Heater and Coolant Temperature

[Pop_Rivit]

I'm having an issue with enough heat in my 1999 Dodge 3/4 ton pickup. There are some wide variations in the temperature of the heat coming out of the vents on cold days.

I have an ultra gauge hooked up, and the coolant temperature fluctuates between 180 degrees and 190.4 degrees. When the coolant temperature is close to 190 degrees the air temperature from the vents is nice and warm. When the coolant temperature drops below 188 the air temperature at the vents gets cool, at around 180 degrees it's almost cold air. The coolant temperature seems to continue to fluctuate back and forth in 10 degree swings.

I think that the OEM temperature for the 5.9 engine is 195 degrees, and the coolant never reaches that temperature according to the ultra gauge, even in the summer.

I suspect the thermostat is either a lower temperature replacement (probably non-OEM), or the original thermostat is starting to fail. I plan to replace the thermostat this week with an OEM 195 degree unit.

I've used the ultra gauge since mid-summer and it has only exhibited the wide, 10 degree fluctuation in cold weather; in summer weather there was only a 2-3 degree difference. I've also flushed the coolant system and refilled it with Prestone antifreeze mixed so it's good to -27 degrees.

Other than an incorrect/faulty/failing thermostat, is there anything else that could cause the coolant temperature to fluctuate so much in cold weather?

 

[The_Jeff]

or you're low on antifreeze... sounds like an air bubble in the heater core.

Where's the thermostat in relation to the temp sensor for the computer? If it's on the other side of the motor, the thermostat opening and closing can sometimes have an opposite effect on the gauge reading.

 

[flwxman]

My vote is that the thermostat is failing. I say replace it!

 

[MrMeeks]

I use a 180 stat.. and the heater works very well. Your trucks heater core could be plugged and need a replacement.Test the heater core by removing the 2 heater hoses at the firewall.Now force water through the core with a garden hose and observe the flow.This could flush the core well enough for it to be of continued service.ok

 

[mechtech2]

A big difference from 190 to 188?
Weird!

But for sure, get that 195 in there. You heat should be better, and cruising gas mileage efficiency should improve.

You could try a back flush of the heater core when you change you thermostat. Even simply running a garden hose backwards from the regular flow could help.

 

[mjoekingz28]

+ on the backflush. I had a Dakota 3.9 and took off a hose (forget its been 5+ years), crammed a hose in/on it and hit the valve and some crud was released. I remember, it looks like soggy toilet paper or something.

Anyways, perfect, hot heat there after.

On a side note, I was in a early 20s performance craze and went to install a cooler thermostat (didnt help anyhow) and the bolts broke off. Had to have it towed. I know you have a 5.9 but its still a Dodge.

 

[Virtuoso]

I remember back-flushing my old SHO's heater-core and blowing out some rust and junk, and the heater worked much better afterwards.

It never hurts to do that. Although, I'm not sure what to make of the air temperature variation. Even with 180 degree coolant, you should get some nice heat through the vents.

 

[Vikas]

It looks like you have air in the system. You should get heat even while the coolant is warming up at 150 degrees.

 

[Pop_Rivit]

Replacing the thermostat corrected the issue.

The existing thermostat had 195 stamped on the bottom, but no other markings. I don't know if it was the original thermostat that was failing, or a poor quality aftermarket replacement.

I put in a Mopar OEM thermostat. The thermostat and gasket were $11 at my local Dodge dealership.

The temperature now varies between 192 and 198 and once it's up to temperature the heat is nice and even.

 

[Vikas]

Do you get heat while thermostat is warming up? When the coolant temperature reaches 150, do you get some heat now?

 

[Pop_Rivit]

Originally Posted By: Vikas
Do you get heat while thermostat is warming up? When the coolant temperature reaches 150, do you get some heat now?


I don't know for sure. I normally don't turn the heater on until the engine is up to temperature.