when im driving my temp gauge will go up suddenly like its running hot for a few seconds and then goes back down to normal real quick. i put a toyota thermostat on about 8,000 miles ago, new radiator this past summer, had the cap pressure tested and its fine. my fan is working properly too, so basically my cooling system is working as it should. no coolant loss anywhere. is it just the gauge or possibly temp sending unit? has anybody else had this problem?

 

A temperature sending unit can go bad, and read to hot or to cold. I had this happen to me, the temp gauge would read just above cold when the engine was at normal temp, replaced the sending unit and that solved the problem.

 

ok thanks

 

Is the new thermostat the dual stage unit? maybe its just a bad thermostat? if it's not the dual stage it could also be the typical temp overshoot as well. Fixed mine by drilling a hole in the thermostat as 4crawler mentions.

 

or you could have a air bubble in the system

 

I'd start by checking the level of the coolant in the radiator. A friend of mine with an MR2 had that same problem just last week and replaced the T-stat just to find that he was low on coolant in the first place. After that I'd suspect the T-stat of sticking.

 

the coolant level in the radiator and resevoir is fine. the thermostat is a genuine toyota that i got from the dealer. it was the only one that was available for my year model truck. on their website it dindnt list a dual stage thermostat just the one i got.

 

what year model is your truck? by the looks of your avatar it looks like an 86-87 like mine. what is the part number? better ye t how much was the thermo?

 

its an 87. not sure of the part number but the thremo was around $18.

 

that sounds like the single stage thermostat. I love these dealers that never mention the temperature overshoot TSB and let people know that there is a dual stage thermostat available. The dual stage should be about $35-$40 only available from the dealer.

here's info on it:

http://www.4crawler.com/4x4/CheapTri...tml#Thermostat

 

the needle on the temp gauge only goes up when the truck is at normal operating temp not when i first start the truck up with a cool engine. then it will go back down to normal real quick. ive had this thermo in for around 8,000 miles and it didnt start doing this till 3 weeks ago. thats why started thinking it was maybe the temp sending unit or a faulty gauge. i dont know why all of sudden the thermo would start doing that.

 

mine was doing the same as yours, I drilled the 1/16 hole in the thermostat and it solved my issues. My guage would almost go up almost to the point of pegging. Also I don't think it's just a cold start issue. It will also happen when the heater is on and in some other cases which I am sure you can find here by searching of temp overshoot. I am not saying that this is your problem but it's something I would look at. Could be just oil on the sending unit wire as well. Easiest/cheapest way to figure it out is the hole in the thermostat. worst case scenario your out $18 for a thermostat.

 

where do i drill the 1/16 hole at?

 

anywhere in the base kinda like the little "jiggle" valve would be if it has one.

 

ok thanks

 

not to thread-jack, but...

I replaced my temp sending unit about 6 months ago, rad and t-stat at the same time.. Temp gauge worked normal for about 1 month. then it read cold all the time for about 1 month. Then normal for a ocuple days. ANd since september or so, it has read pegged off the charts HOT all the time. Even if the truck is sitting for a few days, i put the key in accessory position, and it shoots straight up to hot.

With the frigid winter temps, when it is -20 (celcius), the temp needle doesn't shoot all the way to hot, it will read normal temp, then as the truck warms up in the driveway the needle slowly climbs and pegs itself above the red zone.

It seems like that sensor has gone screwy, and is 100 degrees uncalibrated.. I don't know how it came to be, but it broke itself somehow. The one-wire hookup connection is clean and solid..