Well, we have helped you overcomplicate what is probably a simple problem by giving you obscure scenarios. Try a different thermostat, and check for a head gasket leak. I guess you found out what 40+ psi of water system pressure does to a heater core.

And thr plug in the thermostat housing is not for bleeding air. Get it out of your head that there is anything in that area that has anything to do with air in the system. If after the normal burping sequence does not work and you are still getting air, the most likely thing is a bad head gasket. But don't go teating off the head and asking a bunch more questions until you have tested the cooling system for a head gasket leak. You should be able to buy a test kit at any good parts store.

 

There are two different worlds inside the coolant system in your truck; Behind the thermostat, and ahead of the thermostat.

Behind the thermostat would be the heater core, and the engine block. If you open anything in that area, or drain the engine block completely, you will have air in the system.

I know it's not right, but most older American cars and trucks do not have a bleeder for behind the thermostat. Most foreign cars do and have always had one. A lot of the newer American cars do now.

During your last heater hose replacement, all you had to do is drain the coolant, replace the heater core hoses, and leave one of them off and not connected. Then take your funnel and start filling the radiator. As soon as coolant started flowing out of the heater core hose that you did not connect, reach over and connect it, then tighten the clamp. You will still have air in behind the thermostat, but not much.

You then continue filling the radiator. You do not have to fill it all the way, some will overflow.

Start the engine with the radiator cap off. You can put a little bit more in the radiator as it's running, but as it warms up it will puke out coolant. You can just sit the radiator cap on there, and some of it will go into the overflow bottle. But you will still get some coming out. This is the rest of the air getting out of the engine.

It will take several minutes, and several puking episodes, but finally you will feel the whole upper radiator hose is hot. When this happens, top the radiator off and put the cap on tight.

Take it for a drive, your temp may get up a little bit but not much, because you got most all the air out. During your previous drives you temp went way up because you had so much air in the engine. And hot air does not open the thermostat as well as coolant does.

During your other episodes, the thermostat finally opened, all the air went into the radiator, and then afterward you topped the system off and everything was back to normal, no overheating, but your factory gauge has failed and is not working correctly, I can see that from your posts. If you are over on the "H" on the factory gauge that will be something like 250+ and you will have definite signs overheating.

 

thanks again for sharing your knowledge guys. really grateful for the help here getting this to resolution. it rained all day here, so did not get outside.

tomorrow i will try burping one more time following directions step by step so clearly provided in last post from Franklin and see what we get.

I got this Equus mechanical aftermarket gauge and will install where sender goes here so post burping can find out the real temperature.


One question before doing this tomorrow is about the heater core. since it seemed not to make any difference per se was still hot either way, i prefer to reconnect hoses because the bypass seems to have several bends in the hose vs the passages in both hoses not hindered when going right into heater core.
per this i dont know if you are saying heater core bad now?
I literally just put the garden hose on the inlet from core at firewall with no kind of adapter at all (insert image of choice here, but don't be too harsh)
and ran water. it was not sealed at the inlet. so i am guessing water was likely going into the core and also around it and right into the vents.

meaning heater core still fine vs "blown" by pressure? i read somewhere maybe on an aftermarket gauge thread, franklin said his truck is a tool to do the needed work not for show hence gauges give real info to see what it's doing. also in this case, not a tragedy to see water coming into the vents......
just need to know if reconnecting hoses to core okay because the bends in the bypass position seem difficult to get open/to keep from folding with hose length.
again appreciate your generosity sharing your knowledge....does good out here.

 

p.s. after last try at burping and aftermarket gauge install if no change in high reading- onto try new Tstat and start considering head gasket, WillowBilly. thanks again

 

gathering final thoughts for day here to reference if needed later
1) i did some searching on forums and found info on bypassing heater core, so going to keep it bypassed for now. saw discussion of whether to plug or bypass and am going with bypass for now. will use an elbow if needed to get a better lay of the hoses.


2) this is a camaro forum but showed a good description of what happened with the geyser here
after reading step by step from Franklin2, i am understanding this was likely a pretty good size air pocket from when i swapped the heater hose.


3) also just noting that saturday this happened again- after drive across town, Tstat went to left of center and stayed down all day again. joked that the truck only likes driving on Saturdays (it was previous Saturday we had the "perfect" run all day.)


4) double checked the water pump part number. installed is CP4020N which says it is part for carburetor, not EFI. it matched the one that was taken out though.
had o'reilly look up water pumps and found CP4048 listed as the one needed for EFI. i told the o'reilly guy a forum post said He said when he looked at the pumps both said they ran clockwise. and symptoms have never indicated water pump is completely not functioning/running backwards. the part given was for not EFI, though the truck has the plate on the back that says Electronic Fuel Ignition. he wondered and F150 forum also said someone may have swapped out the engine or put EFI on at some point.


5) Using the Temp Gun per to try to get temps when no aftermarket gauge install time, these were actual temps. learning here appears to be temp guns not as simple as point and have real info.
upper radiator hose 130
heater core inlet (when still connected) 222
heater core return 178
Tstat housing 143
Temp Sensor 214
at surface near spark plugs 202 or 205 (some lower 173)


6) if it goes to get new Tstat, saw some posts and wonder if getting this Murray Plus Ultra vs the Murray 3359 presently on it would be better. Or if it is advisable to go to a 180 like this. Seems like i saw mixed posts of it needing to be only a 195 as the computer is expecting this or some saying they run a lower one (160) on all of their trucks. as said, noting here while i had time to do some looking around. Seeing about Tstat will be after following Franklin2 Dave step by step.

thanks again

 

On my 89 F250 during the summer time I bypass the heater core to keep it heat soaking the heat/AC box inside the cab. With temps in the mid to high 80's here in SW Florida,guess its time to bypass the heater core now.

