Engine Temp Drops In the Cold

[ktmall07]

Tonight, coming over a mountain pass, it was snowing pretty heavily, and coming from the valley (where it was around 60°), the temperature outside dropped to about 25-30°. In the space of a few minutes, *coolant* temperature gauge dropped down to around 120°, and the heater started blowing cold air, and did so until we got home, which was on the other side of the range, snowing heavily, and about 35°. I'm thinking it has to be the thermostat, but is there anything else I need to check? The past couple weeks, I've noticed it takes FOREVER to heat up in the morning, is there anything else it could be?

Edited February 22, 2022 by ktmall07

[gogmorgo]

You have an engine oil temp gauge, or just the factory temperature gauge that measures coolant temp? Thermostat is likely, go with 195°. 25-30° is chilly but not super cold, I wouldn't expect a bad thermostat on its own to have that drastic of an effect on a running and hot engine at that temperature, but you can always experiment by blocking some of the airflow off through the rad with cardboard or something, see if the temp comes back up. Is the fan clutch maybe seized? Low coolant level will also affect heater temp.

[ktmall07]

I'm an idiot. I meant coolant temp gauge, but somehow I typed oil temp both times haha  There's plenty of coolant, I just flushed and refilled a month or two ago, and checked up on it before my trip last week. 

 

Fan clutch seems good. No wiggle room in the fan, and I couldn't rotate it much before starting the truck this morning.  

 

I wonder if maybe an air bubble has gotten stuck and I just didn't spend enough time burping the system after the flush?

 

I'll have to check, but the thermostat could probably use replacing anyways. Thanks for the help!

[rokinn]

Hard to imagine that's anything other than a thermostat that's failed open.  Going up hill will warm it up some and going down hill will definitely cause it to cool.  The hose from the thermostat housing to the radiator after it's run for awhile should feel pretty hot despite the outside temp.

[Minuit]

If you do determine that it's the thermostat, and I see nothing else it really could be, go with Mopar for the new one if you can. They're made a lot better and also make bleeding the cooling system easier.

[ktmall07]

@Minuit any tips on where to find a Mopar thermostat? Everywhere I've looked says you can't buy them anymore.

[gogmorgo]

A seized fan clutch absolutely can cause over-cooling. There should be enough resistance in it on a cold engine that it doesn’t freewheel, but there shouldn’t be a whole pile of resistance either. 

But for sure start with the thermostat. Cars had working heaters long before they had fan clutches. A seized clutch is mostly just going to cost you a tiny amount of power and fuel economy so long as you have a functioning thermostat. I only brought up the clutch because I’ve had thermostats stick open at much lower temperatures and still had the engine get hotter than 120°F.

 

We had a discussion on Mopar thermostats recently, where we established Motorad is the supplier. They sell their own brand and also supply other brands. I wouldn’t hesitate to run one.

[cruiser54]

Local dealer?

 

https://duckduckgo.com/?q=mopar+83501426&atb=v1-1&ia=web

[75sv1]

I've had two thermostats go out on me in the past two years. I think they were both the Mr. Gasket Hi-Flo ones. One came apart. I think I do have a Motorad in one of mine. Not sure of the other, but it has the dingle ball. Here an incomplete cross over list. 

Jeep thermostat:

Stant: S330-195,

Robertshaw 330-195, 330XHT, 335XHT

Delco 131-3, 195P, 229P 229PH, 131-19

Carol 19195

Dole: DVN-1XH, DVN-26XH

DELCO: 195P, 229P, 299P

Gates: 33009, 5509

Robertshaw: 330XHT, 335XHT

STD. Thomson: W117H, 650HT

Sterling: 1X, 2H

AMGAUGE : A309  (192F)

Cresent: 3-9195

Stewart: 300 (160F), 301 (180F), 302 (195F)

[cruiser54]

Finding one that FITS is easy. Finding one that PERFORMS is when the factory stat comes into play.

[schardein]

A story from back in the day-

I put a large 4 core radiator in my 79 K5 Blazer in the fall.  That winter, my heater never really got hot.  It worked fine before the radiator replacement.  I kept telling myself the larger radiator took longer to warm up, even though I should have known better.  When spring finally came, I was thinking one afternoon, and went out and pulled the thermostat.  It was stuck open.  I could only shake my head after freezing all winter in the truck.  Put a new thermostat in and all was well.

 

Since then, I've had two thermostats stick open right out of the box.  Now, I don't trust a new thermostat.  It only takes a few minutes to fill a pot with water, stick a thermometer in it, and put the thermostat in.  I test every new thermostat before it gets installed.  I'd trust a used thermostat from a junkyard before I trust a new one.

[cruiser54]

Beware of CRAP.

ChineseReplacementAutoParts.

[75sv1]

9 hours ago, cruiser54 said:

Beware of CRAP.

ChineseReplacementAutoParts.

True dat. I listed those numbers for people to look at NOS. I did order some Carols off of E-Bay. USA made. I also have some AGS, made in Taiwan. I think Robershaw used them as a manufacture at some point. Robertshaw is back, through Flowcooler. Also, the Stewart is an 'Upgraded' Robertshaw. 

[lippetd]

Be sure to get all the air out of your cooling system after changing the thermostat or you may end up with an overheating problem instead.  On my 4.0 I usually had to leave the truck idling on an incline so that the air in the system would rise to the radiator and be expelled. I sometimes have to cycle through heatup / cooldown 3 or 4 times for the system to burp itself.