water temp dipping

[flight doc89]

engine was rebuilt 3,000 miles ago. Just tonight, it noticed that at about 2k rpms, the water temp guage dips and starts bobbling. it only does it at about 1900-2200 rpms, more or less and the guage indicates normal. When i parked (after 10 minutes of around town driving, probably 60-ish degree weather), i popped trunk and the rad cap wasn't as hot as normal.

I was thinking maybe a fuxored thermostat, but why does it go back to normal when above 2100-2200 rpms?

 

[edi]

maybe you have air in the system...

 

[plaaya69]

Did you burp the cooling system?

 

[TurboWarrior]

yeah id try that first they are a bitch to get all the air out. Mine still has air in the system after the rebuild i can tell. Put the car on a semi-steep slope or jack up the front and run the car with no cap. you should see air bubbles start to bubble to the top of the rad, if thats what the problem is.