I drive a 1999 RS and after warm up and getting on the road the temperature needle climbs up to 3/4 and stays there for about 3 or 4 seconds and then drops right back down to normal so its not staying up at 3/4. i tested the coolant and its working fine and the overflow is where it needs to be and not losing any. what could it be? just want opinions before i do any work

maybe a bad temp sensor... replace it see if its any different

try to replace the temp sensor or the thermostat if still no luck there is a bleeder valve right behind the radiator cap that releases air in he cooling system but the car would have to be running with the defroster turned all the way on hot just loosen the allen head bleeder valve slowly until you see theres no more air bubbles coming out of it then tighten it down and give that a shot just dont take the screw all the way out if the cars hot it could shoot up hot antifreeze all over the place

thank you guys, i was thinking it could be air caught in the system so im going to give that a try and let you know. thank you again

ok i burped the radiator a couple of hours ago with the heater on high and every few minutes a bunch of tiny bubbles would come up so there was air in it then turned it off and replaced radiator cap with a new one. went for a drive and i thought it was ok since the temp needle rises after a few mins, i kept watching it and it slowly climbed up to middle and then close to 3/4...i think burping the system did help a bit but its still doing it. maybe i should do it 1 more time??

Alright yesterday around 5pm i got to work on this problem. Drained the radiator, filled it with water and radiator flush liquid, let than run for 10 minutes with the heat on high. let it cool, drained that and filled the radiator up with just water and ran that again for 10 minutes heater on high. Drained that and took apart the thermostat housing and put in a new thermostat and used high temperature rtv to seal it up again so it doesnt leak. let that dry for about 2 hours and put new coolant in it. let it sit overnight since 1 gallon of coolant wasnt enough to fill the system and went back just now to fill the remainder of the radiator. Ran it on heater high with the cap off to burp the system for 10 minutes, put the cap back on and went on my way with a close eye on the temp gauge. It pretty much stayed below the middle mark and every once in awhile went up slightly above the middle mark and back down to normal. so the new thermostat and coolant did work. gonna top it off when it cools down and burp it again and drive it and see if it stays completely still now.

Sounds like you're getting there...I'd definitely do the coolant temp sensor while you're at it though. It's a cheap enough part, 5 minute job, and could be the culprit

thanks guys, i definitely will do the temp sensor once i dont have to spend every penny on bills haha, but for now the temperature is staying below the middle line so i think i fixed the problem =] i appreciate the opinions!

Get a $30 infrared thermometer and check your radiator's heat pattern.

You can skip turning on the heater. DSMs are full-flow, full-time, and all you're doing is moving an air control flap. They don't run a water valve to the heater core.