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Heat Cold At Highway Speeds

7K views 34 replies 11 participants last post by  davidonmars 
#1 ·
Hey guys,

I;ve been looking all over the place for an answer to this, but it seems people have the opposite problem where the heat is cold/ luke warm at idle/low speed and gets better at high speed. When making my commute back home from Long Island (about 7 hour ride) my heat just started blowing cold air at about 75 mph. I rode home frozen to the steering wheel the rest of the way and replaced the thermostat. Backflushed the heater core, before winter hit, but the problem still developed. I have tried blocking the radiator off, pipe insulation around the heater hoses. Nothing really seems to fix it or improve the quality of the heat. I have to drive it around for about 30-40 minutes for it to blow luke warm heat at city speeds. Any ideas? I wanted to get opinions before I take it to the shop and shell out cash for a new heater core or something. Thanks in advance for your help.
 
#3 ·
195. Up to the point in my trip where the heat stopped working you could watch the temp fluctuate to the temp the t state would open then the temp would drop, then suddenly the temp drooped and stayed low, like 160 or so, so I thought the t stat was stuck open. Didn't seem to fix the problem though.
 
#6 ·
my 98 was a 4 banger, and it did it to me sometimes. i just figured it was the simple fact that at 10 degrees outside and 75 mph, that little 4 cyl didnt make enough heat to keep the coolant system temp up high enough for adequate heat. basically the cooling system was just too efficient in super cold temps.
 
#7 ·
I was concerned about the heater core, but I haven't been losing any coolant, the level as been constant in the radiator and in the overflow reservoir. I'm liking the radiator cap idea, since its just a tad cheaper than the heater core. I've been watching the passenger floor mat for wetness but nothing there either. I'll give the radiator cap a shot, it's never been replaced. Thanks for the advice, I'll report back after.
 
#18 ·
Sorry I haven't posted in a while, med school is a PITA.


hskrRT, the temp was like mid 20s when that was going on, and I did change the T-stat after that happened.

I replaced the radiator cap, and I do have heat at highway speeds, but its still pitiful. I pretty much have no heat the entire way to most local destinations because it just takes too long to warm up, and when it does, its laughable. It seems to get worse when you turn the blower up. on the second setting you can tell the air is kinda warm, but any higher speeds than that it just feels like a cold breeze blowing in your face. It's too cold here to try another flush, everything is frozen. I know the heat in our trucks is not known to be fantastic, but this just doesn't seem right. I'm blown away when I ride in other people's cars by how warm their heat is. I may end up taking it in to get looked at again because I simply don't have time to trouble shoot this thing with classes, especially if there's the possibility of pulling the heater core, as I understand that's not the fastest job in the world.
 
#19 ·
I replaced the radiator cap, and I do have heat at highway speeds, but its still pitiful. I pretty much have no heat the entire way to most local destinations because it just takes too long to warm up, and when it does, its laughable. It seems to get worse when you turn the blower up. on the second setting you can tell the air is kinda warm, but any higher speeds than that it just feels like a cold breeze blowing in your face. It's too cold here to try another flush, everything is frozen. I know the heat in our trucks is not known to be fantastic, but this just doesn't seem right. I'm blown away when I ride in other people's cars by how warm their heat is. I may end up taking it in to get looked at again because I simply don't have time to trouble shoot this thing with classes, especially if there's the possibility of pulling the heater core, as I understand that's not the fastest job in the world.
have you tried sittin still and revving it up for a while, seeing if it starts blowing heat? if its not your heater core its gotta be the pump. didnt you say the engine temp fluctuates, too?
 
#21 ·
My truck doesn't get very hot as far as the heater goes unless I;m sitting at idle for 5-10 minutes after the motor has already warmed up. I can drive the truck for 20 minutes after the t-stat opens, stop the truck, pop the hood and the lower radiator hose is ice cold and the passenger side half of the radiator is as well. The cold water going back into the waterpump gets pushed right through the heater core once the t-stat opens. Try putting a piece of cardboard over half of your radiator and see if that helps any.
 
#22 ·
Step ahead of you on the cardboard. I've been running like that for a while now, no improvement. The temp was originally fluctuating like I had said. Now the engine temp will get up near 200 then fall once (presumably) the T-stat opens. I can't remember what brand the T-stat was, whatever the guy at Advanceauto game me at the time, definitely wasn't a mopar part. I have sat and revved up the engine to get it warm in the morning, but I would need to do so for quite a while do get any kind of warmth out of it, which even then is not that good.
 
#23 ·
hows the coolant level? did u get all the air out? sounds like ur low on coolant to me...
 
#24 ·
have you checked the operation of your blend door? if the tstat is new, the core is new, coolant is full... about all thats left is if your blend door isnt closing off the cold air all the way and the cold air its lettin in is just mixing with the hot air that the heater core is puttin out.
 
#27 ·
Mine does the same thing

i swapped back to a 205 tstat and it never seems to warm up.

I am thinking the heater core simply isnt flowing like it should be even after a flush.
 
#29 ·
Drove back upstate for the weekend, same nonsense with the heat. Just went out and played a bit with the heat. Shut my cooling fan off and revved the temp up to 210, barely any heat. Upper and lower rad hoses were too hot to hold. Passenger side heater hose was too hot to hold, the driver's side I could grab onto and hold comfortably.......
 
#32 ·
Sweet, lets hope its not replacement. Everything is frozen here so I can't flush it. Are there any shops that can flush the core independent of the whole cooling system?
 
#33 ·
if all your doin in the core itself, just disconnect the two hoses at the firewall after claming them shut with vise grips or something, then hook a hose up to each port and put some water to it, see if a bunch of shit (or anything) comes out.
 
#35 ·
as for everything being frozen I had to do the same thing a few weeks back while it was -15... take your garden hose and put it in a sink. run hot water for about 10 minutes till those hose defrosts. be quick with flushing it and youll be fine. The fluctuating in temp is more then likely air in the system. Mine did that too after because it wasnt properly bled. Good luck
 
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