Greetings for 2008! I've been gone for awhile but now I have some questions about the coolant temp on my 99 318. In November I had the hypertech 180 replaced because it was showing a colder temp than it used to. The mechanic said that it was in upside down and couldn't believe it lasted a year so he put in a new Stant 180. When I start up cold (about 20-40 deg) I always turn the heater off to allow faster warm-up and it definitely heats up better to the old "180-ish" mark that I'm used to. Then when it reaches that temp, I turn the heater on to the feet vents and the coolant temp quickly drops to just above the 2nd mark and will not go back up. I have heat fine, but the old stat and the stocker never did this. I understand that when the stat opens the cold coolant in the radiator then mixes causing it to drop but shouldn't it eventually return to 180? After a good 10 minutes on the highway I would think it would return to the higher mark. If I turn the heater off it will rise back up. Those of you with 180 temps and especially the Stant brand--where does your temp read with the heater on? Does the heater really draw that much heat from the coolant? Can someone post pics of their temp gauge with the 180 stat and heater on? Thank you.

(I gotta tell you the old hypertech stat seemed to work so wonderfully as it never wavered in temp no matter the season and makes me wonder how the heck it did that upside down; but that's neither here nor there.)

 

Technically, the thermostat can only regulate the minimum temperature of the cooling system, not the max, so I'm puzzled that you're seeing temps below the rated number of the thermostat. On the other hand, the cooling systems on these trucks, at least on my 2500, are pretty robust and have a lot of capacity. Turning on the heater is like turning on an electric fan behind a mini radiator. The coolant is always circulating through the heater core, by the way. When you turn on the heater it merely opens the air ducts behind the dashand turns on the fan, so you're not really speeding engine warm-up by leaving it off. But a high fan speed can add some small amount of extra cooling to the engine, but not enough to drive the temperatures below the thermostat's rating. The way the thermostat works (I hope this isn't insultingly basic) is by closing when coolant drops below a certain temperature, letting the coolant warm up in the hot block, then opening to cool it off in the radiator. By doing so, it can open and close constantlyto regulate the temperature pretty well. An amazingly effective yet very simple device. Your engine should definitely be making enough heat to keep the coolant above 180 inside the block.

If you're seeing the temperature drop below 180, then perhaps your mechanic installed a 160 thermostat by mistake? Or maybe HE put it in backwards and/or it is stuck open, or he didn't put one in at all. Even with the heater on, it should easily be able to maintain 180 if that's the rating of the thermostat. How hot is the air coming out of the heater? Do you have "the gurgle"? Check your coolant levels. If there's air in the system, the thermostat won't react the same way. Don't use the plastic bottle to check levels, open the radiator cap and see where the coolant is inside the radiator. Make sure you do this when it's cold. I usually fill mine by revving the engine up, which will drop the level of the coolant in the radiator, adding coolant, then closing it up before letting it drop back down to idle. That way you know it's full and any excess will go into the overflow tank like it should.

Double check the thermostat's function by letting it warm up and putting your hand on the upper radiator hose. You'll feel it get warmer quickly when the thermostat opens up. Check the gauge--although it's probably not all that accurate--and see when that happens.

There's also the possibility that the sensor that controls the gauge is going bad. It's just a thermistor that varies resistance as a function of temperature, and a lot of things can knock it out of whack. If you've got heat, the upper hose is warm/hot and it opens and closes like it should, I'd check the gauge next.

Hope this helps.

 

Now THAT'S an informative response! I will check what you recommend today and report back tonight. No, there's no "gurgle" or noise and the heat is decent when it comes out. Not as hot as the stocker stat but fine. That's another symptom the hypertech stat had was that it wasn't heating up like it used to. Maybe I'm just worrying over nothing. The gauge is not accurate in degrees so it could be reading 180 and I don't know. Where should the needle be on the gauge? It's plenty above the 130 mark, just a couple millimeters above the second mark. Secondly, is the sensor easy to change or cheap? Any brand or part number recommendations? Thank you.

