thermostat stuck open?

[FritzTKatt]

94 4.0, truck takes about 20 minutes at idle to crank out nice heat, then still sits at nearly cold on the gauge. It's gotten up to half way once, when I was doing donuts for 15 minutes, holding it at 2k rpm. New water temp sensor, which had fixed a random fluctuation before. Now it's pretty much staying cold. I dont think it's the water pump because it would get hot, not cold, and my cabin heat CRANKS burning hot on defrost let alone panel. Motor doesn't run strange either.

 

[RonD]

Yes probably t-stat from description.
Use a 195deg t-stat with "jiggle valve"

Jiggle valve is a small hole in the t-stat plate that allows air out when refilling system, when placing the t-stat the jiggle hole goes at 12:00 position.

Engines run best at 200-220deg, so 192-195deg t-stat will get engine up to best operating temp faster.

The '94 with 4.0l uses the heater core as part of the cooling system, you will notice there are no heat control valves on the 2 heater hoses in the engine compartment, they run directly from the engine to the heater core.
If you notice temp gauge going up and down in what seems like a random fashion, after t-stat replacement, then I would replace the heater core.
Takes less than 20mins first time, cost is under $30
I would do it just as a preventative measure when doing the t-stat since you have to drain the system anyway.
Remember to fill the core with coolant after install via a heater hose, this helps when purging air after draining.

The core can get partially blocked over time, it still blows heat but limits full flow so engine temp fluctuates.

 

[FritzTKatt]

Ok, thanks. I'll probably get to doing that on Sunday. Will have to look up in the book how to do the heater core.

EDIT: will be nice not having to warm it up for 15 minutes, and it's still not warm.

 

[RonD]

Replacing heater core is pretty simple on the '94

Remove the two heater hoses from core in engine compartment

Under the glove box drop down the fiberboard cover, if it has one, just remove the plastic holders so it drops to the floor from center hump to side.

You will now see the heater core's black plastic case, it is held in place by 4 downward facing screws, 5/16 heads if memory serves, or maybe phillips.
1 screw in each corner.

With the 4 screws removed the case cover will drop out, without the core.

I would get a towel and put it on the floor under the core at this time.

The core slides toward you, out of the firewall, and then you need to tip it to get it out, which is where the towel comes in.

The old core has weather stripping on it which can be reused if your careful or get some so new peel n' stick 1/4" or 3/8"

Install new core

Fill core once it is installed.

 

[FritzTKatt]

Cool, thanks.