Engine/Transmission (1994 - 1998) Truck won't warm up

[pdxkid]

At the end of my normal 10 mile commute, my truck temp. gauge is just barely above 140. This just started happening Monday. It normally warms to full operating temp. well before the end of my commute.

Does this sound like a stuck closed thermostat?

If so, Where is the thermostat?

 

[RikDavis]

It sounds like a stuck open thermostat. probably located in the housing where the upper hose fitting bolts to the engine. (at least on the 99)

 

[SnoKing]

All it takes is a small flake of rust under the seat and that will happen. My truck did that a few months ago, and both of my 4B150's in the boat have done it. Snoking

 

[Missouri Mule]

It seems to me the thermostat is on the side of the engine/ head near the alternator side on the 12 valves.

 

[Cummins12V98]

Do your self a favor and buy the T stat and two O rings from a Cummins shop. I bought a Gates from Geno's and compared to my Cummins stat it looks very CHEAP! I ordered a Cummins Stat. Wish Geno's stocked the Cummins one.

 

[63shult]

I have the same problem put two different T stat in no change when you find the problem post back so i can fix mine

 

[Jeff L]

I wouldn't just assume the truck isn't "warming up". Start with, the "the gauge says......."

What I'm getting at is, the temperature sending unit, wiring harness, or gauge could be the culprit(s). Testing the wiring and gauge is a simple procedure - with one catch. There's one wire attached to the sending unit, and all you have to do is disconnect the plug (at the sending unit), and with the ignitioin switch in the run mode (engine NOT running), ground that single wire. That should swing the temperature gauge to the high side, and light the warning lamp in the dash. If not, the wiring, or gauge is faulty.

Now for the "catch". You'll invent some new words getting to the sending unit and it's connector. It's at the extreme rear, left-side of the head, behind the lifting eye. I mean you couldn't bury it any deeper with shovel.

I'm bettin' thermostat, or sending unit, with the gauge itself as a "huh, that doesn't happen often" entry.

*edit to add 'Nother thing that'll cause low temps is if the jiggle pin (located in the thermostst housing) isn't seating. That one is real easy to troubleshoot - pinch the small hose off that goes from the fitting for the check valve (jiggle pin), to the elbow on the thermostat housing that connects to the top radiator hose. Bring that hummin' Cummins up to operating temp', and see if the thermostat, and gauge are behavin' normally. If so, the jiggle pin isn't jigglin'. You can either leave the hose pinched off, or replace the check valve (which is the right way, and it's inexpensive, and easy).