1998 F150 4.6L Manual Transmission. It's my winter beater truck to be used while my EX is parked. Damn cold here right now and I'm freezing to death and could really use some help figuring out what's causing my problem?

Symptoms
Intermittent heat.
Mostly luke warm with sporadic bouts of actual heat.
Both inlet and outlet to the heater core hoses are hot.
Truck has never even thought about running hot - engine wise.
Have to run the truck 15-20 miles before the upper radiator hose will get hot.
You can idle the truck for an hour and that hose never gets hot.
If you squeeze that hose you hear a small thumping sound.

What I've done to try to fix it so far.

1. Put a thermostat in it - I think it was a motorcraft thermostat but a friend picked it up for me and I honestly never saw it in the box so I am not 100% sure.

2. Blew air both ways through the heater core, got some gunk out but not a lot. Good flow both directions.

3. Reversed the heater core hoses after it was flushed, and it had heat for a short time then stopped heating again.

4. Put a new water pump on it.

5. Ran it with coolant cap loose to burp air (now 4th time coolant has been drained and replaced so I figured air in the lines was a good bet) and it was the one time the truck had screaming hot heat, so hot I had to shut the fan down. Stopped and tightened it up after about 10 miles and heat immediately went luke warm.

6. Took the bottom of the dash apart and checked the blend door, it moves freely and the motor is fine. Removed the motor and turn the blend door manually, no change in heat. Still luke warm just like it is when you use the temperature control to move the blend door, so I don't think the door is the issue.

So #5 stands out but I don't know enough about heating systems to understand what my problem is?

 

Have you tried a new radiator cap?

 

Are you sure your blend door is not broken ? The nub on the bottom of the box will turn , not fall out , with actuator removed even if the door is broken . I have temporarily " fixed " a few with a drill bit ( small hole ) and a piece of wire with a hook on the end to pull the door closed to get heat .

 

Just re read your post ( should have done that first ) What temp is your engine running ? Hook a scanner up or use an IR gun .

 

Yep did that manually closed the door no changes.

I don't have a scanner that will read temperatures, well not for this truck anyway. Nor an IR gun but I can get a gun tomorrow and check. Where do I test it at? I know the gauge gets only to about 1/3 of the way, but I realize that's more of an idiot light than a gauge.

 

Check the temp at the thermostat housing , heater hoses , radiator hoses . Get several readings . That helps check flow and temp drops . Be sure the thermostat housing is getting to operating temp . You may have a thermostat that is opening too soon or stuck open .

 

You mean coolant reservoir cap?

 

Yes...sorry. That holds and releases pressure like a radiator cap on a radiator
.....even though it is not on the radiator proper.

 

It is the heater core. Took it by a local shop owned by a friend of mine, and despite my earlier claim that there was flow through the core, apparently there is "not enough flow". It is not flowing all the way through, just through the top portion. So it looks like a new core. Ugh!

 

I'm sure you know the dash has to come out to get at it. Are you going to do it yourself or have it done?

 

Pondering that one....for now it's parked until I either do it myself or pay to have it done. Way too cold to even think about driving something with no heat. If I do it myself I am for sure going to wait until it warms up a bit.

 

These heater cores are not too hard to change ( after you have done a few ) I think you could do it yourself . Save some money , take your time easy enough . I can do one in about 4 hours .

 

Going to do it myself. If I can do 20 extracted manifold studs I can handle a heater core. I have watched the video and just looks like the key is patience and organization, both of those I have so I'm not paying anyone else to work on my trucks if I am humanly capable of doing it mysefl. I know it's big job for a a "non tech" person but I will get it done. For now I flushed the crap out of the core today and got heat going to get me through hopefully the next week. Opened it up a good deal, the carnage that came out was impressive.

 

Good ! Do it yourself ! You have the mechanical aptitude to preform this task . I Love to see people preform their own work . I work on vehicles for a living , But I understand the $ aspect . One note , I see some that say you have to discharge the AC , NOT TRUE ! A few tricks , and an easy job . 1/4 inch tools and swivels are a must . Best of Luck !

 

I'll be honest, if that crud is still actively coming out of that heater core, I'd keep flushing. It might fix it enough that you can wait until warmer weather to fix it. And, who knows, you may be able to flush it, replace the coolant, and be good to go. If you get adequate flow through it with no leaks, I'd let sleeping dogs lie until it stops working again or starts leaking.