Dodge Demon

Been a few posts lately about faulty heater cores,plugged,leaking or whatever,any year,1st,2nd and 3rd gens, mine is 2001.5, drive it 5 days a week,100 miles a day,when winter set in last fall, it didnt seem to putting out cab heat like it used to,so I changed thermostat (192 degrees) and the engine temp now holds really well, but still not good cab heat,very noticable at 20 below F and colder, so I changed heater core, (I'm the one who cut the pipes on the new hearter core to make instalation easy),anyway now I have cab heat just like it was new again,EXCEPT,whem I'm going down the road at 60 or so MPH and I remove my foot off the pedal or cancel the cruise, and the engine imeaditly goes to idle (800 rpm)the air out of the heater vents,doesnt matter which setting,even can be on recurculating inside air, goes cool within 10 seconds of the engine going to idle, as soon as I speed up the engine to over a thousand RPMs, I again have instant hot air, as this is an auto tranny as soon as one cancels the cruise, engine goes to idle mode,so I have slowed down gradually,from 1600 RPMs to idle speed in maybe 1000 FT and the heater air stays hot, even when its actually down to idle RPMs, this condition only happens when its kinda cold out,from 20 below and lower, to me it seems that for some reason as soon as the engine drops from 1600 RPMs to idle instantly, the water pump is not circulating the water very good or the pump is cavicating or creating a vacum or what, anyone else notice this condition? Had a 95 and that didnt do this, and the only diference between the two was that on the 2001 the water outlet to the heater inlet came from the back of the head and the 95 came from the front of the head,return lines are the same place on both trucks.

YJim

2 guesses...

Coolant level low
Air in coolant system

biododge1

x2^^^might need to burp the system again

Dodge Demon

Coolant level full,cant see how a air bubble could still be in there after a month, but strange things happen, it was doing this even before i changed anything,never ever had to add anti-freeze,am i the only one with this condition.

dieselman2300

What brand thermostat did you install?
If its a parts house thermostat, that is probably your problem.
The Cummins stat opens and closes to regulate temp. constantly.
I ran a parts house stat, and I had lousy heat all the time.

Just an Idea.

dozer12216

The Cummmins stat has dual seals if recall. Defeat that with incorrect stat and you dont have correct coolant flow.

omlambu

Could it be vacuum related? I am still learning about my truck so just a thought.

Fueling around

ding ding ding

slowflyer2112

Get a infrared thermometer and measure the temperature differential between the two heater hoses at idle and under a slight load with the rpms up. This should tell you what the problem is.

nickg

I'm curious do you have the fan speed on high?
If so try it on a lower setting and see if you have the same results
I found when I have my fan on high in extremely cold temps if at idle the fan cools the coolant in the core faster than the coolant is replaced. if I drop the fan speed I have hotter air longer, actually in extreme cold I get more heat on the lower fan speed because the heater uses outside (extremely cold air) air and pumps it thru the heater core so it takes alot of heat to heat the air before it gets into the cab, I've also found I get x2 the heat out of the heater if you run the heater in the MAX AC position, as when in this selection the heater box uses air from the cab(warm air)

I too have a newer heater core (may last yr)