 

Normal is about 11:00

 

Matt--
I checked the coolant cold this morning and it's right up to the top of the radiator. So I started it and waited until the hose warmed up then checked the gauge which was just a couple degrees past the first bracket past 130. Maybe 160 if I had to guess but not as high as I expected. I took some pictures with my phone but I then realized I don't have a way to upload them. Whoops. Here's what I have discovered today while testing around town:
The gauge is definitely erradic in its measurement. But it will rise in temp when the heater is off and at idle. As soon as the heater is on, OR I'm driving at 10mph or higher, the temp drops to just above the 2nd notch at about 1oo. I can make it fluctuate roughly 20-30 degrees by turning on or off the heater. What gives? This is the third t-stat I've had in this truck over 6 1/2 years and never seen the temp waver like that. Is it a bad stat or a bad temp sensor? Thank you.

 

I had some similar problems like what you've have which turned out to be the heater core having a leak in it. I also installed a 2-core brass radiator feeling it would be better than a 1-core stock aluminum. It turns out that this brass radiator has cores which are too small, causing my truck to overheat at times. I am awaiting a gap in my schedule where I can install the new radiator.

You may also have quite a bit of air in your system. It's a bitch to bleed the air from these trucks.

 

That's not good. Wouldn't a leaking heater core cause drips on the ground or loss of coolant? How about noises or odors? I have none of these symptoms.

 

Where are you located? Is it very cold? Do you do a lot of coasting? My wife's car often cools down considerably as it coasts down long hills--air is still cooling the radiator, the pump is still circulating coolant, but the engine isn't making any heat because the injectors are shut off when you're coasting (this is pretty common, and I'm guessing Dodge does it too). There might still be air in the system, but probably not enough to cause such erratic readings and you would probably have the gurgle if there was a lot of air. I'm inclined to think that everything is fine--if it warms up without any airflow through the radiator, but cools when there is airflow, then you're in no danger of overheating. My guess would be that you have a 160 thermostat in there, either the mechanic installed the wrong one or it was mislabeled from the factory (it happens). If the heater blows warm air and the truck isn't overheating, I wouldn't sweat what the gauge says. It isn't very accurate anyway, and is more of a fancy idiot light than an actual reading of temperature.

There's also the possibility that the mechanic used water to fill the cooling system, which conducts heat much better than antifreeze. My race car runs notably cooler with 90% distilled water/10% antifreezein the radiator than the standard 50/50 mix.

If you're not losing coolant or smelling it, then I don't think your heater core is leaking, but it happens quite frequently on these trucks. I don't know how to test it, however.

I wouldn't worry about the gauge reading.

 

Not always. Somewhere around here, in one of these heat related threads is a pic of my heater core. It took quite a while for the smell of coolant to become prevalent; however, even on full heat the truck would barely warm the cab. There was no leaking of any kind evident underneath the vehicle. Once replaced, that thing almost roasts me on even the lowest setting.

 

I live in Washington and it gets maybe 20 at night. During the day it's 30-50. (I'd hate to see what temp I'd get with REAL winter temps.) I'm not that worried because it still runs fine but I've owned this pickup for almost seven years and that coolant gauge has always been rock steady. I'm a stickler for everything to be right on my vehicles and I watch the gauges religiously. So I immediately knew something was up this fall when it wouldn't reach operating temp. like normal.

Here's the revised symptoms: I can make the temp drop by simply turning the heat fan on high. On low fan or at idle the temp goes up a little. As soon as I drive or turn the heat up it drops again. Could there be something electrically wrong with the fan switch? How could increasing fan speed cause the temp to drop? (Referencing your house fan/radiator analogy.)

This pickup has never done this or anything like it. I drove across South Dakota once in a blizzard with negative wind chill and the temp never wavered. I called the shop and set up an appointment for next wednesday. Tomorrow I'm going to check for codes with my PPIII.