I have a 1987 F250 w. 6.9-L diesel, 4WD, 4 on floor which has a heater problem. When I first bought the pickup, I always had heat, winter & summer. Summers are short in Dakota Territory, & I prefer to drive with the windows open, so this didn't really bother me.
Now, however, for the past 2 winters (we're starting the 3rd) I have no heat whatsoever 90% of the time. Today's high is +11F. Last night's low was +3F. Driving with no heat & a blanket over my legs is not fun.
I have: flushed & reverse-flushed the radiator & the heater core half a dozen times, replaced the antifreeze (60-40 mix), replaced the thermostat.. last Saturday. My non-contact thermometer read 140F on all hoses & the top of the block after running 20+ minutes. When I read it on the top of the radiator, it read 125F. When I flushed, the fluid came out as clean-looking as any I've ever seen.
The air conditioner is disconnected - I don't like AC.
The deflector(s) under the dash are not operating correctly in that when I put the lever to "floor" I get air from the panel vents, but that shouldn't matter to the amount of heat I get. Or don't..
I can't find any kind of valve under the hood that might operate the heat, but did find a small "cannister" above the heater housing that has a wire coming off of it. Can't find anything for it to operate though, to or from. My meter doesn't tell me anything except that the wire is entire.
I finally gave up & took it to the 2 best mechanics in the area & neither of them could find anything either.
Has anyone out there got an answer for this? It's c-c-c-COLD out there & this is the end of November. Tain't gonna improve until something like April at the earliest. Thanks!

 

Do you put stuff on your dash.. paper, pens, random junk? You should be able to see where I'm gong with this, if something fell down into the vents it could be blocking the blend door in next to the heater core. The other possibility is you have a broken vacuum line inside the dash, the temp control level is vacuum actuated. What you should do is remove the cover off the heater core enclosure and inspect for the above problems. The heater core is accessed by removing the glove box and then removing the heater core cover panel.

 

In addition to the deflector that moves the air from defrost, dash, or floor vent I believe there is also one that determines whether or not the air passes over the heater coil. It is how the desired temperature is changed from cooler to warmer. Check to see if it is working correctly. I think it is operated by a wire-type cable that can break or come loose.

 

Before I put any junk on my dash, I glued some screen in place over the vents at the bottom of the windshield to prevent anything interfering with the door you speak of & trust that obviated the problem. However, I bought this truck 4 years ago & maybe something got in there before I did this. I took the cover off the heater core today & looked in there & everything looked fine. I tried reaching up behind the core, since I have medium-slender fingers, but found nothing. I wonder if the door isn't blocked anyway. so tomorrow I guess I have to bite the bullet & put him in the shop a-gain & drain antifreeze a-gain so I can pull the heater core & look at the door. If this thing is vacuum-operated, as old as this truck is, I wonder if I'm not looking at a vacuum leak. There is that 'cannister' on the heater fan housing with the plunger on one end & a wire running to a bundle.... I worked that plunger all the way 'in' today & wired it in place.. I got tepid air into the cab, which is something of an improvement, but it's still between +3F - +15F out there, not counting wind.... Pretty quick I'll get tired of all this entertainment & go find a job so I can pay bills all winter. I'm not much on sitting around on unemployment.. and auto mechanicking is entertaining, but I really prefer doin' it in warmer weather. The shop is at +50F, which beats outside ambient, but it still isn't 'warm', you know... Thanks for your efforts, guys. I'll keep you posted.

 

First of all after a long drive you should check the temperatures like you did in above post.
Also check the heater hose temps. If both are hot then its what ever flapper valve in the unit that directs the heat, I haven't played with the heaters in these trucks "yet".
If the engine temp isn't comming up you have a regulator (thermostat) problem. Does the fan motor come on?

 

Are you getting good heat, and no volume, or lots of cold air?
My diesel has a penchant for expanding the tubes in the heater core, and blocking off the fins, so, I'll get good hot heat, but, almost no volume from the fan. It's on it's 4th heater core in the last 2.5 years, with no explination whatsoever for it's doing this.

 

I'm getting medium to low volume & usually, c-c-c-cold air unless or until I have driven the truck more than 1/2 hour. But sometimes I get medium to low volume & wrist-temperature heat.
Today's efforts were: (1) to remove the heater core & look for a blocked gate. None. There are an inner & an outer 'door' to the left of the core cavity as I look in thru the glove box hole.. The door closest to the core moved appropriately as I ran the lever from defrost to floor. The inner 'door' didn't move except when I put my hand in & moved it; then it rebounded to its original position, which was against a hole, closing that hole. Figuring I had little to lose by blocking it open, I took 2 alligator clips & bent the ends where wire would go so the door would be kept open. Not much - 1/2" or so - but it's not what I first had. (2) Since I put the weather front someplace safe a couple of years ago & can't find it, I put overlapping cardboards in front of the radiator & blocked it off completely. I can always slide the pieces to give the radiator more air if need be.
My friends' shop is 18 miles from my digs & I came on into town, making my total run about 25 miles. At 20 miles, I began to get heat out of the panel vents (easiest to reach while driving). It's a little hotter than I got before, but not much. Volume from fan is no better. Dash gauge reads a little hotter than before I put in the cardboards, but not a lot.
Unless I can find a winter job, I can't afford a new heater core ($65.00) so I guess I'm facing another winter with a blanket over my knees & the seat piled with stuff so I don't have to try to heat the cab with my own heat. Not my idea of fun, but it still beats walking. Wind chills here tonight are predicted to be -15F to -25F.
About your heater core tubes expanding -- if your fluid is too hot & not moving fast enough thru the system, the expanding fluid could expand your tubes, hydraulics being what it is coupled with the lack of space for expansion.. Which leads me to.. check your thermostat & your water pump (??)..
Thanks.. DW