Is my thermostat stuck open

[sleonard13]

I have a 91 c-1500,,,,Its about 60 here in Indiana over the last couple days...When i first start my truck when the engine is cold will the top radiater hose be hard or should it remain cool and soft until the thermostat is open..My temp guage after 20 min at a idle about 1400 rpm is only i would say 120 and i have not seen it rise..The thermostat in it is a 195 degree one... so the engine should warm up quicker I think...I do get heat after about 10 min. Its warm not real hot...Please let me know..What is the operating temp of a 91 chevy truck.

[Palf70Step]

It sounds like it to me. My 02 had the same issue. I never really notuiced it until it got really cold down here and saw that my truck never was making it up to operatiing temperature. Flsuh your system, put in new stat and coolant and you'll probably be back to normal.

[metalgod]

should be rite at 210 and the hose will stay cold and squishy for a few mins before getting hard then it will get hot enuf to open the stat and then the hose will be squishy and hot

[sleonard13]

I can fill mine get hard as soon as i start the truck..I think with it running cold its throwing my idle off..The coolant temp keeps telling the computer the engine is still cold...I drove it for about a hour today and it would not come off 120-130 degrees...when i shut the truck off the temp guage did climb abit since water was not going through the motor..

[sleonard13]

I took the t-stat out and checked it and it would open and close..i thought maybe i had a faulty temp guage..i went ahead and put the new one on since i already bought one...I started my truck and it went to about 1400 rpm,,,The guage slowly started to move up to around 190 after about 10 min of idling and the rpm was dropping also instead of always staying high...Could the one I took out been opening to early or could it of just been the wrong one..The one I picked up at autozone was a 192 temp...

[metalgod]

yes they go bad ive replaced a few of them, they really do some weired thing's some times to mess with your head but it sounds like you fixed it.......

[ChevLoRay]

Seems I have heard Click and Clack on NPR talk about some other problems that can happen when good thermostats go bad. One of them is poor gas mileage. If the temps never get to the right place, other inputs to the CPU don't happen, either.

So, any issues with your fuel mileage?

Did the new thermostat help it?

We're all curious, so clue us in.

[sleonard13]

Yeah the thermostat made a hughe difference...before I replaced the t-stat I could here apinging in the motor when the car was in drive it was more noticable..No mater how much i drove it the damn temp guage would not move..i had heat somy uncle wasconvinced it was a faulty guage..After about a week I noticed when I would shut the motor off and leave the key to run position i would see the temp climb..So I went ahead and replaced it...Now with 10-15 min after start-up the idle rpm drops to about 1000...When i shift it into gear it drops to about 700-800rpm and sounds real nice idling with the dynaflow exhaust..I no longer hear the pinging or knocking or something coming from the motor..Also before i replaced it after i would be driving, giving it plenty of time to warm up..I could shut the engine off,,start it back up and the rpm would go right to 1500...Now when I do the same thing it will jump up to 1300, 1500 rpm but drop quickly to idle about 900 in park,,About 750, 800 in gear...She runs great now..Don't know about mpg,,,Don't drive it much..

[ChevLoRay]

So, it sounds like you had drivability issues in addition to temperature issues. The CPU's in these vehicles, especially the OBDII (1996 and up), are much more dependent on data from sensors than something that is old-school, like my '69 C-10.

[sleonard13]

Really it always ran good it was just the idle always being off on start up and warm up and when shifted into gear...

[69-350]

Originally Posted by ChevLoRay

So, it sounds like you had drivability issues in addition to temperature issues. The CPU's in these vehicles, especially the OBDII (1996 and up), are much more dependent on data from sensors than something that is old-school, like my '69 C-10.

Indeed. It's called the Closed Loop and Open Loop, and deals with your O2 sensors. When you first start your engine, it's cold, and your O2 sensors go into the closed loop, yadda yadda, as it warms up, it goes into the open loop, changing some things around. It's been a few years since I've been taught about it though, so I can't remember everything what it does. That's the jist of it though; open loop, closed loop. Matters on emissions and general operations of the engine, like what sleoard13 was saying.