my '99 f-350 has no heat. i can't get the motor hot enough to open the oem temp thermostat. i replaced the thermostat thinking it was stuck open but, the new one isn't opening. what would keep the engine from reaching operating temp or at least hot enough to open thermostat? i'm in florida so, it's not below freezing ambient temps

 

Are you positive you didn't put the new one upside down? (And don't ask how I know this can be a problem....)

Obviously if you have combustion you have heat, which will transfer from the cylinders to the oil and from the oil to the coolant. Its not like there's a mechanical device that could break... What does your heat gauge say? Surely your head passages aren't all plugged. What's your mileage?

I believe the answer to the question "what would keep the engine from reaching operating temp ". is as a practical matter, "Nothing"

 

You're right that lack of heat problems are typically in the thermostat. But on older vehicles your heater core could be plugged, or the switch/valve for diverting hot water into the cab could be broken.

 

from what i see, there is no way to put it in upside down. the temp gauge only goes up about a quarter inch from the low line ( not the very bottom ). i can hear water/ air mixture @ heater core so, i tried to overfill the coolant to be over the upper hose on core. still hasn't done anything different. when it reaches highest temp, the upper rad hose and both heater hoses all feel the same. the lower rad hose is cold--that's how i know the thermostat isn't opening. i've let it sit and idle for about 45 min. and it won't heat up. the truck has 131,000 miles on it

 

Well I'm certainly confused. If you have a hot upper hose and a cold lower hose then either the thermostat is closed, or the radiator is plugged. Yet if the upper hose is hot, you should be getting heat in the cab heater, which I believe you aren't.

Well okay, here's what I would do. Remove the thermostat. Then if your heater works just fine after a long warm up, you know that the thermostat is the problem. If it doesn't, feel the radiator hoses again to determine whether the radiator is plugged. If the top and bottom are the same temperature, then the problem is a plugged cab heater. Otherwise its a plugged radiator/valve.

Makes sense to me..... Anyone else?

 

Well on reflection maybe there are some other points. I know you said you topped off the coolant, but after you've worked on the cooling system and run the engine, its not uncommon for the coolant level to drop maybe even as much as a gallon, depending upon what you've been doing. I'm assuming you have rechecked the coolant after running the engine for 10-15 minutes.

I'm also assuming that your temperature gauge reads normal after the engine is totally warmed.....

 

Sounds like you need to bleed the cooling system.

By the way, the bottom radiator hose is SUPPOSED to be cooler than the upper one. But in your case, it should be somewhat warm to the touch, at the very least.

 

Noting that Irishkev ran his engine for 45 minutes, practically speaking, what more bleeding can he do?

 

Aha...... Another possibility might be a water pump wearing out. We really need to know what that temperature gauge is showing!

Also, can you visually see coolant motion at the top of the radiator?

 

To TK1251k -

Sorry I can't respond to your e-mail because I get a message that you haven't enabled your e-mails.

In my personal experience most heater problems are just a plugged heater core and the solution is to get it backflushed someplace like Rapid Oil or a mechanic, etc. I.e., Coolant is forced through the heater core backwards, which pushes debris out the way it came in. However, to answer your question as to what the valve looks like, bear in mind that its generally a vacuum switch so there will be an upside-down-tea-cup-looking piece of metal with a vacuum line attached at the center. That in turn will be attached to two heater hoses. Certainly it would be possible in a high mileage vehicle for that vacuum line to have a leak, or maybe the vacuum canister could have a rusted-through spot. I would presume though if you have such a vacuum leak that your engine would not be operating normally.

 

thank you for all suggestions and advice. i think i'll try pulling the t-stat and see what happens.