inconsitent high rev's

:flash: Have a 89' nissan max SE, and just have come across a problem with my idle. After the car has risen to it's operating temp the car idles high around 1500 rpm's. Sometimes it will lower back to 500 rpms or normal idle, but sometimes I have to turn the car off to get the idle to resume back to normal!!! Have changed the water sensor temp, and the battery is fine along with the alternator!! Not really sure what the prob is if anyone has some suggestions!!! Thanks!!

There are two small rubber tubes that go from the engine to underneath the throttle body and back (recycling hot water from the engine), there is a mechanical temp sensor there, which opens the throttle a little bit on the cold mornings, but as the temp reaches it’s proper level, it closes the throttle to its normal position. If the water from the engine doesn’t get there as it should, the air coming through the throttle might lower the temp, causing the mechanical sensor to open the throttle, having a higher rev idle.