I've replaced the thermostat twice.
The second time I got the 203° Thermostat and the Billet housing from Dieselsite.

I put in a new rubber thermostat o-ring seal too.

I can go start the truck and let it idle for 1/2 hour or more, and it is
still cold when I get in (Temp gauge at the bottom & no heat).

The water pump has been replaced once as well, but that wasn't related.

Anyone know what else could cause the engine not to warm up?
It acts like the thermostat is wide open, even with a brand new one in it.

 

Are you sure its not the gauge or some sensor malfunctioning?

 

Where are you at? The north slope?

 

Theres a sensor next to the thermostat that needs to be replaced, my guages is running lower than it should even if the truck is at opeerting temp.

 

Once I get out on the road it warms up and the heater works.

But starting it up and letting it warm up in the driveway doesn't work.

Our morning temps are still in the mid forties right now (Seattle area)

 

I can let mine idle forever and it's still "cold". Mine needs RPMs in order to warm up. Same with my Excursion.

I'd say yours is working normal.

Stewart

 

Agreed. Ya' gotta' drive it to temp it up.

 

They heat up better/quicker on the road for sure,but my truck will make a little heat sitting there idling at start up,lets make sure we go through the checklist with him before we say its normal.I would say maybe the heater core might be clogging up,I'm just shooting from the hip of course but I don't want him to have an issue and the brass from FTE let him down with a quickie........Sorry guys...Just my.02 worth

 

It seems like if the thermostat is closed and no water is going through
the radiator, even a highly efficient engine should warm up (?)

My old Suburban 6.2L warmed up after about 5 or 10 minutes,
but I had to put cardboard in front of the radiator when I drove
it to get the heater to be hot enough to defog the widows and
warm the interior.

That was a good truck. I love the Excursion, but I miss my old Suburban too.

 

I live in Seattle too and don't bother to try and warm up the truck in the driveway.
Only way that I can get some serious heat out of it is to drive it.
Both my truck and my Jetta TDI are the same way. Idle forever and the guage barely moves.
The diesel engine has no throttle plate so it's gulping tons of cool air through the engine and that will carry a lot of heat away.
On a freezing morning in the winter you can watch the gauge go down a little if you pull up to a stop somewhere after driving for a while and leave the engine running.
I've had three diesels that will do that.
I had an old '84 VW Rabbit diesel that I had to put at least ten miles on it before it fully warmed up and if I let it idle for any length of time the temperature would start to drop.
I had a 92c thermostat in that thing and it was a bear to defrost on a freezing winter morning.
.
I run the factory Ford 192f thermostat in my truck.

 

i had a bad fan clutch that would blow fust enough air over the moter to keep it cool when the temp was 45 or below replaced it still took a long time to heat up but it helped

 

I hadn't thought about the air going over the engine - I figured since
coolant isn't circulating through the radiator it wouldn't help to put
cardboard in the grillle - now I see why it worked when I did that
on my Suburban. I always wondered why it worked - I never considered
how it would affect the under-hood temperatures. That's a good point
and now makes perfect sense.

It's just hard for me to fathom any engine running that cool.

Are you sure there's no throttle plate? That doesn't sound right,
but I'll do some research to satisfy myself (I want to know for sure).
Si I guess I'll before I fully believe it.

 

Absolutely positive.
If there were, the engine would be able to generate vacuum at the intake manifold.
That would eliminate the need to include the vaccuum pump that is installed on top of the right front fenderwell.

 

Ditto on what Kwik said....

The only reason you have a throttle plate on a diesel is because you own an early 2004 MY 6.0. The throttle plate was deleted thereafter....