hi guys, i'm having a wierd heating problem with my new 1996 f250 4x4, 5.8 auto, 110k miles. it won't blow much heat inside the cab, and the temp wouldn't really come up, so i changed the thermostat, but that didn't help at all. i then realized that my factory temp gauge is sitting at about 10:00 o'clock, about half way to where it should sit. even if i let the truck cool down completely (for a few days), the gauge will not drop. when i first got the truck, it was the whole way to cold. it slowly climbed to where it sits now after about 1/2 hour driving, and now it won't drop. i replaced the temp sender (the one for the gauge, not computer), and it still wont drop or warm up with the new gauge. with a non contact thermometer, i get about 180 degrees around the water neck, which seems normal. as far as the heat in the cab, i'm baffled there too. the thermometer says one hose to the heater core gets to about 140 degrees, and the other to about 90 degrees. if i'm driving in town, sometinmes it will blow a lot of heat, and sometimes it won't. on the highway, its got no heat. i assumed a vacuum leak, so on the highway i turned off overdrive and didn't touch the accelerator. this should kick the vacuum way up, but i still got on heat. where is the actuator for the heater box? where should i check for vacuum leaks? what else could this be?

 

On my '92, there's a white (at least it's white now) plastic line that goes in through the E-coil box for the AC. My guess is it hits a reservoir or something inside the cab. One of the most common problems with the heating systems on these trucks is that little line will break. It's far enough down-stream it probably won't cause a fast idle problem or anything, but if it breaks, it certainly won't operate the blend door. Others have found anything from deck screws, ink pens, inspection mirrors, you name it that get into the blend door and won't let it close. Yank the cover off the duct for the heater core, and see if you see anything in there. The difference in driving around town and running highway speed is a LOT more air comes through that blend door at 70 mph than will at 20 mph. The fact you're getting a 50 degree drop in the coolant temperature through the core says it's probably OK. Could just be dust bunnies clogging the core, but that usually is accompanied by low air flow, which also reduces the drop in temperature across the core. If you turn the fan on High, you'll probably see a little more drop with your thermometer.

You're hot on the trail (excuse the pun). Keep diggin.

 

With a new heater core in my truck, and a heater that will run me out, the hoses run real close to the same temperature. If you are getting that big of a temp drop, I'm guessing you are not flowing very much coolant thru the core. I agree you probably have other problems, like vacumn leaks, blend door stuck, maybe a bad temp gauge, etc, but I bet your core is almost plugged. Changing the core is about $25 and 1/2 hour. It is one of the easiest things you can do on the truck.
Good luck Frank

 

i thought it could be a clogged heater core also, so last night, i hooked it up to my garden hose and flushed it (reverse flow). it definately was not clogged at all, it flowed perfectly and no gunk came out. i'll look into vacuum and blend door, i can't see what else it could be. i still have no idea whats going on with the gauge. i drove the truck around thinking cycling the engine temp would make the gauge start working, but still no change. any ideas?

 

Good temperature drop, not drastic, with the fan on high is a good thing. Means good air flow through the core. Obviously, if you slow the fan down, the drop will be less. Huge drop typically means no coolant flow, as fmr9 said. Yours was about 50 degrees from what I remember, which is a good drop. The core is nothing but a mini radiator. That's one of the reasons turning the heat full tilt on in summer can help llimp an overheating vehicle home.

I've never touched my core nor had any problems with heat. I'm the OO on the truck in my sig. Just used the heater yesterday, and yup, it still gets hot enough to nearly melt the louvers in the vents. Like fmr9 said, it the heater's working right, it'll run you outta the truck. My AC, on the other hand, finally died last year. 15 years of service and it's already quit. They just don't make stuff like they used to.

 

Couple months ago we were having this same discussion, and I did some temperature measuring.

I have a thermocouple that attaches to my multitester. It appears to measure temps reasonably well. It measures air temp accurately, and when pressed against something hot , or cold, it responds as expected. Let's assume it is accurate enough for our purpose.
I got the truck idleing in a garage, heater turn full warm, and fan on high. This is a few months old heater core. As the engine came back up to temp, I could tell with the thermocouple when the thermostat opened. I'm measuring the surface of the heater hose, not the water temp inside the hose. I believe if the water temp goes up 10 deg, the heater hose also goes up about 10 deg, but that is an assumption. The heater hoses were measured a few inches from the firewall. I checked one and then quickly switched to the other, over and over. Temps overall varied from about 150 just before the thermostat open, to a low of about 135 deg. The temps were constantly changing as the thermostate opened and closed trying to maintain 190 deg internally.
The difference in temp from one hose to the other was neglible. I could not even tell if it was one deg. Holding it in my hand was even less telling. They are both hot.
Theroetically there has to be a temp difference between the in and out hose, because we have captured some of the heat inside the cab. In reality, there are so many BTUs flowing thru the hoses, and the heater core is so ineffecient, the temp difference is very small.
It was kind of cool watching the temp go up and down with the thermostat.
Just a test on my truck. Just for fun, a few days latter I grabbed the hoses on my Grand Marqui. My uncalibrated hand gave the same results. Both hoses are hot, no noticeable difference.
Frank

 

The gauge problem is interesting. I know how my 90 works, and how to trouble shoot it. I guess yours is similar. With the ignition on, if I take the wire off the sending unit, the gauge goes all the way to the left. If I then ground the wire, the gauge goes all the way to the right. That means the wiring and the gauge are ok. The ground for the gauge cluster could cause a problem, but I would think it would show up in other instruments too.
If your needle is not moving at all, perhaps it is dragging on the face plate of the temp gauge? I'm not familar with your instrument panel, but I had a similar problem with my gas gauge after removing my gauge cluster. Had to take it back out and get the face in right.
Sorry can't be of more help.
Frank

 

The gauge itself can be replaced don't know if it comes by itself or as a pair with whatever is next to it in your dash. I do remember replacing a gauge unit on 88 but have not been in 95 dash.
EVTM says temp sender should read 74 ohms cold and 9.7 ohms hot (probably full scale or in that vicinity.
I would think it would read 74 ohms with engine cold and somewhere around 50 ohms at operating temp (just a guess).
If your sender is showing these readings, either wiring or gauge is bad, and since it is not moving would be inclined to say movement is sticking and/or bad.
Hope this helps.