we had it on the stand and it was pretty much 185 psi across the board except for cylinder 3 which was at 179 psi. I'm going to repeat the compression test this morning so I'll read the new plugs then. I'm a retired Fire Chief who still volunteers so I have a FLIR so I'll take several readings on the block and head.

 

Is that the 1993 EFI head or the 1985 pedestal mount carburetor head?
If the engine is in a 1985 truck, what are you using for an ignition system?

 

The kids I bought it from had butchered the original engine and support wiring. The replacement was a 93 EFI engine but I converted it to a 84 carb system and an aftermarket HEI. It ran beautifully until we put it back in.

 

When you checked the timing did you check to see if the timing is advancing with rpm.
The EFI engine with a stock cam needs 10 degrees initial timing at idle and no more than 30 degrees by 3000 rpm.

 

Okay, at this point I'm 99% certain it's a head gasket.
Compression test this morning:
Morning Evening
1-190 psi 190 psi
2-180 psi 180 psi
3-180 psi 180 psi
4-175 psi 178 psi
5- 190 psi with coolant system filled but 170 psi nine hours after draining and see plug photo
6-180 psi 180 psi
Cylinder #5 plug came out wet but not gasoline

 

There is something going on with #5 looking at it and the rest look like they are running lean, #5 rich.

I would do your compression test empty and then fill the cooling system but leave the radiator cap off and see what you get.
I would even look to do a leak down test, cooling system full & cap off and see what you get.

They say there is a test where you can check for exhaust gasses in the cooling system like you dip a paper strip in the radiator, I have never looked into this.
The other thing you can do is with the spark plugs removed put pressure on the cooling system and see if it holds.
If the head gasket or a crack in the head or block the pressure would not hold and if done long enough and it leaks into a cylinder if you were to crack it over, no plugs, coolant would push out the plug holes on the leaking cylinders.
That is how I would go about testing.
And coolant in the oil I have hardly seen but dont count it out if the block is cracked or coolant sits in a cylinder for a long time to leak past the rings.
Dave ----

 

Number 5 came out wet with coolant. Replaced the head gasket and now the gauge comes up to the M in Normal then settles back to the R and stays there. That was an odd one as there was no white smoke and the coolant looked normal. I'm just glad it's happy again. I am having a dieseling run on issue but I can't find my timing light. It's gotta be around here somewhere.

 

It was the head gasket. Replaced it yesterday and finished hooking everything back up today. Running great and keeping cool. The dieseling was timing. The plugs are looking good and all cylinders are within 5 psi of each other.

 

Thaks for the follow up as I would never have guessed it was the gasket without more testing.
Did you see where it was blown out?
Any clue why it went bad?

I was going to say the run on could be to high of a idle speed.
What was the timing set to that it ran on? I have had mine up to 14* BTDC and not had any run on only pining so I am at 12* BTDC now but still get a little so I been running higher octan gas as I dont drive it much but when warmer will go over the timing curve again to dial it in better.
Dave ----

 

The run on was just timing. It's happy now. Couldn't really see anything on the gasket but it must have just happened. The coolant on #5 plug was the give away. As I'm about to get the AC updated to R134 and running again I'm installing a 3 core radiator and electric fans. That should keep it nice and cool.
Thanks for all the suggestions.

 

It most have just started if it was not really showing any real signs of a bad gasket

I am not a fan of electric fan to replace the motor driven one.
Most posts you see about running hot and when you finely get it out of the person they just installed electric fans.
To have them cool they need to be good, read $$$, fans to pull the needed air through the AC condenser & radiator.
And after you spent the money for the good fans you find the charging system is not up to the job as the fans pull a lot of juice from the battery.
So now you have to spend more money to up grade the charging system.
Most go for a 3G ALT and that then leads to rewiring to take the AMP meter out of the picture, can be replaced with a volt meter from Rocket Man.

A lot of work and money for something that may not work and for what to save the load on the motor?
You have just changed it from the motor driven fan to a motor driven ALT so have not really saved anything.
Besides being you have AC you should have a clutch fan so when it is not needed to pull air it unlocks and free spins saving power.
Cant really beat the factory set up in my book but it is your truck do as you wish.
Dave ----