Im thinking this is a bad thermostat but wanted to confirm before I ripped this thing apart in the cold. Here are the symptoms : After running for 20 mins, No heat, temp guage stays on cold, high idle for duration of trip(in the summer too). Im thinking this all relates to a bad thermostat.... Could anything else cause these symptoms?

 

Thermostats are generally designed to fail in the open position to avoid overheating. Sounds like yours failed as they're intended to.

 

Perhaps I should have worded it..... "Failed but failing as designed"

Thanks im probably going to order one today....

 

You'll be best getting an OEM Toyota unit, I've found the typical autozone ones to be sub par in comparison. Then again, I'm very picky.

 

Yea possible thermo. Thats the easiest part replacement type deal. I'd say replace it with a "Toyota" one like "shaeff" said. Check coolant and ad as neccesary. High idle is another issue on its own. Fix t-stat first then we will talk about your idle. The engine not warming up quick enough could cause the high idle due to the computer going "wtf heat up already"

 

I almost 100% sure that is the thermostat issue because I got that exactly same problem then gone after I replaced the thermostat. The high idle because the O/D is not working at all. The O/D is related directly with the temp.

 

The computer only responds to sensor inputs and does not control the idle speed. The high idle would be caused by the IAC/AAV not closing properly thus allowing air to bypass the throttle body, which would be due to the cold coolant; and the ECU would keep the fuel mixture rich in accordance with the feedback from coolant temperature sender- basically the truck would be stuck in a cold-start loop of sorts and not enter closed-loop mode.

To the OP-
Yes, some thermostats are designed to fail-open. However some don't fail "fully" open meaning that they allow coolant to flow to help prevent overheating but they also don't allow full flow. Now, depending on where you are, a failing fan clutch could over-cool the coolant in the radiator, and if both fail at the same time, you would be hard pressed to get warm even at idle since the fan clutch would not be releasing and would be pulling substantial air over the radiator even when unnecessary.

 

^Basically what i meant in that sentance but in lazy terms.....

 

Right on. The ISC (doesn't Toyota call it Idle Speed Control?) is totally mechanical, and bases idle speed off of coolant temperature. If the ISC is removed completely, and the coolant lines looped, you will have a low idle forever, no matter how cold it is outside.