mattches

So this is not really affecting anything except it does make me wonder why. When I start the truck, say in summer and I have the temp set to cool, it takes a while to get to temp. Like 5 miles or so. But if I turn it to hot (even with the fan off) while it is getting up to temp say like after a mile, then the guage goes really fast to the regular temp. If I leave it in heat when I start it, then it warms up quickly.
Why would this do this?

abecedarian

Winter or summer, both my trucks "warm" up to where the gauge moves to almost normal "warm" within 3-5 miles of driving, so what do you mean by "takes a while to get to temp"?

Having the heater "on" / set to warm lets coolant bypass the thermostat and radiator and go through the heater core.

Maybe your thermostat is stuck "open"?

mattches

By a "while" I mean that I can drive most all the way into town (about 6 miles) and the gauge still shows just below normal temp, if it is set to cool. Whereas if I have the heater set to "warm" from startup, it only takes about 1.5 miles. Same stretch of road, same driving habits.
The odd thing is that if I start off with the temp setting at cool, and drive about 1.5 miles or so, then turn the temp setting to warm while I am driving, the temp gauge moves very quickly up to the normal temp.
The temp sensors for the motor (3vze) and the in/out for the heater are at a manifold at the back of the engine. There is two sensors there, one for the ecu and one for the gauge. The coolant feed for that manifold is by a crossover tube from the front of the motor to the top of the radiator. The thermostat is at the lower side of the motor just past the water pump. The thermostat is working fine, brand new. The motor never overheats, overcools or anything like that.
It is just odd that by opening the valve to the heater core allows coolant to rush past the sensors and show that the engine gets to temp faster, or slower, depending on the position of the cab temp control.
I would guess that there is a bypass hose or something in that stupid manifold is blocked.
I guess I am wondering if anyone else has ever experienced this before.

abecedarian

I'm not familiar with the 3VZE engine but if it's like the 22RE in this regard....

The water pump pulls from the radiator.
Coolant passes through the engine and goes to the radiator in parallel with the heater.
Depending on the "heater" setting:
heater set to cold- coolant has no circulation until the t-stat opens and then circulates through the radiator. That would cause a long warm-up period.
heater set to hot (or warm or somewhere other than cold)- coolant circulates through the heater core and returns back to the water pump inlet, bypassing the radiator.

Theoretically, it should take longer to warm up with the heater on since the heater will scrub heat from the coolant to blow it into the passenger cabin. Since having the heater on causes coolant to bypass the t-stat and return to the water pump inlet, I think you have a bad t-stat- stuck open, even if only a little.