Hey fellas, I have been nursing this truck back into daily driver condition from its much neglected farm truck days and it has been a long road. However at this point it is finally starting to come around, but this cooling issue seems to be one of the last things I need to address.

The truck is a 93 1st gen, but has a 94 motor in it. Since I bought it it has cooled erratically (running cool but occasionally the gauge will go up to a bit past mid way, then go back down to normal cool levels, like it has air in the system). It has never overheated but just cools funny (according to the gauge anyway). Most of the cooling system components has been revamped so I am hesitant to just start tearing it apart after draining the coolant four times now in the truck for various reasons.
Cooling setup:
BTR radiator
Dieselsite Coolant Filtration System (made it work for the truck)
new NAPA waterpump
new NAPA thermostat
Cummins block heater replaced last week (it seemed to have gotten worst since then)

The block heater doesnt seem to be leaking and I have let the truck reach operating temps then spray soapy water on the block heater. Other than the bubbles simmering from the heat nothing seems to be leaking. I burped filled the truck by taking the coolant filtration hose off at the top of the head and then filled it with my NAPA yellow radiator funnel while squeezing the top rad hose multiple times.

I keep checking for leaks but I dont see anything. Any suggestions or advice on how to diagnose this would be much appreciated. Thanks in advance,
--Garrett

My setup:

 

---____---

Well, after freshly detailing the interior yesterday and having the truck plugged in overnight I was welcomed to this when I hopped in the truck a few minutes ago this morning. Most likely explains things:

Guess the heater core took a dump on me last night:


Freshly detailed interior:


Any suggestions now? How hard to replace heater core on these things?? Need to get this thing back on the road before tomorrow as this lemon is my DD!

 

Heater core> https://www.dieseltruckresource.com/...d.php?t=229112

Totally normal for the designed for a load cooling system on these engines to fluctuate up and down between 140-180, it's just the thermostat opening and closing.
Trying to fix it is futile.

When the 12 valves were sold new the dealer was supposed to tell the new owner the following and a sheet came with the owner's manual that said:

NOTE: THE COOLING SYSTEM USED ON THE CUMMINS DIESEL ENGINE EQUIPPED VEHICLES PROVIDES THE CAPACITY AND PROTECTION OF THE HIGHER GVWR AND GCWR RATINGS AVAILABLE ON THESE VEHICLES. OWNERS SHOULD BE ADVISED OF THE NORMAL SLOWER WARM UP AND LOWER ENGINE OPERATING TEMPERATURES ASSOCIATED WITH DIESEL ENGINES.

THE COOLANT TEMPERATURE GAUGE WILL FLUCTUATE ON DIESEL ENGINE EQUIPPED VEHICLES. IT IS INFLUENCED BY VEHICLE LOAD, OUTSIDE AMBIENT TEMPERATURE AND EXTENDED IDLE TIME. THIS FUNCTION IS CONSIDERED NORMAL

GAUGE FLUCTUATION WILL BE NOTICED MOST OFTEN UNDER STEADY STATE (HIGHWAY) DRIVING.
PLEASE ENSURE THAT YOUR CUSTOMER IS AWARE OF THIS NOTE PRIOR TO RELEASING THE VEHICLE.

In 1998 after years of complaints Cummins electronically turned the coolant gauge into basically an idiot light.
The temp still fluctuates, you just can't see it.

 

Infidel thank you very much for the informative post! Makes me feel alot better about the cooling fluctuations.

Started to attempt to take the dash apart. The instructions you posted are for a 2nd gen. I did some searches and it seems the screws to take the dash out on the 1st gen are under the windshield weatherstripping? Is this true? I just dont wanna go cutting up a very expensive weatherstrip if I dont have to.

 

scratch that! Found a great thread by Jim:

https://www.dieseltruckresource.com/....html?t=248580

Thank you DTR!

 

Haha ya, I know my owner's manual says that too. What a load of hog wash. It's such a poor excuse of an attempt to cover over a design flaw! First they say the fluctuation is influenced by vehicle load, outside temp etc, and then they say it's most likely to happen at steady state highway driving - where load, outside temp etc. DOESN'T fluctuate (much). Haha that gets me every time. It's never good to cycle hot/cold/hot/cold through a working engine - hard on heads and head gaskets. It's a bit of a poor design the way the thermostat is set up on the 12valve - it can't sense temperature quite right, so it needs a little help. This problem is very easily fixed.

Pull your NAPA stat out and throw it in the bush. Get yourself a STANT thermostat and drill one or two holes in the rim about midway from the valve to the outside of that stat with the smallest teeniest drill bit you have. They can be side-by-side or on opposite sides of the stat. My truck only needed one hole, my father-in-law's needed two. Install and refill with coolant and enjoy nice consistent cooling. The holes bypass a wee bit of coolant past the thermostat which helps it sense temperature better. Since I did this on my truck, the temp gauge hits 180* and just sits there.

 

After doing alot of research it seems that drilling holes in the thermostat is a popular method of solving the erratic cooling these trucks tend to have with the stock thermostat.

What I wound up doing:
I ran the truck without a thermostat for a day just to see if it was really the thermostat that was giving me hell. (after I replace the heater core and evaporator coil I would burp the truck but it would overheat no matter what I did). Didnt over heat running no t-stat, so called my local cummins dealer (which is on the other side of the earth for me) and ordered a cummins t-stat.

The cummins T-stat looked completely different in every way. Burped the truck, and immediately drove it off. Coolant gauge has ran cool and completely consistent with no fluctuations since install of cummins t-stat.

So to everybody who reads this, throw that NAPA garbage in the trash!!

Also to note, the NAPA stat that came out of the truck has ZERO jiggle pins.

Here is the new cummins t-stat that went in the truck (not my picture but same t-stat.):
http://www.google.com/imgres?q=cummi...:17,s:100,i:55

 

Yep, the OE Cummins stat is pretty different. The fluctuating stats in Father-in-law's truck and my truck were both 180* OE Cummins thermostats. The Stant works great and is cheaper and available all over the place.

Also, the reason the aftermarket stat's for this application don't have a "giggle pin" is because the pin is for bleeding air, and the 12V Cummins T-Stat housing already has an air bleed at the top.

I also run a 45479 Stant in winter. It's a 190* stat for a 440 Big Block Dodge, but with the hole drilled in it, it works great for a little extra heat in winter.

 

Interesting...You have a very good point 98....