On the 4.9L I6 engine with EFI, you MUST BYPASS the heater core and NOT plug them off. The reason is the ECU temp sensor is located in the heater supply line and needs the coolant flow across it to operate the engine correctly.

The 1st time I did a bypass, the bypass hose folded up and would not let coolant flow past the ECU temp sensor. Got some bad MPG's and poor running engine till I figured out what happened.

 

There is a trick you can do and avoid swapping hoses each season. You can go to the store and buy a heater valve for a 96 Ford ranger. It has 4 ports on it with a vacuum can. When you apply vacuum, it closes the heater core off and routes the coolant in a circle back to the engine.

What I did on my 89 diesel is inserted this ranger valve in the heater hoses, and then run a vacuum line over and put a tee into the actuator over near the pass side hood hinge. This actuator is the inside/outside air door. When you put the dash control on MAX, it puts vacuum on this actuator to swing the door so you use inside air. Since I put a tee in this line, when I put my control in MAX, it also applies vacuum to the ranger water valve, which cuts the water to the heater core so if I have a minor air leak around the core, it will not affect my A/C in the summer.

The valve costs about $20.00 at the part store.

 

I cannot figure out how to get to the temp sender location.

I read this "You have to pretty well stand on your head in that corner of the engine bay to see it."
I read this "I go through pass wheelwell to access these," and

Either way, it's probably easier from the top but you can't really see it so you kind of just have to do it by feel. You can see it from underneath, but it's hard to reach up there. I can get to mine through the wheel well, but I have a 3" body lift.

Thanks Raven, I removed the plastic cover over the tire and was able to reach the temp sender. That fixed the gauge problem. Thanks again for all your help.

and i got this picture.

 

have old thermostat off right now and tried testing it in pan of boiling water.

it doesn't appear to open at 195. doesn't open until 212 and not even right when it first hits....about 10 to 15 seconds at 212 before it opens.

i already got a murray plus ultra replacement and it also doesn't appear to open at 195. used this video
guess both of these are bad and onto number three.

there is a motorcraft at another parts store for $18. wasn't wanting to pay 3x as much if not needed.

 

Shop around for price for the Motorcraft.
Or whatever else you want to try.
Just do the test at least 3 times making sure it does what it
is supposed to do.
A shame something as important as a thermostat
has gotten to be of such poor quality.

 

With the current no longer made well parts situation, ALWAYS TEST THERMOSTAT BEFORE INSTALL is the LESSON LEARNED.

After the heater core bypass and proper burping still hot and couldn't get to the sender to install the aftermarket gauge went for remove and test the Thermostat.As reported, it did not open until 212 for a while. So it was opening late and perhaps not completely. I couldn't find the full lift measurement in the manual to check.

I then tested 2 more brand new Murray Themostats (a new 3359 and the ultra MU55359). Neither opened at 195 either! The "better" one didn't open until 205. The replacement 3359 didn't open until 210. 3 tests each. I did not have $20 for the motorcraft that day and couldn't wait to order it online because must have vehicle for work. So I got a Murray Ultra 180 MU55358. When I tested it, it opened just over 190. Tested 3 times. So the 180 opened at 190. Went with that.

PROBLEM SOLVED. Did the burping properly with radiator T in the highest point on the heater hose. Also did drain and fill radiator a few times before filling with clean 50/50. Got in and gauge climbed to O/R. Tstat opened and went down to N/O. Never went past the middle again. In the mornings first few days, was up to O then down to N when Tstat opened, so thought maybe it was going to run too cool. Now it has settled and is running pretty much right in the middle R/M as before this learning experience. When it's chillyish first thing in morning, lower. Will keep an eye and maybe see about ordering the Motorcraft if it seems to be running too low.

Also going to leave heater core bypass as is for now. Next winter if temperatures press for it, I will reconnect and see what happens..if it is blown or that water was just leaking around and in. See what temp does, if we smell coolant, etc. if called for and still here and running. That was a neat trick given for bypassing it by season without having to disconnect and reconnect hoses.

Really grateful for all the bits and pieces gathered here learning. I decided to get back the $20 I spent on the aftermarket gauge for now since the factory gauge seems to have been telling it's "best truth" all along. I did figure out that removing the plastic wheel well would allow access to the sender if I have a problem in the future and need to install for real temperatures. I can see how useful those actual temps would be. After what turns out to be a $250 thermostat though, I will need to live with the factory gauge's less specific communication for the time being.

I am truly thankful to all who shared knowledge. And were patient with the level of knowledge I have and don't have. It was great to be able to get some important help even if I wasn't quite up to par for the course here with my own mechanical knowings. I am curious and I like to learn new things and how things work and you can't learn if you don't ask (even if some of these forums seem to use your asking to judge you somehow less than for not knowing the same things they do.) And it's great that there were answers when I did ask and I was able to learn. Thanks again very much to everyone for sharing your knowledge.

This was my Dad's truck and he died this year. We took him to the cemetery in the back of his own truck and backed it right up to his grave, so it means a lot. I hope to keep it running if I can so would that it be my sons can toss me in the back and drive me over to the burial ground same way when it's my turn. We'll see. Thanks for helping keep it going through this EASY! $250 thermostat.