Running too cool?

[Ausfahrt]

I am the third owner of a well maintained 1996 Jeep Cherokee that I purchased about a month ago. The elderly gentleman that I bought it from took good care of it and all the fluids looked fresh.

But...when I got home I noticed that the temp gauge was in the low range. The OEM thermostat is 195 degrees so I figured that the previous owner put in a 180 degree thermostat to compensate for the Florida heat.

Since I am a loyal BITOGer, I know that an engine that runs too cool can cause sludge and reduce fuel mileage.

I bought and installed a NAPA premium 195 degree thermostat along with a new gasket and the result was? No difference.

So then I thought that the temp gauge may be off....until I noticed that the heater was not working well. It was warm but not hot.

This is my temp gauge after a 20 mile drive and a 5 minute idle to take the pic. What does the collective wisdom of BITOG think?

 

[UNC017]

Dashboard gauges aren't exactly precision instruments. Its likely either a bad sending unit, or else just a poorly calibrated gauge. Both of my Jeeps actually read pretty close to 200 degrees, but I know I'm just lucky.

You can try replacing the temp sending unit and see if that corrects the reading, but otherwise just learn where it normally runs and look for abnormal variations.

 

[Chris142]

It's a Stant stat. They use small V shaped areas where the piston seals to allow a slight amount of flow. They allow too much flow instead.

Same thing with my Jeep with a Napa stat. My mechanical guage reads 160-170F.

 

[SteveSRT8]

you cannot rely on factory gauges. Use an IR thermometer or a stick type and determine the real temp.

Then you'll know what to do.

 

[rslifkin]

If the factory gauge is accurate, then it's running around 165*. That's definitely too cool.

I'd try flushing out the heater core (and the rest of the cooling system), as the heater circuit the only bypass when the t-stat is closed on those engines, IIRC. I've seen that cause some weird stuff on the 4.0. If that fixes the heat, but not the temp reading, then definitely look into the gauge and sender.

I also agree on the cheap Stants tending to run cool. The one that was in my Jeep when I got it was a 195*, Jeep ran about 185* normally. Same thing with another one of the same. I popped a Stant superstat in about a month ago, once warmed up, it runs about 200*. An OBD scanner hooked to the computer showed 197* cruising with the heat blasted in 25* ambient temps.

EDIT: After flushing, use either old school green, or G-05 for coolant.

 

[daman]

That's showing plenty hot,check your heater core for flow restrictions.

slightly low on coolant??

 

[Pop_Rivit]

That's right about where the gauge on our 1996 Cherokee stayed when fully warmed.

You probably need to backflush your heater core-I had to do that on my 1999 Dodge pickup and it made a huge difference in the temperature the heater put out. The factory gauge on my pickup looks like it reads around 180 degrees, but my UltraGauge shows that the actual coolant temperature is between 194 and 200 (according to the computer).

 

[Ausfahrt]

Thanks for the replies.

I forgot to mention that I did do a flush and put in new green Prestone when I replaced the thermostat. The premium NAPA stat was a lot beefier than the old one FWIW. I like the idea of using the IR gun, I have one of those and will try that.

 

[daman]

So you did back flush the heater core it's self?

 

[Ausfahrt]

Backflush?

I'm not sure what that is or how to do it.

 

[daman]

Remove the hoses from the HC and shoot water/air backwards from normal coolant flow/direction..

 

[Ausfahrt]

Ah. I will try that, after reading some other threads I guess that I used the wrong antifreeze as well. Thanks for the tip.

 

[turbochem]

Well, the guage doesn't look like it had a liner scale. Going from the lowest reading to midway it changes by 110F, from midway to the highest is only changes 50F.

That makes trying to determine the actual temp difficult (when does the scale change? Does it change slowly throughout its range, etc....)

 

[rslifkin]

Prestone doesn't make true green coolant anymore. The "green" prestone "All Makes, All Models" stuff is a dex-clone, and does not belong in there.

I'd flush it out again and go to G-05 or an old school green (Zerex still sells it, not sure who else), whichever is easier to get (use distilled water for your final flush).

While doing that, it's easy enough to flush the heater core. Just run a hose through it backwards for a couple minutes, then blow the air out of the core (your mouth provides enough pressure for this), fill with distilled water, and blow it out again (to get the tap water out of the core).

An easy way to do a system flush on a 4.0 is to disconnect the heater return line, put it in a bucket, plug the port it connects to, and let it idle while adding distilled water to the rad. Then, put it all back together, drain the rad, add 50% of the system capacity worth of concentrated coolant, then top off with distilled water until full. Drain the overflow bottle and refill with a 50/50 mix of coolant and water.

 

[mstt55]

Any update on this issue? Did you get it fixed?

I have the exact same problem on a 95 Cherokee. I assumed I had the wrong temp thermostat in. So I put in a new 195 one from Napa. Didn't fix it.

Ran right at 210 all summer, but as soon as it got cold it won't warm up all the way. Gas millage went from 17mpg to 13mpg.

 

[ahoier]

Some others to check....coolant temperature sensor......and temperature "sending unit" (another sensor....).

lol, we ran into a similar scenario with my 88 dodge aries 2.2l - always stuck in the "Cold" area....but it was "deep cold" as I coined it lol.....as if the gauge was "disabled"....

We changed out the Temperature Sender, Temperature Sensor, and then finally, when those didn't fix it...the Thermostat to find the previous owner COMPLETELY removed the thermostat....